Vehicles are equipped with a variety of sensors that are used by the PCMs to perform their intended functions. Since the introduction of GM electronic fuel injection, the sensors have changed in form, but not function. In most cases, early GM sensors are directly compatible with the Gen III PCMs. They all have significant importance to the PCM.
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Vehicles are equipped with a variety of sensors that are used by the PCMs to perform their intended functions. Since the introduction of GM electronic fuel injection, the sensors have changed in form, but not function. In most cases, early GM sensors are directly compatible with the Gen III PCMs. They all have significant importance to the PCM.
Throttle Position Sensor
The purpose of a TPS is to indicate to the PCM the angle of the throttle blade. For cable throttle systems, this sensor directly represents the driver’s expectations related to acceleration and deceleration. At closed throttle, the TPS supplies a signal of approximately .5V to indicate idle and a signal of up to 5V to indicate WOT. The PCM commands additional fueling during WOT by entering the power enrichment (PE) mode.
Early sensors have slotted mounting holes that allow the sensor to be mechanically zeroed; that is, set to a specified low voltage (approximately .5V) at closed throttle to indicate 0 percent TPS to the PCM. Late sensors are nonadjustable and if TPS voltage at closed throttle is within an allowable range, the PCM auto-zeros the TPS signal (percentage of calculated throttle angle).
TPI Throttle Position Sensors
The early TPS (1985–1989) allows for adjustment so that a closed throttle signal can be adjusted to a recommended .54V and sweep to more than 4.0V at WOT. The late TPS (1990–1992) is not designed to be adjustable because the ECM auto-zeros the TPS. This TPS uses a different harness connector than the LT1and LS-series sensors.
LT1 Throttle Position Sensors
The LT1 throttle body changed to accept a new style of TPS. General Motors installed nonadjustable sensors. The aftermarket has adjustable sensors available to allow for signal adjustment if, after an aftermarket throttle body upgrade, the PCM cannot auto-zero the TPS. An adjustable TPS may be appropriate for poorly manufactured aftermarket throttle bodies known to require TPS adjustment. This type of TPS housing uses a different connector than the early TBI and TPI sensors.
LS-Series Throttle Position Sensors—Cable
The LS-series (4.8L, 5.3L, 5.7L, 6.0L, and 8.1L) TPS is nonadjustable and uses the same harness connector as the LT1 sensor. This TPS is interchangeable among 1999–2002 LS-series engines with the exception of electronic throttle systems.
LS-Series Throttle Position Sensors—Electronic
Early LS-series throttle position sensors for electronic throttle systems are unlike any TPS used with a cable throttle system. Electronic throttle systems use an APP sensor, TAC module, and TPS to electronically operate the throttle blade. For these systems, the TPS is used by both the PCM and the TAC module to indicate throttle angle. These sensors vary in appearance, but have the task of reporting two opposite voltages. (See Chapter 8 for more information about electronic throttle systems.)
Idle Air Control Valve
Cable-driven throttle systems require air to bypass the closed throttle blade(s) for an engine to idle well. The amount of air required is adjusted by the PCM through the IAC valve. The IAC is an electrical valve that moves a pintle (pin/bolt) one direction or the other to add or subtract airflow around the throttle blade(s) through a small passage within the throttle body housing. Although some IAC valves appear to look the same, they vary in pintle shape and spring tension. It is important that the IAC valve matches the housing. The LS-series PCMs control all TPI, LT1, and LS1 IAC valves. The only exception is PCMs from vehicles not equipped with cable throttle.
Electronic throttle systems do not use an IAC valve. The Gen III electronic throttle systems use the TAC module to add or remove throttle blade angle to adjust the amount of incoming air at idle. The Gen IV systems have throttle control built within the ECM; they also adjust the throttle for idle. Because the throttle area is so much larger than an idle air passage, electronic throttle systems can adjust the amount of incoming air more quickly than cable throttle systems. For engines with aggressive camshafts, this means better control of the engine at idle.
TPI IAC Valves
At first glance, all TPI IAC valves appear to be the same. However, the clate TPI IAC valves do not interchange with the early TPI throttle body lower IAC housings. Be sure that the IAC valve matches the year, make, and model of your throttle body.
LT1 IAC Valves
The late TPI IAC is used with 1992 and 1993 LT1 engines. This IAC is functionally the same as the 1994–1997 IAC valves, but they do not interchange due to the differences in the throttle body lower IAC housings. The 1994 IAC stands alone while the 1995–1997 IAC is interchangeable. The late LT1 IAC valve interchanges with the Ram Jet 502.
Gen I Vortec Truck IAC Valves
General Motors used the same IAC from 1996 to 2005 for the 4.3, 5.0, and 5.7L Vortec trucks. This IAC does not interchange with LT1 or LS1 throttle bodies. Having the same throttle body housing as the Vortec engines, the Ram Jet 350 IAC interchanges with the Gen I Vortec truck IAC.
Gen III IAC Valves
Among the Gen III IAC valves (4.8, 5.3, 5.7, 6.0, and 8.1L), the only one that stands alone is for the 1998– 1999 Camaro and Firebird. All other IAC valves are interchangeable.
Ram Jet 350 IAC Valves
The Ram Jet 350 uses the same throttle body housing as the Gen I Vortec truck. The IAC valve interchanges with the Gen I Vortec throttle bodies.
Ram Jet 502 IAC Valves
The GM Ram Jet 502 throttle body uses the same IAC as the 1995–1997 LT1 engines.
Engine Coolant Temperature Sensor
The PCM relies heavily upon the temperature of the engine. The engine coolant temperature (ECT) sensor is commonly located in either the intake manifold or the cylinder head and measures the temperature of engine coolant. The PCM uses the ECT signal for turning on and off electric fans, ignition timing, fueling, and many other functions.
ECT with 3/8-Inch, 18 NPT Thread
The most common ECT sensor among Gen I engines installs into a 3/8-inch, 18 NPT threaded hole in the engine’s intake manifold. The Gen II (LT1) uses this sensor, but it is located in the front of the water pump. This sensor is used with all TPI, LT1, and Gen I Vortec engines.
ECT with 1/4-Inch, 18 NPT Thread
While not used with any Gen I, II, or III engine, Delphi sensor # 12146911 is functionally the same as the sensor used with Gen I and II engines, but installs into a 1/4-inch, 18 NPT threaded hole.
ECT with M12 x 1.5 Thread
All Gen III engines have an M12 x 1.5 thread in the cylinder head for the ECT sensor. There are two sensors: one for the 1998 Camaro/Firebird and one for all other Gen IIIs. The ECT sensor used with the 1998 Camaro and Firebird has a built-in sending unit for the coolant temperature gauge in the instrument cluster. These sensors are functionally the same as the sensor used with Gen I and II engines.
Air Temperature Sensor
The intake air temperature (IAT) sensor is used by the PCM to determine the temperature of the air entering the engine. This temperature data is used to determine fuel and spark calibrations. In most applications, the IAT is located ahead of the throttle body and in the airstream.
TPI engines, however, locate a manifold air temperature (MAT) sensor in the bottom of the upper plenum. The IAT and MAT produce the same signals and are used in the same way with the PCM. Because a sensor mounted in the aluminum plenum becomes heat soaked, a better location for the sensor is ahead of the throttle body.
Intake Air Temperature Sensor
A variety of IAT sensors are used among the many GM fuel-injected engines. These sensors differ in form only, as their function remains the same. The GM IAT sensors may be used interchangeably. The late IAT sensors are located within the mass airflow (MAF) sensor assembly. If a MAF sensor is your preference, it is easiest to use a MAF sensor with integrated IAT. For speed density configurations (no MAF sensor), there are several sensors to choose from, so finding the right sensor to fit your intake air tubing should be easy.
Manifold Air Temperature Sensor
The MAT sensor is used with TPI engines to determine the temperature of air entering the engine. This brass sensor has a 3/8-inch, 18 NPT thread and is located in the bottom of the upper plenum. The Camaro and Firebird use the same ECT sensor as the MAT sensor while most Corvettes use a similar brass sensor with a visible thermistor. Although the signals are the same, these two sensors are not interchangeable without changing the harness connector. These sensors become heat soaked as the aluminum intake manifold increases in temperature.
Many TPI owners relocate the MAT sensor ahead of the throttle body or replace it with an IAT sensor ahead of the throttle body. The MAT sensor signal is the same as the IAT signal.
Mass Airflow Sensor
The MAF sensor is used by the PCM to directly measure weight (or mass) of air that enters the engine to control fuel delivery. The MAF signal, measured in hertz (Hz), is interchangeable among the LT1 and LS1 sensors. The early 1985–1989 TPI MAF sensor is not compatible with the LS-series PCM. In fact, the Gen IV MAF sensors are also compatible with the LT1 and Gen III PCMs.
TPI MAF Sensor (1985–1989)
The TPI MAF sensor outputs a 0–5V analog signal to the ECM. This signal is not compatible with the Gen III PCMs.
Three-Wire MAF Sensor (1994–2002)
The common three-wire MAF sensor was introduced with the 1994 LT1 vehicles. It can be found in many 1996-2002 vehicles, including Camaro, Firebird, Corvette, and GM trucks. This sensor measures 78 mm in airflow diameter.
Five-Wire MAF Sensor (2000–2007)
Perhaps the most desirable MAF sensor for Gen III LS engine conversions is the five-wire version that was introduced with the 2000 GM trucks. Its extra two wires are used for the internal IAT sensor. Having the IAT within the MAF housing is very convenient. This sensor is also used with the 2001–2007 Corvette and 2005– 2006 GTO. The inside airflow diameter of this sensor measures 85 mm. Engine calibrations that were set for the earlier 78-mm MAF requires updates to the MAF frequency table for proper fuel delivery.
Slot-Type MAF Sensor
The LS7 6.2 and 7.0L Corvette engine introduced a five-wire MAF sensor that inserts in the side of the intake air tubing ahead of the throttle body. This sensor outputs a frequency that is compatible with the Gen III PCMs. It may be mounted in any size of inlet tubing ahead of the throttle body. For big-cubic-inch engines and/or forced induction, this sensor is a good choice.
Manifold Absolute Pressure Sensor
The manifold absolute pressure (MAP) sensor measures manifold pressure. Its signal is used by the PCM to control fuel delivery and ignition timing. The Gen III PCMs rely heavily on the use of a MAF sensor for fuel delivery; if the MAF fails, the PCM looks to the MAP sensor to calculate fuel delivery.
It has become rather common for engine conversions to eliminate the MAF sensor and for tuners to calibrate the PCM for MAP use only, otherwise known as “speed density.” This system requires several factors to calculate fuel delivery, where a mass airflow system uses a direct measurement from the MAF.
Most MAP sensors are suitable for up to 1-bar intake manifold pressure. Without a supercharger or turbocharger, intake manifold pressures do not exceed 1 bar. The turbocharged GMC Syclone and Typhoon, along with the supercharged 3.8L Grand Prix GXP, require a 2-bar sensor because the turbocharger is capable of generating more than 1-bar pressure within the intake manifold. When forced-induction pressures exceed 2 bar, such as with the 1989 Turbo Trans Am, a 3-bar sensor is required.
Remote-Mount MAP Sensors
The most universal 1-bar MAP sensor is GM# 12569240. It is mounted to a bracket on the side of the 1990–1992 TPI plenum and uses a vacuum hose elbow connection to a fitting in the intake manifold plenum chamber. This sensor can be mounted to a vehicle’s firewall while using a vacuum hose connection to a fitting on the intake manifold. If you supercharge or turbocharge your engine, a 2-bar MAP (GM# 12569241) supports up to about 14.7-psi boost, and a 3-bar MAP (GM# 12223861) supports up to about 29.7-psi boost. These 2- and 3-bar MAP sensors use the same housing as the 1-bar MAP sensor, but because the connector key configuration is not the same, the 1-bar green harness connector must be changed to the orange harness connector.
Be careful to choose the proper MAP sensor for your engine configuration. The PCM simply sees 0–5V from the MAP sensor. Regardless of MAP sensor (1-, 2-, or 3-bar), the output range is 0–5V. Choosing a 3-bar MAP sensor when MAP values do not exceed a 2-bar MAP sensor’s range results in loss of lookup table resolution/ definition for proper fueling.
Be sure to take advantage of a PCM custom operating system upgrade when using a 2- or 3-bar MAP sensor so that the PCM works best with the 0–5V range of the MAP sensor.
Manifold-Mount MAP Sensors
The manifold-mount MAP sensors are available in three different housings. The earliest MAP sensor is the same as the remote-mount TPI sensor, but is used with a seal. The OBD-II 4.3, 5.0, and 5.7L Vortec engines use a sensor housing that is also found on the 4.8, 5.3, and 6.0L LS-series truck engines. The sensor housing used with the LS1 Camaro, Firebird, Corvette, and GTO attaches slightly differently than the LS-series truck sensor.
The LT1 engines use the same remote-mount 1-bar MAP sensor as the TPI engine. However, the sensor is bolted to the front of the intake manifold and sealed with a rubber seal (GM# 1635948). General Motors used this same design with the Ram Jet 502 intake manifold.
For supercharged or turbocharged engines, a 2-bar MAP (GM# 12569241) supports up to about 14.7-psi boost, and a 3-bar MAP (GM# 12223861) supports up to about 29.7-psi boost. These 2- and 3-bar MAP sensors are a direct fit to the LT1 and Ram Jet 502 intake manifold, but because the connector key configuration is not the same, the 1-bar green harness connector has to be changed to the orange harness connector.
The OBD-II 4.3, 5.0, and 5.7L Vortec engines use the same sensor as the 4.8, 5.3, and 6.0L LS-series engines. These intake manifolds use plastic attaching tabs that break if you are not careful. The Ram Jet 350 engine also uses this MAP sensor, but is attached to the side of the intake manifold with a metal bracket. For supercharged or turbocharged engines, a 2-bar MAP (GM# 12615136) supports up to about 14.7- psi boost, and a 3.3-bar MAP (GM# 12623671) supports up to about 30-psi boost. These 2- and 3-bar MAP sensors are a direct fit to these truck-type intake manifolds and Ram Jet 350 intake manifold, and do not require a harness connector change.
The LS1, LS6, and LS2 intake manifolds use their own MAP sensor. It mounts to the intake manifold with one plastic clip. The MAP sensor is found at the rear of the LS1 and LS6 intake manifolds. The LS2 intake manifold relocates the MAP sensor to the front of the intake manifold with a plastic attaching clip.
For supercharged or turbocharged engines, a 2-bar MAP (GM# 12615136) supports up to about 14.7- psi boost, and a 3.3-bar MAP (GM# 12623671) supports up to about 30-psi boost. These 2- and 3-bar MAP sensors require slight modification to the housing to fit one of these intake manifolds. A harness connector change is not needed.
With the LS3 engine, General Motors introduced a Bosch MAP sensor in the same location as the LS2 manifold, but with an attaching bolt. While electronically compatible with the LS1/LS6/LS2 MAP sensor, the LS3 MAP sensor is not a direct fit to the early intake manifolds, uses a different harness connector, and its pin configuration is unique.
Knock sensors are used by the PCM to reduce ignition timing if detonation occurs. With varying frequencies produced by the engine, the knock sensor vibrates to generate a small voltage that is interpreted by the PCM as normal (no ignition timing retard necessary) to severe (retard ignition timing by predetermined amount).
For best results, the knock sensor(s) should be located where detonation-related frequencies can be detected. The sensor should be of a design that accurately generates the appropriate voltages to the PCM when those frequencies occur. With the introduction of aftermarket valvetrain upgrades, larger-cubic-inch engine builds, and other aftermarket parts that alter engine frequencies, matching a knock sensor, its location, and PCM calibration details can be difficult.
There are two types of knock sensors used with the LS-series PCMs: resonant and flat response. The resonant sensor is a single-wire connection to the PCM and the flat-response sensor is a two-wire connection to the PCM.
Resonant Knock Sensor
The single-wire resonant knock sensor was introduced with early small-block engines. This sensor typically threads into one of the coolant plug holes near the center of the block just above the oil pan. TPI engines use only one knock sensor and, with the exclusion of the Camaro and Firebird, LT1 engines use two knock sensors.
The sensors look the same, but are not interchangeable. LS-series engines use a similar knock sensor with a straight M10x1.5 thread. In the LSseries engine, resonant knock sensors are found under the intake manifold.
Flat-Response Knock Sensor
The two-wire, flat-response knock sensor requires either an attaching bolt or stud for attachment to the engine. The flat-response knock sensor was introduced with the 2005 LS2 engine and with the 2001 4.3L Vortec engine. With proper calibration changes and a little engine wiring work, the early LS-series PCM can be configured to use the flat-response knock sensors. It’s a convenience for LS2 engine swaps into LS1 vehicles.
The purpose of an O2 sensor (often abbreviated O2S) is to generate a signal voltage to the PCM that represents the air/fuel ratio. The target air/fuel ratio for engines burning pump gas is 14.7 parts air to 1 part fuel. This balanced reaction of combustion is referred to as the stoichiometric point. This is where the engine burns fuel most cleanly while generating low emissions.
But don’t write this off as being important for emissions only. A controlled 14.7:1 ratio keeps an engine clean. You can also hear the engine run well at idle when it is producing exhaust gases at the stoichiometric point. During the tuning process, and while at idle, as the fuel calibration is changed to target the stoichiometric point, the engine begins to sound unstrained and RPM increases.
Upon engine startup, the PCM does not immediately use the O2S signals to adjust the injector pulse times. This operation is called “open loop” because the feedback from each O2S is ignored while the PCM calculates injector pulse times based on calibrated values. After a predetermined amount of time and engine coolant temperature, the PCM enters “closed loop” and monitors the O2S signals. In closed loop, the PCM applies fuel trims to each bank of injectors in an effort to achieve the point of stoichiometry.
When the O2S signal voltage is low (below 450 millivolts, or mV), the PCM detects a lean air/fuel mixture and adjusts the injector pulse times to add more fuel to the bank of cylinders that is running lean. When the O2S signal voltage is high (above 450 mV), the PCM detects a rich air/ fuel mixture and adjusts the injector pulse times to take away fuel to the bank of cylinders that is running rich. At an O2S signal voltage of 450 mV, the PCM zeros the fuel trims and does not apply adjustments to the injector pulse times.
There are two types of fuel trims: short-term fuel trim (STFT) and longterm fuel trim (LTFT). The PCM uses STFT to make quick, and continuously changing, percentage adjustments to the delivered amount of fuel by adjusting fuel injector pulse times while targeting the stoichiometric point. LTFT is less responsive than STFT and is used to make broader air/fuel mixture adjustments to compensate for wear and tear type conditions; for example, a small air leak from an intake manifold gasket.
For safety, STFT and LTFT have limits. It may seem beneficial to allow the fuel trims to replace calibrated fuel delivery values, so that a camshaft upgrade would not require custom tuning. However, over time O2S performance degrades (slow response) or even fails, so this could lead to the demise of an engine that is controlled by a bad O2S. Although the advertisements of aftermarket engine control units (ECUs) may seem to suggest it, no engine computer can (fully) tune itself.
All Gen III production vehicles have four heated oxygen sensors (HO2S). Each sensor is electronically heated because the internal zirconia-sensing element only generates voltage when it is hotter than about 600 degrees F. The exhaust temperatures alone may not keep each HO2S hot enough, especially when the engine is operating at idle for an extended amount of time.
Early LS-series production vehicles use 12V ignition and a battery ground to heat each O2S. Later LSseries production vehicles use 12V ignition and a PCM-applied ground to heat each O2S. With the PCM monitoring the O2S heating circuit, a malfunction can be diagnosed through any OBD-II scan tool.
Placement of Oxygen Sensors
The terms bank 1 and bank 2 refer to a set of engine cylinders. Bank 1 represents the side of the engine that begins with cylinder 1, or cylinders 1, 3, 5, and 7. Bank 2 represents the side of the engine that begins with cylinder 2, or cylinders 2, 4, 6, and 8.
Each bank contains two HO2S:one before the catalytic converter and one beyond the converter. Each HO2S before the converter is used by the PCM for fuel trims and each HO2S beyond the converter is used to monitor the change in air/fuel mixture through the converter for emissions purposes.
Most “off road only” vehicles eliminate each rear HO2S when catalytic converters are not installed.
Oil Pressure Sensor
The oil pressure sensor is a threewire sensor that returns a 0–5V signal to the PCM representing engine oil pressure.
Do not confuse it with an oil pressure sender used with the oil pressure gauge only, or an oil pressure switch that allows electrical current to pass when oil pressure exists.
The oil pressure sensor is used by the PCM for calibration logic and to broadcast the engine oil pressure through the OBD-II data stream to be used by the instrument cluster (not all vehicles) and displayed through an OBD-II scan tool.
Although the oil pressure sensor is not required by the PCM, it is a useful signal to have available through a scan tool. For engines on a test stand or dynamometer, this sensor eliminates the need for a separate oil pressure gauge. When using scanning software such as from EFILive, the oil pressure PID can be selected among the other PIDs that are monitored while the engine is running.
M16 x 1.5 Threads
The first oil pressure sensor was used with the 1997 Corvette. Until 2003, all other LS-series engines were fitted with an oil pressure sender used by the instrument cluster. The oil pressure sender was eliminated in 2003, and the sensor became standard with LS-series engines. For retrofits, the oil pressure sensor can be removed and replaced with an oil pressure sender compatible with the vehicle’s oil pressure gauge.
The oil pressure sensor is located at the rear of the engine near the intake manifold. All LS-series engines have M16x1.5 threads for the oil pressure sensor.
1/4-inch, 18 NPT Threads
A sensor with a 1/4–inch, 18 NPT thread was used with the 2003–2007 8.1L engines. This sensor is an excellent solution for any early small- or big-block Chevy engine. For small-block installations at the rear of the engine, a 1/4- to 1/8-inch, 18 NPT adapter is required.
Vehicle Speed Sensor
The vehicle speed sensor (VSS) is a magnetic pickup used by the PCM to indicate vehicle speed. The sensor is commonly mounted in the transmission tailshaft housing and gets its signal from a toothed ring installed on the transmission output shaft. The early VSS (TPI and early LT1) mated to a plastic gear that meshed with another gear on the transmission output shaft.
Calibrating the VSS after a tire size change or gear ratio change used to be a big deal with TPI engine computers. The early TPI engine computer required a 2,000-pulse-per-mile signal and the late TPI engine computer required a 4,000-pulse-per-mile signal. To achieve these signals, you had to install a different set of internal VSS gears or use an aftermarket VSS interface module. The LS-series PCMs accommodate all of these GM VSS signals because the pulse count, tire size, and gear ratio are configurable within the PCM.
In situations where you do not know the pulse count from the VSS, use a chassis dynamometer to calibrate the PCM’s VSS input. As long as you know tire size and gear ratio, the pulse count can be changed so that the VSS PID value matches the wheel speed of the dynamometer. Your local tuning facility can assist with VSS calibration.
Camshaft Position Sensor
The camshaft position (CMP) sensor supplies a 50-percent duty-cycle signal from the camshaft to the PCM to indicate the stroke (intake or exhaust) for any given cylinder. When the ignition key is turned the engine is started, the PCM monitors engine speed (RPM) and expects to see an increase in RPM to indicate that the engine has started. If this RPM increase does not occur, and the engine has not started, the PCM considers the cam signal faulty and assumes the opposite stroke in another effort to start the engine.
Symptoms of a bad CMP sensor or misalignment of the cam reluctor wheel include backfiring while cranking the engine to start and/or extended cranking before the engine starts. In addition, the PCM may set a DTC related to the cam sensor.
The PCM supplies a 12V reference, low reference, and receives a signal. General Motors offers several Hall effect (magnetic trigger) sensors; the following sensors perform the same function.
Rear-Mount CMP Sensor
Early LS-series engines have a CMP at the rear of the engine where a distributor is traditionally located. The sensor gets its signal from the camshaft. Although the harness connector is the same as the front-mount CMP, the connector cavity assignments are not the same.
Front Timing Cover CMP Sensor
Late LS-series engines have a CMP mounted in the front timing cover. The sensor gets its signal from a pattern cast into the camshaft timing sprocket. The harness connector is the same as the rear-mount CMP; the connector cavity assignments are not the same.
Vortec Distributor CMP Sensor
The 1996–2002 Vortec distributor outputs the same 50-percent duty-cycle signal as the Gen III engines. The Vortec distributor requires proper orientation adjustment so that its signal falls correctly within the 24x crank signal. This CMP uses a different harness connector and its cavity assignments are different than the LS-series CMP.
Crankshaft Position Sensor
The CKP sensor outputs a square waveform to the PCM to indicate crankshaft position. Although there is no PID to monitor crankshaft location, the engine-speed (RPM) PID value is generated from the CKP sensor signal. The PCM supplies a12V reference, low reference, and receives a signal voltage. General Motors offers several Hall effect sensors; the CKP sensors in this book perform the same function.
Gen III Engines
All LS-series CKP sensors are located in the side of the engine block, just above the starter. The CKP sensor gets its signal from a reluctor ring that is pressed onto the back of the crankshaft. (See Figure 5.1.)
The 1997 Corvette LS1 engine introduced a crankshaft reluctor that generates a 24-pulse square waveform to be used by any Gen III PCM to control a V-8 engine. The CKP sensor reads from this 24x reluctor, which has two rows of 24 teeth that are out of phase. The resulting signal consists of two different width pulses (12 and 3 degrees) that are 15 degrees apart.
Gen VII 8.1L Big-Block Engines
The 2001–2007 8.1L Vortec engines use the same PCM as the Gen III engines and have the same 24x crank signal requirement. The CKP sensor is located at the rear of the block, just ahead of the transmission, and extends toward the crankshaft to read a 24x signal from the crankshaft reluctor.
4.3L Vortec V-6 Engines
The 1996–2007 4.3L Vortec V-6 engines have a CKP sensor mounted in the timing cover. This CKP sensor reads from a three-tooth crank reluctor (3x) that is pressed onto the front snout of the crankshaft. (See Figure 5.2.)
All 4.3L Vortec V-6 engines use a single coil and distributor because the low-resolution 3x signal does not allow for coil-per-cylinder ignition. The 2001–2002 4.3L Vortec V-6 vehicles use the same PCM as the 2001–2002 LS-series vehicles (GM# 12200411).
5.0L and 5.7L Vortec V-8 Engines
The 1996–2002 5.0L and 5.7L Vortec V-8 engines have a CKP sensor mounted in the timing cover. This CKP sensor reads from a four-tooth crank reluctor (4x) that is pressed onto the front snout of the crankshaft. (See Figure 5.3.)
All 5.0L and 5.7L Vortec V-8 engines use a single coil and distributor as the low-resolution 4x signal does not allow for coil-per-cylinder ignition.
The 2001–2002 5.0L and 5.7L Vortec V-8 vehicles use the same PCM as the 2001–2002 LS-series vehicles (GM# 12200411).
The 1996–1997 LT1 engines have a CKP sensor mounted in the timing cover. This CKP sensor reads from the same four-tooth crank reluctor (4x) that is used with the 5.0L and 5.7L Vortec V-8 engines. (See Figure 5.4.) Although the LT1 PCM looks to the Optispark distributor for crankshaft position, the 4x CKP signal is used for misfire detection.
7.4L Vortec V-8 Engines
The 1996–2000 7.4L Vortec V-8 engines have a CKP sensor mounted in the timing cover. Light-duty trucks (with engine RPO code L29) use a four-tooth crank reluctor (4x) pressed onto the front snout of the crankshaft. (See Figure 5.5.)
All L29 7.4L Vortec V-8 engines use a single coil and distributor because the low-resolution 4x signal does not allow for coil-per-cylinder ignition.
The 1999–2000 medium duty trucks (with engine RPO code L21) use a dual-pattern 24x crank reluctor with teeth that are out of phase. (See Figure 5.6.) This 24x signal is identical to the Gen III LS-Series and Gen VII 8.1L big-block crank signal.
Because of crank reluctor differences, the L21 engines use a different CKP sensor than the L29 engines. However, both L29 and L21 engines share the same timing cover.
Gen I Small-Block and Gen II LT1 Engines
To bring coil-per-cylinder ignition to the small-block and LT1 engines, EFI Connection designed two similar 24x crankshaft reluctors that produce the CKP sensor signal requirement of the Gen III PCMs.
For Gen I small-block engines, this crank reluctor is used with the 1996–2002 5.7L Vortec timing cover and 7.4L L21 CKP sensor. (See Figure 5.7.) For LT1 engines, this crank reluctor is used with the 1996–1997 LT1 timing cover and 7.4L L21 CKP sensor.
The 1996–1997 LT1 timing cover has limited clearance near the crankshaft seal, so one reluctor design allows for the LT1 timing cover and the GM single-row timing set. The 1996–2002 5.7L Vortec timing cover has additional clearance near the crankshaft seal and allows for the other 24x crank reluctor that can be used with double-row timing sets.
TPIS has designed billet-aluminum timing covers for both small-block and LT1 engines that have adequate clearance for doublerow timing sets and any EFI Connection 24x crank reluctor.
Accelerator Pedal Position Sensor
The APP sensor is only used with electronic throttle systems. The APP signals are received by the TAC module to indicate the angle of the accelerator pedal. The APP sensor receives several 5V references, several low references, and outputs two or three 0–5V signals.
With the exception of the 2005– 2006 E40 engine computers (Corvette and GTO), all APP signals are received by the TAC module. The E40 engine computer has the TAC built within and directly receives two APP signals. (See Chapter 8 for more information about the APP sensor.)
The park/neutral (P/N) indicator signal is a ground signal from a gear select switch that is used by the PCM to determine if the vehicle is in park/neutral or in gear. A ground signal indicates park or neutral, and an open signal indicates that the vehicle is in gear.
The PCM uses the P/N input for idle characters and as a requirement to perform the crankshaft variation learn procedure. The P/N signal is only required for automatic transmissions. The only LS-series vehicles with a P/N signal are the 1998–2002 LS1 Camaro and Firebird. The LS1 Camaro and Firebird have a P/N switch attached to the gear select lever in the center console. All other LS-series vehicles are equipped with a gear select switch on the side of the transmission case that indicates not only P/N, but also the selected gear.
When using a P/N switch, the calibration must be set to expect the P/N-only signal. If your base calibration is from a truck or Corvette, you must change the gear select type to P/N only. This setting is not available in all tuning software packages.
Gear Select Switch
The gear select (PRNDL) switch is mounted to the side of the transmission case. The transmission’s gear select lever passes through the PRNDL switch. The PRNDL signal is based on a combination of four wires that, depending on the selected gear, output either a ground signal or an open signal. This combination of ground/open signals is used by the PCM to determine the selected gear.
When using a PRNDL switch, the calibration must be set to expect the PRNDL signals. Base calibrations from a Camaro or Firebird must be adjusted to change the gear select type to PRNDL. This setting is not available in all tuning software packages.
Although the same gear select switch was used across the same model year, not every vehicle used the switch in the same way. For example, some wire harnesses use the neutral safety switch for starter crank inhibit and some wire harnesses use the neutral safety switch for a starter relay control signal.
Early Gear Select Switch
The first transmission-mounted gear select switch carries GM# 24229422. This early PRNDL can be found as early as 1995 on 4L60-E and 4L80-E transmissions. This switch is easily identifiable as it has two connections. The four cavity harness connector provides the PCM with a combination of ground signal wires to indicate the selected gear. The seven-cavity harness connector provides neutral safety, P/N indicator, and backup lamp signals.
Many of these switches, and their harness connectors, are considered unusable (cannot be modified or used in other applications) since adhesive at the connections has bonded the harness connectors to the switch assembly.
Late Gear Select Switch
A revised switch assembly was introduced in 2004 and carries GM# 24221125. This switch assembly is functionally the same as the early switch assembly but it accepts an improved lever lock type connection.
The brake switch has been used since the 1980s to control 700R4 torque converter clutch (TCC) lockup. In these early applications, the brake switch receives 12V ignition power and passes 12V to the TCC solenoid when the brake pedal is not depressed. With the brake pedal depressed, the TCC solenoid loses power and inhibits TCC application.
Rather than providing 12V to the TCC solenoid, the LS-series PCMs receive this brake switch signal and apply additional logic to TCC control.
Cable Throttle Systems
For applications with cable throttle, this switch is only required with 4L60-E and 4L80-E transmissions. For manual transmission installations, this switch is not used. (See Figure 5.8.)
Electronic Throttle Systems
For applications with electronic throttle, regardless of transmission type (automatic or manual), the brake switch signal is used by the TAC system for cruise control logic. When the brake pedal is depressed, cruise control is disabled. Without this brake switch signal, cruise control is inhibited. (See Figure 5.9.)
Clutch Pedal Position Switch
For vehicles fitted with a manual transmission, a clutch pedal position (CPP) switch is used to control deceleration
fuel cut-off (DFCO) events. For such vehicles also fitted with an electronic throttle, the TAC system uses the CPP signal for cruise control logic. When the clutch pedal is depressed, cruise control is disabled. Without the CPP signal, cruise control is inhibited.
12V Ignition Signal
Some vehicles are equipped with a clutch switch that passes 12V ignition to the PCM. These vehicles require 12V ignition power when the clutch pedal is not depressed. The circuit is open when the clutch pedal is depressed. This signal is not received at the PCM in the same pin location as vehicles equipped with a CPP ground signal. (See Figure 5.9.)
The LS1 Camaro and Firebird are equipped with a clutch switch that passes a ground signal to the PCM. The PCM requires ground when the clutch pedal is not depressed. The circuit is open when the clutch pedal is depressed. This signal is not received at the PCM in the same pin location as vehicles equipped with a CPP 12V ignition signal. (See Figure 5.8.)
Fuel Enable Control
The Camaro and Firebird require a 5V 50Hz pulse width modulated (PWM) signal to the PCM’s fuel enable circuit to disarm the antitheft system and allow the PCM to command fuel delivery. Other vehicles with a Gen III PCM use Class 2 communications for disarming antitheft.
For Camaro and Firebird only, the early 1998 PCM receives this signal at pin 11 of the red connector and 1999 –2002 PCMs receive this signal at pin 30 of the red connector. VATS bypass signal generators are widely available on the market, but they do nothing more than satisfy this antitheft signal requirement. Because conversions always require custom tuning, your tuner can disable the VATS at that time within the calibration— if for nothing more than to disable unneeded DTCs.
Written by Mike Noonan and Posted with Permission of CarTechBooks