GM ECMs have come a long way since the introduction of the first multi-port injection V-8 ECM used in the 1985 TPI Camaro, Firebird, and Corvette. These early ECMs require a PROM (or “chip”) that contains the engine’s calibration data. While all TPI ECMs are for use with high energy ignition (HEI) systems, which requires a single coil and distributor, there were a few ECM updates before the introduction of a new ECM and ignition system used with the 1992 LT1 Corvette engine. LT1 engines use a unique distributor assembly driven by the front of the engine’s camshaft and output high- and low resolution signals to the ECM (or PCM). All 1996 and newer vehicles have a flash-based PCM that is programmable through the vehicle’s data link connector (DLC).
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GM ECMs have come a long way since the introduction of the first multi-port injection V-8 ECM used in the 1985 TPI Camaro, Firebird, and Corvette. These early ECMs require a PROM (or “chip”) that contains the engine’s calibration data. While all TPI ECMs are for use with high energy ignition (HEI) systems, which requires a single coil and distributor, there were a few ECM updates before the introduction of a new ECM and ignition system used with the 1992 LT1 Corvette engine. LT1 engines use a unique distributor assembly driven by the front of the engine’s camshaft and output high- and low resolution signals to the ECM (or PCM). All 1996 and newer vehicles have a flash-based PCM that is programmable through the vehicle’s data link connector (DLC).
Tuned Port Injection ECMs
In general, all ECMs used with the Camaro, Firebird, and Corvette TPI engines require a single coil and distributor for engine operation. These early ECMs are a batch-fire system, meaning that injectors are not controlled individually (or sequentially). Since the TPI Camaro, Firebird, and Corvette were equipped with either a manual transmission or 700R4 automatic transmission, the only electronic control related to transmissions is 700R4 torque converter lockup. Due to limited calibration capabilities, these ECMs are not well suited for forced induction and high-horsepower engines. Despite the limitations, many continue to use these ECMs for most basic multi-port fuel injection engine installations.
LT1 PCMs
The 1992 Corvette introduced the LT1 engine and a new ECM that requires high- and low-resolution signals from within the distributor. This new distributor design, used through 1997, limits the LT1 ECM and PCM as LT1 engine use only. In 1994 the 4L60-E transmission was a big leap forward in technology and requires the use of a PCM that handles both engine and transmission control. The 1994 1997 PCMs are desirable to the LT1 owner because they allow for custom transmission programming to adjust shift behavior. The OBD-II implementation in 1996 added a four-tooth crankshaft reluctor and crankshaft position (CKP) sensor within the engine timing cover for misfire detection.
The earliest electronically controlled multiport fuel injection for V-8 engines uses a PROM-based ECM and single coil with distributor ignition. The early TPI ECM (left) is shown with the cover removed to reveal the MEMCAL, which holds the PROM containing the engine calibration. The late TPI ECM (right) continued to use a MEMCAL, but relies on engine vacuum readings (through a MAP sensor) to calculate fuel delivery rather than measuring the amount of incoming air (through a MAF sensor).All TPI ECMs support only single coil and distributor ignition. All Corvette TPI engines use a large-cap distributor with coil on top (not shown).
Vortec PCMs
The Vortec 4.3L, 5.0L, 5.7L, and 7.4L engines received a significant ignition system update with the OBD-II implementation. The traditional HEI pickup was left behind and replaced with a CKP sensor that determined crankshaft position and was used for misfire detection. From within the timing cover, all Vortec 4.3L V-6 engines are fitted with a three-tooth crankshaft reluctor; all Vortec 5.0L, 5.7L, and 7.4L V-8 engines are fitted with a four- tooth crankshaft reluctor. Many of the PCMs used to control these Vortec engines offer support for 4L60-E and 4L80-E transmissions. Enthusiasts have been replacing these “black box” Vortec PCMs with the Gen III PCM for higher RPM capability and forced-induction support. (Read more about the Gen III PCM upgrade in Chapter 15, Project 5, on page 136).
In 1994 General Motors introduced a flash-based PCM in the Camaro, Firebird, and Corvette. This LT1 ECM is unique to the LT1 engines because it requires the high- and low-resolution signals from within the LT1 distributor. Although this early PCM is not fully OBD-II complaint, it has much in common with Gen III PCMs. GM trucks were fitted with an OBD-II compliant flash-based PCM in 1996 (not pictured). With no coil drivers, these early PCMs support only single coil and distributor ignition.
Generation III Powertrain Control Modules
Not unlike the preceding PCMs, the Gen III PCMs were used to control and monitor the engine, transmission, emissions equipment, electric fans, charging system, and air conditioning system. While several updates and improvements were made to the Gen III PCMs, the can be used almost interchangeably for modern V-8 engine conversions. The versatility of the Gen III PCM makes it an excellent candidate for use as a standalone controller for just about any GM V-8 conversion.
The Gen III PCMs were released in two different cases. The 1997–1998 PCMs (left) use the same wire harness connectors as other LS-series PCMs (right), but they are not pinned the same. The 1997- 1998 PCMs are scarce and generally not commonly used with LS conversions. All other LS-series PCMs are plentiful and widely supported among engine tuners.
512-Kilobyte PCM
The first Gen III PCM was introduced with the newly designed Gen III LS1 V-8 engine. This PCM (GM# 16238212) was used with the 1997 Corvette and was configured for electronic, drive-by-wire, throttle control. In 1998 the Camaro and Firebird received the LS1 engine and Gen III PCM, but it was configured for traditional cable throttle control using a throttle position sensor (TPS) and idle air control (IAC) valve.
What made this PCM unique to the LS1 engine was the 24-pulse crankshaft signal input. The PCM monitors the 24-pulse crankshaft signal to determine when to fire ignition coils and injectors. Early small block and big-block Chevy engines do not produce this 24-pulse crankshaft signal. The wire harness for this PCM is the most unique among all Gen III–equipped vehicles. It shares the same wire harness connectors as all other, newer, 512-kb PCMs but it is pinned very differently.
You must look closely at the PCM service number located on the bottom of each PCM. While the 1999–2000 PCMs are interchangeable with the 2001–2002 versions, the 2003s incorporate a few important changes. The 1999–2002 PCMs (left) use connectors with blue and red retainers, while the 2003- newer PCMs (right) use connectors with blue and green retainers. The red and green retainers are keyed differently. Not all 2003-newer PCMs support an IAC valve, a limitation that does not allow for a cable throttle body.
Notice the difference at the rear corner of these two 2003 PCMs. The one on the left (GM# 12582605) is missing the chip that handles the IAC stepper motor. The one on the right (GM# 12576106) includes the chip that handles the IAC stepper motor. Pay close attention to GM service numbers to be sure you have a PCM that works with your throttle type.
In 1999, the GM# 16238212 PCM was replaced with the GM# 09354896 PCM. These PCMs are not interchangeable and the wire harness was pinned differently. Vehicles using these PCMs included the Corvette, Camaro, Firebird, and full-size trucks. The full-size trucks were available with the 4.8L, 5.3L, and 6.0L engines. The only GM calibrations used with these PCMs were for Gen III engines, which were all equipped with a 24x crank reluctor. The Corvette continued to use an electronic throttle body, the Camaro and Firebird used a cable throttle body, and the trucks were available with either a cable or an electronic throttle body. The electronic throttle systems required a throttle actuator control (TAC) module, a small black module that communicates with the PCM to control the throttle body and cruise control system. The electronic throttle body and TAC module was different between Corvette and trucks.
The final update to the 512-kb series of PCMs was the introduction of the GM# 12200411 PCM in 2001. With the exception of the 2003 Corvette, this PCM was used through 2002. This PCM is considered the most desirable because of its versatility. General Motors used this PCM with Gen III engines (Corvette, Camaro, Firebird, and trucks), Gen I small-block engines (5.0L and 5.7L Express Van), and Vortec 4.3L V-6 engines (S10 pickup, Blazer, and Express Van).
General Motors released these PCMs with calibrations that support V-8 one-coil-per-cylinder ignition (24x crank signal only), V-8 single coil and distributor ignition (4x crank signal only), and V-6 single coil and distributor ignition (3x crank signal only). While this PCM was used with both cable and electronic throttle systems, electronic throttle is not available for engines with single coil and distributor ignition systems because General Motors never released such a configuration.
1-Megabyte PCM
General Motors released several different hardware numbers to identify the 1-mb Gen III PCMs, but any of them can be used with an engine equipped with a 24x crank signal. All 2003-newer Gen III PCMs (excluding the 2003 Corvette) were fitted with additional flash memory capacity. These 1-mb PCMs use the same harness connectors as the earlier 512-kb PCMs, but the color and key configuration of the removable retainers were changed. All 512-kb PCMs use one 80-cavity connector with two blue retainers and one 80-cavity connector with two red retainers. All 1-mb PCMs use one 80-cavity connector with two blue retainers and one 80-cavity connector with two green retainers. The 1-mb PCMs look the same as the 512-kb PCMs and operate in much the same way.
Notice this PCM (GM# 12582605) is missing the chip that controls the IAC motor. This PCM does not allow for use of a cable throttle body. Avoid this GM service number if you plan to use a cable throttle with your engine.
This PCM (GM# 12576106) has the IAC chip installed. This PCM can be used with either a cable or electronic throttle body. This PCM is commonly found in the 2004 GTO and Express Van.
Most 2003-newer Gen III engines are equipped with electronic throttle. The few cable throttle exceptions are found in the Express Van and 2004 GTO. The Corvette continued to use the same TAC module through the Gen III engine run, but the trucks received a new TAC module that is not interchangeable with the early version. Although all Gen III TAC modules use the same two harness connectors, they are not interchangeable.
Take caution to identify a 1-mb PCM that is compatible with your throttle system. Some 1-mb PCMs are unable to control the IAC valve on a cable throttle body. A visual inspection of the PCMs circuit board reveals whether the PCM is capable of controlling the IAC. Some 1-mb PCMs are not fitted with a driver (a chip located at the far corner of the circuit board). Other 1-mb PCMs, such as GM# 12586243, were used with both cable throttle engines(2004 GTO) and electronic throttle engines (2004 Corvette).
Moates 16-Bit Real-Time Emulation PCM
Craig Moates, a prominent GM enthusiast, developed an add-on emulation board that fits within the 1999–2003 512-kb PCMs and 2003– 2007 1-mb PCMs. Rather than flash memory access through the serial data stream at the OBD-II diagnostic connector, the PCM’s flash memory is tied to the tuner’s laptop through a USB connection. This real-time tuning solution is compatible with EFILive and TunerCat OBD-II tuning software. The process of tuning an engine with this emulator is fantastic. Combined with EFILive, the tuner can watch engine parameters on a laptop PC and make instantaneous corrections to the calibration without a single hiccup from the engine.
EFILive has added a control panel to their software for toggling emulation, reading the calibration file from the PCM, applying calibration changes to the PCM, and rewriting the PCM’s entire calibration content.
The tuning process can be a breeze when you have realtime access to the PCM’s flash memory; it allows updates to the engine calibration while the engine is running. Craig Moates developed a flash memory add-on module that installs within the case of the 1999-newer Gen III PCM and allows real-time access to the PCM’s calibration through a USB connection to a laptop or PC. Both EFILive and C.A.T.S. OBD-II Tuner have made a provision in their software for use of this PCM. Although the PCM can be permanently installed in a vehicle, it is most often used as a tool to quickly dial in the proper PCM calibration and then flash the final work into another PCM.
EFILive allows access to the Moates Emulation PCM through its tuning software. The user can retrieve a stored calibration, flash in a new calibration, or set the PCM for emulation mode to make real-time updates while the engine is running. If the setting option is enabled, the PCM makes instant changes as the tuner updates values within the tables in the tuning software; the vehicle experiences changes in engine/transmission behavior as updates are being made.
Throttle Actuator Control Modules
All Gen III electronics require a TAC module when using an electronic, drive-by-wire, throttle system. The responsibility of the TAC module is to receive inputs from the accelerator pedal position (APP) sensor plus TPS and operating parameters from the PCM (through two serial data lines) to control the throttle body motor.
Perhaps most significant to enthusiasts, the TAC module is responsible for cruise control operation. With the elimination of a throttle cable, cruise control module, cruise control cable, and all associated wiring and vacuum hoses, the engine bay is free of the extra clutter without cruise control compromise. The TAC module simply receives 12V signals, the same as many early GM cruise control switches, to signal cruise control on/off, set/coast, and resume/accelerate driver requests.
All Gen III engines equipped with an electronic throttle body use a TAC module for throttle and cruise control operation. This is a TAC module from a 2004–2005 Cadillac CTS-V and its APP sensor assembly (accelerator pedal). All TAC modules have two wire harness connections: one dedicated to the APP sensor and the other for power, ground, PCM communications, cruise control, and throttle operation.
No matter which TAC module you work with, it looks like one of these. The 1999–2002 GM truck TAC modules use the same black plastic housing (left) as all Corvette and LS6 Cadillac CTS-V TAC modules. The Corvette and LS6 Cadillac CTS-V share the same TAC module. In 2003 the trucks received an updated TAC module with an aluminium back plate (right). It is always best to keep matching electronic throttle components (TAC module, PCM, accelerator pedal, and TAC to pedal harness). A mismatch of electronic throttle components can cause a frustrating troubleshooting experience.
With required brake and clutch switch signals, the PCM and TAC module know when to cancel cruise control operation. Compared to early, cable-driven, cruise control systems, the driver’s experience is noticeably improved. With no vacuum actuator and cable, the acceleration lag experienced with early cruise control systems is replaced with a smooth, modern feel.
General Motors did away with the external TAC module with the introduction of the Gen IV E40 ECM. All Gen IV systems use electronic throttle and are controlled by the ECM. With no provision within the Gen IV ECMs for an IAC motor and the requirement of two throttle position signals, cable throttle is no longer an option with Gen IV ECMs. With the exception of the E67 ECM (and supported calibration), cruise control is a function of the body control module (BCM) and requires the controller area network (CAN) communication with the ECM for cruise control operation.
With the exception of the 2004 GTO and 2003- 2006 van, all Gen III systems use a TAC module and electronic throttle body.
1997–2004 Corvette and 2004–2005 Cadillac CTS-V TAC
Electronic throttle was the only option with all C5 Corvettes; this meant the elimination of throttle and cruise control cables for a quick and responsive throttle and cruise control system. While there have been several hardware updates, all Corvette TAC modules are interchangeable. The last year for the Corvette to use a TAC module was 2004. In 2005, the C6 Corvette was released with a Gen IV E40 ECM that integrated the TAC within the ECM.
The 2004–2005 Cadillac CTS-V are fitted with the 2004 Corvette’s LS6 engine, PCM, and TAC module. The CTS-V has a different throttle pedal, but its APP sensor is a functional equivalent to the Corvette’s APP sensor.
Truck TAC for 512-kb PCMs
Electronic throttle was introduced with the 7.4L gasoline engine in the 1999 medium-duty GM trucks. Although these trucks share the same PCM as the 1997–1998 Corvette (which is also electronic throttle), the TAC module does not interchange. This first GM truck TAC module (GM# 19245410) was used with the 16238212 PCM.
The more common truck TAC module was introduced in 2000 (GM# 19245406). This TAC module was used through 2002 with Gen III engines equipped with electronic throttle. General Motors used this TAC module with two different PCMs: GM# 09354896 in 1999–2000 and GM# 12200411 in 2001–2002.
Truck TAC for 1-MB PCMs
With the 1-mb PCM came a new TAC module. The 2003–2007 truck TAC modules are identified easily as they have an aluminum back plate and plastic housing. While this TAC module accepts the same harness connectors, it is not interchangeable with the early truck TAC modules. However, it does use the same pedal assembly as the early drive-by-wire trucks. The early TAC-to-pedal harness uses nine wires (for three APP signals); the late TAC-to-pedal harness uses only six wires (for two APP signals).
The early truck throttle pedal assembly (GM# 15177923) is used with 2000–2005 trucks. An alternate pedal assembly (GM# 15264643) is used with 2004–2005 trucks. These pedal assemblies are quality pieces with a metal mount, metal arm, and traditional spring-loaded foot rest. The APP sensor contains three sensor signals that provide pedal position details to the TAC module (only two are used). The APP sensor is connected to the TAC module through a separate wire harness.
Reducing costs, General Motors began using a different throttle pedal (GM# 15107594) in the 2006–2007 trucks. Used with or without power adjust, this pedal assembly is plastic with a plastic arm and fixed foot rest. The APP sensor is built within the pedal assembly and contains only two sensors that provide pedal position details to the TAC module. The APP sensor is connected to the TAC module through a separate wire harness. There are other truck pedals used with a power adjust. Because of their large size and extra electronics involved, however, they are not well suited for conversions.
Written by Mike Noonan and Posted with Permission of CarTechBooks
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