All too often PCM-controlled (or monitored) air conditioning is overlooked after the engine has been swapped, the new wire harness has been installed, and custom calibrated PCM has been received from the tuner. The A/C system is commonly one of the many “I’ll think about that later…” tasks as the excitement builds for the first engine startup.
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I recall an LS1 engine conversion in an early Corvette that followed this scenario. I discussed the project with the shop owner before building the engine harness so that nothing would be missed. He wanted to keep the wiring “clean” and not have A/C wiring related to the PCM. He neglected to think through the aftermarket A/C system wiring and did not realize, until the car was days from scheduled delivery, that there was no way to turn the electric fans on when the A/C system was on.
Being pressed for time to meet the deadline and out of money to make further changes to the car, he compromised with an undesirable workaround, by turning the fans on at 30 degrees F through the PCM. Now he had a $100,000 Corvette with an electric fan solution that General Motors never would have implemented, even in a $10,000 car!
Base Calibration
General Motors implemented the A/C system in several different ways among Gen III vehicles. The PCM calibration is specific to each of these implementations. Of most importance is the consideration of electric fan control.
If your project uses electric fans, begin with a PCM calibration from a vehicle that has electric fans.
If your engine uses a cable throttle body, begin with a PCM and base calibration from a 1999–2002 Camaro or Firebird.
If your engine uses an electronic throttle body, begin with a PCM and base calibration from a 1999–2002 Corvette. (See Chapter 8 for more about Corvette electronic throttle equipment.)
If your project uses electric fans and the PCM and calibration is based on a truck, the PCM does not turn on the electric fans when the A/C is on. Truck PCMs older than 2003 can be reflashed with 1999–2002 Camaro, Firebird, or Corvette calibrations for proper electric fan operations.
Operation Overview
In the 1990s, General Motors introduced PCM control of the A/C compressor clutch. While the operation of the A/C system hasn’t changed significantly, there’s been a few improvements. Fuel injection enthusiasts should consider several things such as the diagnostic feedback, functional benefits, and safety of PCM-controlled A/C.
The Gen III PCM supports DTC processing for the A/C system. The DTCs shown here may be enabled to report a fault in the A/C pressure sensor or A/C compressor clutch (or related wiring). Notification of a fault is presented to the driver through the MIL or a scan of the OBD-II diagnostic port.
EFILive tuning software presents calibration details for the operation of the A/C compressor clutch. Notice that the A/C compressor is disabled above 4,895 rpm or a TPS value of 100 percent (also known as WOT).
In a production GM application where electric fans are used and the A/C is on, the PCM commands the Fan #1 control circuit on (which is actually going to turn on both fans at low speed; see Chapter 10) through calibration {G0903} Fan #1 Turn-on By A/C Temp. With the electric fans on above the specified coolant temperature, the PCM monitors the A/C pressure signal and uses calibration {G0911} Fan #1 A/C Pressure Enable to allow electric fan operation, and calibration {G0912} Fan #1 A/C Pressure Disable to prohibit electric fan operation.
OBD-II diagnostics can be helpful in troubleshooting your vehicle’ A/C system. With a handful of A/Crelated DTCs, the first step in identifying a problem may be a quick look through the OBD-II diagnostic port at DTCs that have been set. A brief description of a logged DTC malfunction may point you directly to the source of the problem. Depending on your PCM’s calibration settings, your scan tool may display very useful information about A/C faults.
It is not always appropriate to engage the A/C compressor clutch when the driver commands the A/C compressor to turn on. The PCM monitors the pressure of the A/C system to determine if the A/C compressor clutch should engage with the driver’s turn of the A/C control knob. More than that, the PCM also considers other engine operating data such as RPM, throttle position, and intake air temperature.
To prevent damage to the A/C system, it is crucial that air passes through the A/C condenser when the A/C compressor clutch is engaged. Because the PCM controls the operation of the electric fan(s), it is capable of turning on the fan(s) when the A/C is on to pass air through the A/C condenser. The PCM applies coolant temperature and A/C pressure logic to the operation of the electric fan(s).
Request Input
A 12V signal wire is monitored at pin 17 of the red PCM connector. With no voltage applied, the circuit is open and the PCM does not attempt to turn on the A/C compressor clutch. When 12V is applied to the circuit and based on the pressure in the system, the PCM commands the A/C compressor to turn on.
Pressure Sensor
Common among Camaros, Firebirds, and Corvettes, the A/C pressure sensor provides a 0 to 5V signal to pin 14 of the red PCM connector. Based on this signal, the PCM does or does not command the A/C compressor to turn on when the driver requests A/C operation. A common location for the A/C pressure sensor is the passenger’s side of the engine compartment ahead of the A/C accumulator/drier.
This sensor is in a 1997 Trans Am. In this vehicle, the LT1 PCM also uses the same 0 to 5V A/C pressure signal as part of the logic that determines whether the A/C compressor clutch is engaged.
Low-Pressure Switch
Common among trucks and vans, the A/C low-pressure switch is used as an input to the PCM (up to 2002) or HVAC control module (2003-newer) as part of the logic that determines whether the A/C compressor clutch is engaged. This switch provides a ground signal to the PCM or HVAC control module when the A/C pressure is not low. When the A/C pressure is low, the circuit is open and the A/C does not turn on.
When the driver turns on the A/C system and the PCM determines the system is ready for A/C operation, the PCM activates the A/C compressor clutch relay. If the relay does not supply 12V to the A/C compressor clutch, then the PCM sets DTC P1546. This indicates a problem with the A/C compressor clutch relay or related wiring.
This switch is in a 2003 Silverado A/C accumulator/drier. Be sure to check the wiring schematics for the vehicle type your PCM calibration is based on because this switch is not used with all LS-series vehicles.
Clutch Relay
When the driver commands the A/C on from the control unit and the system pressure criteria is met, the PCM grounds the A/C compressor clutch relay coil to turn on the A/C. This relay is most often found within a fuse block assembly.
In the 1997 Trans Am, General Motors located the A/C compressor clutch relay within the fuse and relay box in the front driver’s side of the engine bay.
On Input
As a secondary check, the PCM looks for A/C compressor clutch voltage on pin 18 of the red PCM connector. If the PCM has commanded the A/C on and 12V is not found on this input, the PCM sets DTC P1546 to indicate that the circuit is unexpectedly open. A likely cause for this DTC is a faulty A/C compressor clutch relay.
Compressor Clutch Solenoid
Located within the A/C compressor, this solenoid is responsible for engaging the A/C compressor clutch. A diode is installed either in the wire harness or within a fuse block assembly to block a high-voltage spike as the A/C compressor is cycled on and off.
While there is a variety of A/C compressors, they all use a solenoid to engage the compressor clutch to pressurize the system. This A/C compressor is included with the Chevrolet Performance small- and big-block Chevy front accessory drive kits. Notice the wire harness connection for the clutch solenoid at the top of the compressor just behind the pulley.
Electric Fan Wiring
For proper electric fan control when the A/C is on, the best PCM and calibration choice is 1999–2002 Camaro/Firebird/Corvette. In these vehicles, the PCM receives a 12V request from the control head, checks the refrigerant pressure, activates the compressor clutch relay, and monitors the voltage on the compressor clutch to verify that the compressor has been turned on.
Because the A/C compressor cycles on and off, a diode is found in GM wire harnesses to block a high-voltage spike. This diode is sometimes found within a fuse block assembly
When the A/C system is on, the PCM activates low-speed electric fan operation for series/parallel operation. It turns on only the electric fan 1 relay for on/off operation. (See Chapter 9 for electric fan schematics.)
Eliminating the Pressure Sensor
The A/C pressure sensor can be difficult to install in early vehicles because there may be no provision for it. An alternative to using this pressure sensor is to fake the refrigerant pressure signal by replacing the sensor with an 80,000-ohm, 1/4-watt resistor. The resistor can be installed between the 5V reference (pin 45, blue PCM connector) and signal (pin 14, red PCM connector). The compressor clutch engages regardless of refrigerant pressure.
An alternate method of logic should be applied to the 12V A/C Request Signal on pin 17 of the red PCM connector. Install some type of safety pressure switch(es) between the A/C control head and the PCM. An existing or aftermarket system may already include logic to the 12V A/C compressor clutch supply circuit. This circuit can be used to supply the 12V A/C request signal on pin 17 of the red PCM connector.
Compressor Clutch Control
Early LS-series trucks and 2001– 2002 vans were not equipped with electric fans. These vehicles use one or more switches as part of the logic that determines if the compressor clutch should turn on when the driver commands the A/C on through the control head. This system does not use the 0 to 5V pressure sensor found in the Camaro, Firebird, and Corvette.
When using electric fans, it is best to choose a PCM and calibration from a 1999–2002 Camaro, Firebird, or Corvette. General Motors fitted these vehicles with electric fans and made a provision within the PCM for A/C operation that relies on the use of the electric fans for keeping the condenser cool. This wiring schematic represents the A/C system in the 1999–2002 Camaro, Firebird, and Corvette.
For vehicle installations where the A/C pressure sensor cannot be installed, the voltage necessary to indicate proper line pressure can be created through the installation of a resistor. By installing an 80,000- ohm, 1/4 watt resistor across the A/C pressure sensor, 5V reference, and signal circuits, the PCM thinks the A/C pressure is always adequate for A/C compressor clutch operation. This workaround should be used with extreme caution as it eliminates the safeguards General Motors has implemented through the use of an A/C pressure sensor.
The A/C system wiring for the 1999– 2002 trucks and vans is represented here. This system does not use the 0 to 5V A/C pressure sensor found in the Camaro, Firebird, and Corvette. These trucks and vans were not fitted with electric fans. If you plan to use electric fans, consider using the wiring and PCM calibration from a 1999–2002 Camaro, Firebird, or Corvette
In 2003, General Motors introduced HVAC control modules in trucks and vans that replaced the traditional 12V A/C on signal with Class 2 serial data messaging. For conversions requiring a 2003-newer Gen III PCM, the A/C system has to be wired independent of the PCM.
There are variations on the implementation of this A/C system, so refer to the GM schematics when necessary.
A benefit of implementing this wiring in your non-electric fan project is that you can make use of
PCM control of the A/C compressor clutch based on engine RPM, throttle position, and so on.
A big problem with PCMcontrolled A/C was introduced in 2003. In an effort to reduce the amount of wire in newer automobiles, serial communication lines were established for modules to communicate across the vehicle. Most 2003-newer PCMs receive Class 2 serial data messaging from the HVAC control module. The early 12V A/C request from the A/C control unit is no longer used and has not been used in a GM vehicle since.
LS PCM conversions using the 2003-and-newer PCMs must disable the DTCs related to A/C and not
bother with wiring in the A/C pressure sensor. If the vehicle is equipped with electric fan(s), they must be turned on when the compressor clutch is engaged. A trigger wire from the A/C wiring must be wired into the electric fan relay(s) to pass air through the condenser when the A/C is on. If the PCM has not commanded the electric fan(s) on based on high coolant temperature and a ground is applied to the fan relay signal wire(s), DTC(s) set to indicate that the fan(s) have been turned on unexpectedly. The DTC(s) can be disabled through tuning software.
PCM Conversion Recommendations
For LS PCM conversions with cable throttle, the most desirable PCM and base calibration is the 1999– 2002 Camaro/Firebird. Those calibrations support A/C compressor control and electric fan control. The Camaro/ Firebird calibrations allow you to choose between manual transmission and 4L60-E automatic transmission. Should your LS PCM project require control of a 4L80-E transmission, a segment swap does the trick. Not all tuning software packages allow segment swaps but those that do, such as EFILive, provide necessary details.
The A/C pressure sensor (left) reports a 0 to 5V signal to pin 14 of the red PCM connector that represents the pressure of the A/C system. The A/C low-pressure switch (right) indicates whether the A/C pressure is low by applying a ground to pin 55 of the red PCM connector.
For LS PCM conversions with Corvette electronic throttle equipment, a Corvette base PCM calibration must be used. The Corvette calibrations allow you to choose between manual transmission and 4L60-E automatic transmission. Should your LS PCM project require control of a 4L80-E transmission, a segment swap does the trick.
For LS PCM conversions with truck electronic throttle equipment, the corresponding truck PCM and base calibration must be used. The early truck calibrations support A/C control, but not electric fan control. The 2003-newer truck calibrations do not support A/C control without the enormous task of integrating the HVAC control module for Class 2 communications with the PCM. For these types of LS PCM conversions, when the A/C system is on, electric fan operation has to be implemented without control from the PCM.
Written by Mike Noonan and Posted with Permission of CarTechBooks
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