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C-68 Electronic A/C SystemPosted 6/7/1999By Dave Martin
Many shops servicing air conditioning systems this summer will encounter problems with no compressor engagement on General Motors' C-68 Electronic A/C system. You will be able to diagnose and repair the system more efficiently if you understand its operation. First, determine if the problem is on the "request" or the "command" side of the system. The request side usually consists of the heater A/C control head, the programmer, high pressure switch, pressure cycling switch, PCM, and related wiring. The command side consists of the PCM, compressor relay, compressor clutch, clutch diode, and related wiring. To quickly check the basic operation of the request side, install your scanner, start the engine, turn on the A/C and check the data for an A/C request. If the scanner reads "yes," skip over the request side tests. If it continues to read "no," keep testing. Here's how the sequence occurs on most systems: The control head sends a request to the programmer over a data line. The programmer then applies a low amperage, 8-12 volt signal to the high pressure switch. If the system pressure is under 430 psi, the switch is closed and the signal continues to the pressure cycling switch. If the system pressure is over 25 psi, the contacts are closed and the signal is sent to the PCM where it is recorded as a request for compressor engagement. If you do not have a scanner, check the high pressure and cycling pressure switch operation with a DVOM. Note: A conventional test light will not light on this circuit because the signal sent by the programmer is very low amperage. If you rely on a conventional test light, you will misdiagnose the system. Both the PCM and programmer monitor the cycling time of the compressor. In most cases, if the compressor cycle time is less than 1.5 seconds for 10 consecutive cycles, the PCM will shut off the compressor for that key cycle. If this problem occurs for three key cycles in a row, both the PCM and the programmer will set A/C DTCs, and will discontinue all compressor engagement until the DTCs have been erased. Be sure to repair the problem so the DTCs do not reset in the future. An A/C DTC in the PCM will not turn on the MIL/SES light. If an A/C DTC is set in the programmer, the A/C control head will switch from "Auto" to "Econ." There are occasions when the system will still not respond after clearing DTCs from both the PCM and A/C control head. In these cases, remove the battery negative cable and turn on the headlights for 10 seconds. This will discharge the memory in both the PCM and programmer and cause the system to "wake up" in a normal state. Be sure to record your customer's radio stations and any other electronic pre-sets as these will be lost. The PCM will only operate the compressor if:
A quick test of the command side can usually be done by jumping the DLC (ALDL) A to B with the key on and the engine off. Besides causing the MIL to flash a DTC 12, this will issue a command to the PCM to turn on all the PCM-operated solenoids and relays, including the A/C relay. Note: The fuel injectors and the fuel pump relay will not turn on in the "field service" mode. If the compressor does not engage, switch the A/C relay with a relay with the same part number from the vehicle. Check the inputs/outputs at the relay and all related wiring, and repair the defective circuit, if necessary. Finally, to check the amp flow of the compressor, simply install a 5 amp in-line fuse to power up the compressor for two to three minutes. If the fuse doesn't blow, the compressor clutch circuit is not shorted. If the compressor diode is burned out, replacement parts are available from General Motors as well as retail electronic stores.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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