HVAC-NO A/C CLUTCH OPERATION DISPLAY SWITCH (INFORMATIONAL)

SUBJECT: HVAC HAS NO A/C CLUTCH OPERATION AND DISPLAY SWITCHES TO ECONOMY MODE (INFORMATION)

VEHICLES AFFECTED: 1991 "C" MODELS

CONDITION:

---------- A/C does not operate, no HVAC codes are stored and controller switches to "Econ" Mode.

CAUSE A: Pressure cycling switch cycles too fast. --------

CORRECTION:

----------- 1. Verify A/C System state of charge is correct. 2. Correct intermittent electrical condition (Refer to text). 3. Repair short to ground in the compressor circuit.

CIRCUIT OPERATION:

The programmer sends a voltage (9-12V) out terminal D4 through the Hi Pressure cutout switch, through the pressure cycling switch, to terminal D11 of the PCM requesting the PCM to turn on the A/C compressor clutch. When the PCM sees this voltage, it turns the clutch on and when the voltage disappears, the PCM disables the clutch. Normal operation results in a steady voltage out of the programmer when it wants the clutch on. This voltage is turned on and off by the pressure cycling switch as required by the refrigerant pressures. The PCM also monitors how fast the pressure cycling switch cycles. If the cycling occurs too fast, the PCM sets a Code 66 and disables the compressor until corrective measures have been affected. If the PCM sees the failure in three consecutive ignition cycles, the code will permanently disable the AUTO mode until repairs are made.

The PCM assumes that the rapid cycling of the voltage is due to a low state of refrigerant charge in the system causing the cycling switch to open and close rapidly. The PCM will keep the clutch disengaged and send the programmer a message over the serial data stream (circ. 800) telling the programmer why it has turned the clutch off. When the programmer receives this message, it will stop sending voltage out terminal D4 and force the control panel to "ECON" mode. If "AUTO" is then attempted to be re-selected, the programmer will force "ECON" to continuously reappear until the problem has been corrected and the Code 66 cleared from the PCM along with going through the "Clear Codes" function (OC3) in the climate control on-board diagnostics. The "Clear Codes" function must be completed even though the on-board diagnostics show "00" in the code display. The problem could be the result of a low state of charge, an intermittent electrical connection, or a short to ground on the compressor request circuit.

CAUSE B:

The programmer believes the system has no refrigerant because the feedback signal from D4 was not received at Pin C3 after 30 min. of operation.

CORRECTION:

----------- 1. Verify A/C system state of charge. 2. Repair feedback circuit wiring (see text). 3. Clear codes

CIRCUIT OPERATION

The other condition that will cause the programmer to force "ECON" when "AUTO" is selected is when the programmer does not get the correct feedback signal on the low refrigerant input. When the programmer requests "A/C" (sends a voltage out on terminal D4), it looks at the feedback signal (terminal C3), that runs from splice S294 in circuit 67 to terminal C3 of the programmer. If the programmer is sending out a voltage on terminal D4, then it should see a voltage come back on terminal C3.

If, after 30 minutes above 70 degrees, the programmer has not seen a voltage on terminal C3, it assumes there is no refrigerant in the system because the pressure cycling switch is always open. Under this condition, the programmer forces itself into the "ECON" mode and stops sending a voltage out of terminal D4. This condition will be present until the problem is corrected and the programmer "Clear Code" (03C) function in the on-board diagnostics is completed. The condition could be caused by a very low state of charge, or an open or short to ground in circuits 762, 66, 204 or 67. The "Clear Codes" function must be completed, even though the on-board diagnostics show 00 in the code display. Note that it will take 30 minutes before this condition can reoccur and there will be no codes set in the HVAC or PCM.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.