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HVAC |
Overview of A/C System ElectricsHow It Works (refer to pages EWD-131 to 133 in Bentley)The Integrated Climate Control Module in the center console takes numerous signals to determine when the A/C compressor should be engaged. These are (at minimum) the AC (or Recirc) pushbutton, evaporator temperature, and blower switch position (via a pot on the same shaft).When the A/C is switched "on", the light on the appropriate button is lit. Then, provided sufficient fan speed is detected by the fan knob position, the compressor clutch is cycled such that evaporator temperature stays above freezing. (As the evaporator is usually wet, allowing ice to form will prevent air flow and may damage the unit.)The compressor clutch signal (a ground from the controller module at pin 22) is passed first through the pressure switch pair on the receiver/dryer (under the air cleaner). These switches break the circuit if refrigerant pressure is either too low (insufficient R-134a) or too high (excessive R-134a or more likely insufficient cooling of the condensor, located in front of the radiator).The signal then proceeds through the DME which can break the circuit as required, for example when the engine is stopped or being cranked, and possibly at full throttle to improve power.The signal goes from the DME to the Compressor Control Relay (pin 2 on relay) in the main fuse box (engine compartment, rear left). This small relay is located at the front-middle of the box. See Bentley page 610-1, Fig. 2, item 5. The other connection to the relay coil (pin 1 on relay) is supplied with +12V via fuse 29, hot only when the car is running.Pin 5 of the relay contacts receive high-current power from fuse 9, live at all times. Pin 3 goes to the A/C compressor. Pins 5 and 3 are connected when the relay is energized.
How I am Troubleshooting an Intermittent A/C Electrical FaultI am currently experiencing an intermittent loss of A/C. The light comes on when the button is pushed but the compressor clutch does not engage. Idle speed does change slightly, but not as much as it should.There are three parts of the circuit: the HVAC controller, the compressor relay control circuit, and the compressor relay power circuit. I initially suspect the two latter circuits. As the control circuit has more switches and connectors I'll tackle that first. If the problem turns out to be in the controller, it's inputs, or the relay power circuit, I'll cover that in a future addition.The first step is to "instrument" the control cicuit between the various control points.Place a length of very fine solid wire (about 30 gauge) at each of the following terminals. Wrap the bare end around the terminal and replace the relay or connector, allowing measurement at any time on the road. Bare the other end and place such that it can't short against any metal parts.1. Pin 3 at the compressor control relay (Bentley page 610-1, Fig. 2, item 5.)2. Pin 5 of the auxillary fan low-speed relay (rear relay in the small relay box, see Bentley page 170-4 fig 3.)3. Pin with Blk/Gry/Yel wire attached at the pressure switch connector (under the engine air cleaner box, see Bentley page 640-11, Fig. 23.)When the problem surfaces I plan to park the car, leave the engine running, jump out and measure the voltage present first at fuses 9 and 29, then each of these four points.
Two Weeks Later...We'll, it seems I must have disturbed something as the problem has not surfaced again. My guess is that the pressure switch connector was flakey, as all the other connections seemed secure.
Twenty-Two Weeks Later...I think I've finally found the problem - and it's so embarassing! The temperature wheel on the center dash vent needs to be on "cold" to produce cooled air. Since the heater was not on I didn't see why that would be an issue. With no outlet for cooled air, the compressor cycles on and off quickly as the evaporator gets too cold. |