AC Fan and Flap Control Circuit
 

Hi,
Thank you all for your help, especially Dr Bob. You all have been very helpful. After tracing everything as Bob said, and reading Dwayne's tutorial, I'm pretty sure my relay in the controller is shot and am going to have to change that. Also, I am not sure if my flap and fan control circuit is working. Maybe I also have an issue with one of the poles in the AC button (if not both). Does any one know what plug and wire position on the CEB where I would find the fan and flap wire so I can check if it has any voltage?
Thanks in advance
Randy

1986.5 928S Automatic

Your car doesn't have the flaps, and has a much simpler control circuit for the fan.

Fuse #29 feeds power to Relay XVIII, which, when triggered, feeds full 12 vdc power to the fan.
Relay XVIII is triggered by:
- Coolant temp switch, located in the lower corner of the forward face of the left radiator side tank.
- Refrigerant temp switch, located on the receiver/dryer. This doesn't work very well.
- ATF temp switch, located on the forward part of the transaxle (autos only).
All of these sensors function only when the ignition is on.

In addition, there is a temp sensor in the top of the intake manifold that is powered when the ignition is off. It powers the fan thru a resistor, so that the fan runs at a slower speed if the engine is very hot after shutdown. This circuit does not go thru Relay XVIII.

Go to our website, then the tips section on the left, then Wally's world, then the HVAC link. That tutorial might give you some assistance.

Start testing at the evaporator anti-freeze switch, located under the (very fragile) plastic shield at the back of the engine compartment. If you use this test sequence, you must do the steps in order! With the engine running and the AC on, you should have 12vdc from each of the switch terminals to ground.
If you do, the fuse and control head relay are OK.
If not, check Fuse #8. If you have power thru Fuse #8, but not at the antifreeze switch, the problem is in the control head - either a switch, a connection, or the relay.
If you have power thru the antifreeze switch, check the single connector in the black wire near the oil dipstick. If you have power there, but the compressor clutch doesn't engage, the clutch or one of its connections is faulty. If you have no power there, the problem is the refrigerant pressure switch - either the refrigerant is low, or the switch has failed.