1978 - 1985 Aux-Fan On with A/C ON

1979 - 1986 928s with air conditioning have an Auxiliary Fan that is installed in front of the A/C condenser.  The fan doesn't come on when the air conditioning is turned on.  It is switched on when the coolant temperature exceeds 88C, or by the high-side refrigerant temperature.  Normally it takes some time before the Aux fan turns on and by that time things are hot and may not cool down.  

In hot climates, by turning on the Auxiliary Fan when the A/C is turned on, the Freon temperatures are lower and at lower RPMs the A/C blows cooler.  I noticed that at highway speeds, the radiator coolant temperature was lower too.

This modification requires no wire cutting.  The new circuit and relay just act as parallel circuits and do not inhibit the other switches from activating.  There are two separate types of circuits, both use the same relay and relay socket:

- 1978 - 1984 connects to the Supplementary Air Valve and Refrigerant Temperature Switch
- 1985 - 1986 connects to the Low Pressure Switch and Refrigerant Temperature Switch

Wires with Piggyback Vinyl Insulated Spade Connectors need to be fabricated.

If you'd like to learn more, send me an email at electronikrepair@earthlin.net and I'll provide a PDF document with schematic details. 

I'm interested in figuring out my overheating in traffic. AC not working at the moment. To me it  looks like PO bypassed the normal operation. Your thoughts    ? 

Hello,

I need to know the model year of your 928 and I'll reply with the wiring diagram.  That is non-stock wiring as the 20A fuse in the photo isn't original. 

What is the coolant temperature indication on the gauge are you getting when in traffic?  I the needle at the 2/3 mark or higher?   Does it move into the red or stay below the red mark?  Do you see the temperature decrease once you're moving above 45 mph, it it much lower at speeds of 55 mph or greater?  Have you used an IR temp measurement at the coolant intake to the engine to get an idea of how warm the water is?

The pre-1987 928s used a fluid fan clutch.  It is quite possible that the fan isn't moving enough air through the radiator at low speeds to cool the engine.  When increasing the RPM with the hood open, you you feel the airflow increase when throttle opened?  It is necessary that the upper and lower fan shrouds are in-place or the fan will not be effective.

It's a 1985 928S 32V . It goes to past the second white line towards the red. I blast the heat and roll the windows down to pull heat off the engine to get to a place to park. I don't hear the fans coming on. Fan shrouds are in place. When driving sub 45 it creeps up and doesn't go down much,.when going faster if it crosses the last white line. 
Had a new tb and water pump.   This happened a while back and I parked her  and started a couple of DIY projects but then I was sick and had some surguries and kind of left it alone until my health improved.  Trying to get it back on the road , now that I'm doing better. won't start right now, so I can tell you if the air increases with hood up. I just saw youre post and felt like mine had something off with that fused wire. 

I attached the wiring schematic for the Auxiliary Fan.  There are multiple items that will turn on the fan.  In the case hot engine coolant, look to the light blue dots over the circuit for the Thermoswitch Cooling Water (located on the forward lower driver side of the radiator).  It provides a ground that energizes the Relay XVII Auxiliary Sensor relay (relay pin 85).  If the fan isn't running when your 928 coolant temperature is too hot then check Fuse 29, Relay XVII, the fan motor or the wiring to/from the fan motor.

I'm not sure what is unplugged in the photo that you attached.  The schematic may assist you with figuring that out.

Thanks. I'll take a look. To me it looks like the unplugged prongs  I'm holding  some from the dryer and should have the spaded wires that are  plugged into that fuse  on the prongs.