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maddhatter1 · Joined: 20 Jul 2010 Posts: 19 Location: New Jersey

I am trying to figure out if I have a cooling problem or not on my 80 931 with AC. After starting the car up in the morning when completely cool, the radiator fan will come on after about 3 or 4 minutes and stay on. The temp gauge at this point has barely moved, and will never go up past the quarter mark when running. So I replaced the thermostat thinking it was not opening and it made no difference. While bleeding the system, I looked through the bleeder valve opening and could see coolant flowing freely past into the radiator hose, so it seems like the pump is functioning ok. After running the car for a half hour drive, I checked the radiator and found that the lower hose was meerly warm to the touch at best, while the upper was warm but could be squeezed quickly without too much trouble without burning myself. Do most cooling systems in 924 run this cool that the lower hose can be barely warm after a run? It makes me think my radiator is partially blocked, but my gauge says otherwise so I am a little concerned that the gauge is wrong. Also, the fan does not stay on after the car is shut down as I think they are supposed to do. Any thoughts?

1980 Porsche 931

datatrain · Posted: Sat Oct 30, 2010 5:06 am Post subject:

I assume you are aware the primary fan is controlled by a thermo switch that come on after the radiator is heated. The Secondary fan comes on as soon as you switch on the AC.

The lower hose should be cooler than the top one. It's supplying the cooling liquid to the engine. The top hose should be hot but touchable. Whatever the thermostat rating is will determine that.

At shut down I would expect the primary fan to stay on at low speed but again it's controlled by the thermo switch.

I'd check the switch and resistor one of them sounds faulty.

But it could also be the control relays. Start with the switch.
_________________
'78 924 NA with Collector plate
33 year old car, with me for 21 yrs
Mint '92 318i BMW
Near mint '98 Buick LeSabre
VE7HFR

maddhatter1 · Joined: 20 Jul 2010 Posts: 19 Location: New Jersey

Sure, the thermoswitch in the upper left part of the radiator I think controls the fan on this car, but I doubt that the thermostat has even had a chance to open in only a few minutes time let alone the temp reach the trigger point in the radiator switch. I'm thinking it has to be the thermoswitch that is acting up and is clicking the fan on way too early, and anyway they are cheap to replace so I will give that a try first and then see if it makes a difference. Thanks.

datatrain · Posted: Sat Oct 30, 2010 8:40 am Post subject:

Before you buy a thermo switch test it with a meter when it's cold. Should be open curcuit.
_________________
'78 924 NA with Collector plate
33 year old car, with me for 21 yrs
Mint '92 318i BMW
Near mint '98 Buick LeSabre
VE7HFR

Grenadiers · Posted: Sat Oct 30, 2010 8:58 am Post subject:

Also, check that the wires to the switch are not touching anything metal. Does not account for a short delay and the fan switches on. The terminals have a rubber covering that disintegrates over times, and it's had lots of time! I removed the issue by loosening the air cleaner mounts on the right side and shoving air cleaner housing back. Now the rubber terminal covers can disintegrate as much as they want, nothing touches them now.
_________________
'83 944 Track car.
'88 924S Track car.
'89 944 Turbo
2004 Winnebago Vectra monster RV
2012 Jeep Wrangler
2014 Kia Soul
2001 Ford F350 powerstroke

Paul · Posted: Sat Oct 30, 2010 9:36 am Post subject:

Yes all that fan switch on the radiator does is provide a ground, so if the wire shorts to ground, the fan runs.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

maddhatter1 · Joined: 20 Jul 2010 Posts: 19 Location: New Jersey

Thanks, that sounds like a good place to look at considering the wiring is 30 years old and there might just be a few unintended grounds lurking there somewhere in the wiring.

maddhatter1 · Joined: 20 Jul 2010 Posts: 19 Location: New Jersey

It seems you were right on the money with the wires shorting to ground and causing my problem. The plastic cover on the wires that connect to the thermo switch were rubbing on the metal air cleaner bracket and grounding causing the fan to go on. Still am not sure why it took a few minutes to turn the fan on, but problem solved for now.

Paul · Posted: Mon Nov 01, 2010 2:27 am Post subject:

I'm guessing the PO disabled the resistor so the fan would not run even after the car was shutdown. If that wire is shorted, the fan (when everything else is working correctly) would run the fan on slow speed until the battery went dead.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.