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Cooling Fan(s) Run When Car is Off - Update


Model: 968, Year:1992, Mileage:112000, Type of use:Both Street & TrackI'm updating this post, which now contains all correspondence and the resolution (scroll all the way down for the final resolution!) I have a 1992 968, and when I went in the garage the other day the fans cycled briefly (10-30 seconds) a few times. It was 40 degrees (Fahrenheit) or less and the keys were in the house. I do not know if this was an isolated incident, the car was sitting for a few weeks until I noticed it. I drove the car today and it seemed to function properly, i.e. the fan(s) ran for a brief time after shut down and then stopped. Do you know of a successful fix or can you suggest a diagnostic plan? How often would this need to occur before draining the battery? Please let me know if you need more information before suggesting a course of action. Glenn MacMoyle [NNJ] 12/30/2009 3:17:05 PM Answer: You've described a condition that is quite well-known to the 968 community, where the coolant temperature switch in the radiator is getting ready to give up. Let me be clear that it is normal operation for the fans to run at low speed with the key off, but when the temperatures are low and you're still experiencing fan operation after shutdown, you're getting the early warning signs of a switch going bad. This switch is relatively easy to replace given enough time and patience by anyone with a modicum of mechanical aptitude and basic hand tools; look for a relatively large device on the lower portion of the radiator with 3 good-sized wires running to it.Bill BurrisPCA Tech Committee Thanks for the response. Unfortunately, changing the switch did not resolve my problem. The replacement part is the same 85/93 switch that came out. All seemed well as I warmed up the car and bled the air from the system; the fans seemed to cycle normally. However, as the car cooled, the fans continued to cycle on and off. I disconnected the battery (to prevent drain) and came back about an hour later (ambient temps in the 40s). As soon as I connected the battery the fans came on. They "pulse" in short bursts of a few seconds. Do you have any further suggestions?Glenn MacMoyle [NNJ]OK, here's where we're at: The cooling fan power is delivered by a 2-stage relay on the central electric board at position G10. This relay delivers power for full speed and low speed through external step-down resistors located under the cowl on the passenger side behind the battery. This relay gets power all the time through two fuses, fuse 15 (a 25 amp fuse) at terminal 30M1 on the relay, and fuse 10 (also a 25 amp fuse) at terminal 30M2, plus unfused key power at terminal X on the relay. There are other inputs to the relay's logic (yes, it has a small microprocessor in the relay) some of which should only be responded to when the key is on (forexample, terminal AC gets a signal when the AC button on the dash is pushed.) Other inputs come from the radiator switch you just replaced: A ground signal goes to terminal TS when the engine coolant reaches 92 degrees centigrade, and ground to terminal TF when the coolant temperature is over 102. What happens inside the relay logic is simply to respond to inputs and trigger the low speed or high speed on the fans (which coincidentally can operate independent from each other since they have unique power supplies from the relay. Terminals V1 and V2 provide low speed to the fans, terminals M1 and M2 for high speed.) I'd continue with a diagnosis of your problem by getting a look at the relay itself (check for burning or other challenges) and the wiring to the radiator switch you just replaced. Start looking for anything suspicious, previous accident damage, wiring loom adjustments, funky audio installation wires to the bottom of the central electric board, etc.) Let me know what you find and we can go from there. If it ends up that you want to try a replacement relay, you should be able to get on easily from a recycler, these relays are not known to go bad so there should be plenty of inventory out there. Also, if you don't mind re-posting your follow-up question on, I'll put my response there for others to take advantage of as well - should theyrun into the same problem you're experiencing. Bill Burris PCA Tech CommitteeChanging the relay resolved the problem. I went with new, about $120. I decided that was inexpensive enough not to risk getting a bad used one. While that was about $100 more than the sensor, considering the savings in time and mess, I would suggest others starting with the relay if they experience a similar issue.Thanks,Glenn

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