Wicked Strange Electrical problem


97 Audi A6 wagon - My coolant fans don’t work. I found a relay that I can apply voltage to the control terminals (small wires) which will make the voltage (big wires) flow to and run the fans. So I know the relay and fans work. When I check what is supposed to be 12v supplying one of the control terminals - it shows NEGATIVE voltage. If I switch my tester electrodes, it shows positive 12 volts. What could be going on?

Thanks for your thoughts…



What are you using for a ground when you take these readings? Sounds like you have a bad ground. One of those control wires is ground, the other becomes hot to energize the relay. You might try giving the relay what it wants, a good ground connection…


Thanks for the post. Yes - the ground actually goes to a temp switch on the radiator that activates when things get too hot. So the logical test, which I have done, is to bypass that switch and just go directly to ground (directly from the relay terminal). Fan doesn’t run. I get the “negative” voltage off what is supposed to be 12v - when it is disconnected from the relay. I believe it is supposed to supply 12v all the time - or at least when the car is running. Not sure if it is a “overrun” fan, which will still run after the engine is shut down. Don’t think I ever heard the fan run after shutdown.


Road maps are nice to have; so, are wiring diagrams. If you are having an electrical problem, why AREN’T you using the wiring diagrams? There are two relays, which one did you see the reverse polarity phenomenon on? And, what are the color of the wires?


Thanks for your comeback. Yes I do have wiring diagrams. Didn’t think any one else would on this forum. Blue/Red, terminal 85 (standard relay terminal number) goes to 12v. Same for both relays. Actually I assume that it goes to 12v. Would have to for it ever to work the relay to ever work. You’d have to see the diagram to understand. Or to be confused. Anyway it just seems odd that it should show neg voltage on my voltmeter. (the ground is term 86, which goes to therm switch on radiator.)


The temp sensor on the radiator is dead. You can bridge the wires going to the sensor. I did my friend’s A6. 3 terminals. One is a 12V+ the others go to relays. Bridge them and the fans will run full time when the ignition is on. Turn off the ignition and they stop. Radiator fans are 100% duty cycle rated so they can handle it. It’ll save you from overheating if you do a lot of low speed driving. Alternatively, just get a new radiator thermal switch.


Much appreciate the post. Wiring diagram is pretty clear that the 3rd wire goes to ground from the temp sensor - not to 12v. Good that it worked out for you, but I think I have taken the temp sensor out of the picture (but not necessarily proved it works or doesn’t) by putting that relay terminal directly to ground. You would have to see the diagram to see what I mean. But to give you the benefit of the doubt, I will check for 12v on the 3rd wire tonight - in case the wiring diagram is wrong.


The power has to go through the Coolant Fan Resistor after it leaves the relay IF it is turned on via the Thermal Switch. The brown wire from the thermal switch is the ground wire. The A/C high Press Switch turns om the cool fan to HIGH speed (it doesn’t go through the resistor). Voltage polarity is relative. If one wire has a higher voltage than another, it might appear positive when compared to the lower.


Thanks to all who replied. I haven’t had a chance to get back to this, and won’t for a few more days. Last post was interesting about the AC. Wondered what might trip the high speed. I will be checking and rechecking all connections, but I still think it odd that to control 12v wire, when disconnected from the relay buries my voltmeter in the wrong direction. I should be not related to any of the rest of the circuit, but I do not think I have detached the wire from BOTH relays when testing. They both go to the same connector according to the wiring diagram. I may end up dedicating an all new fused 12 connection if I see no apparent causes.