I think I have found a new failure mode for the cooling fans!
I have a customer’s car here that has a fan issue – the cooling fans run full blast any time the key is ‘on’, they turn off when the key is off. Yes, the AC is off. In fact if you pull the AC relay the fans still stay on.

Any body else run into this one?

 

Had the same problem a few weeks ago, turned out it was the fan switch. It had failed and was constantly running at high speeds, exactly as you described. What was weird was i started the car and was doing a walk around (cold start) and right by the drivers left wheel i heard the relays click and the highspeed fans turn on... they stayed on the rest of the trip home. Kind of funny i knew exactly when that fan switch failed..

 

+1

started last week

2 weeks ago twice fans wouldn't turn off and on of which I found out when I woke up and it had killed the battery

fan switch on the list of to do's next week as usal among other fix's.....

 

Remember - it turns off with the key, fans switch failure should keep the fan on with the key off - thats what different about this one!

 

My current problem is exactly that for the moment but a week before that it would just not turn of period but it wasn't a constant problem hence I only ever caught it twice and this week I was working on cleaning up some temporary connections I made into permanent ones and I noticed when the key was on the fans were as well but I hadn't started the car and when the key was off the fans were off.

 

Not exactly, the fans will turn on with the key off on low speed at any temperature regardless. Say the car is running hot and the high speed fans are on and you shut the car off.. the fans will switch to low speed. Same as if car were running and the low speed fans were on..then shut car off, low speed fans will continue to run.

Its confusing, but basically what i am trying to say is the high speed fans only run with the key 'on', so even with the failed switch the fans will still shut off when the key is moved to 'off' ..unless of course its the low speed fans coming on.

 

+1 The Duck's right. Try jumpering pin 1 & 3 on a good car with the ignition off -- fans only come on with the ignition. IIRC.

 

This is a 951 right, and not an Early N/a or a 924?

 

Yes, '86 tubro
if the fans are running correctly they will continue to run on low after the key is taken out if the engine is hot. If your thermo switch fails it can keep the fans running with the key off - this is the usualy battery killer - I have seen it many times. My current problem (electrical pun!) is that the fans act like the AC is turned on - allways on high when the key is on. The strange thing is that they still run with the AC relay removed.

 

Plug and unplug the fan relay. If it doesn't make a clicking noise, the relay is "welded" together.

 

Fan relay is fine...fans shut off when key is off - usually when the fan realy goes bad you have fans on with key off.
I did swap out the relay just to check - no change.

 

What about the high pressure switch on the dryer bottle?

 

How about the fan resistors below the windshield? These are known to rust out and give problems with cooling fan operation. Just a thought.

 

And so... if the low-temp circuit in the thermoswitch is stuck in the "no continuity" mode and the high-temp circuit in the thermoswitch is stuck in the "always continuity" mode, then the car will behave as Chris is describing.

 

Nope, it the high temp circuit was on then the fans will run with the key off. Nice try - thanks for playing!