fraggle

augh! mechanic for throubleshooting this is probably going to be spendy!

Nightfly

Alright, this might seem like a really stupid question, but why are the fans running after the engine stops? Billions of cars were made with mechanical fans, that stopped when the engine stopped. I owned many of those cars, and never had an overheating problem unless a hose split or thermostat stuck or something. Besides, the fans cool the fluid in the radiator, not the fluid in the engine; as far as I know, the water pump stops when the engine stops unless there's some witchcraft going on here. So, assuming virtually nil coolant movement, how are the fans doing anything other than cooling the radiator which does nothing to cool the engine when it's not running?

Leon Speed

When there's a difference in temperature in a closed system, warm water will tend to rise and cold water will tend to stay low, so maybe there is some circulation within the system. However it appears unlikely that it's much at all.

Nightfly

That's what I'm saying. Kind of makes me think the 'running fan syndrome' may very well be the reason our batteries die a premature death.

blitz928

Not really, my mechanic is a good friend, he owns my old 993, we hang out, he won't charge me much.

JG928S4

When water stops flowing through radiator is when it will heat very quickly (why you do not remove radiator cap after shutoff, safer w/car running), fans get that water down under temp of shutoff switch on radiator, then off, shouldn't take long.

blitz928

Just thought I might update this thread, for those who may have had a similar issue. Problem is fixed. The problem was the Fan Speed amplifier. Black finned amplifier looking thing under the hood near the hood latch. Swapped it out with a different one(thanks, Kevin Michael) and all is good...

dr bob

Failure mode is typically the FET's in the amp module. Usually open but sometimes closed. The parts are available to replace, with the only difficult part being the way the heatsinks attach. I replaced the rivets and drilled/tapped the blocks for #4 screws.

fraggle

Where might you get these FETs? I'd feel comfortable re-soldering these in.

blitz928

I have the FETs. I didn't use them and got a different unit instead. I felt uncomfortable soldering them as well. I have the old unit as well as the replacement parts I may go ahead and fix it now, though.

The FETs were available through Radio Shack.

David L. Lutz

Matt

Glad to see you got it fixed. I guess sometimes its caused by different problems.

I will see you at the show. Thanks for the pm. I am calling Russ this week.

GP White 928 S4

Bumping this old thread as the symptoms seemed closest to my current scenario. Car is an '88 S4 Auto. Would appreciate any guidance!

This will probably work best if I explain in timeline form:

This past Saturday: Power washed car exterior in a brushless bay to prepare for local PCA car show the next morning. Did not spray under hood but possible some water seeped in with hood closed?
Saturday night: Hear cooling fans running while car is parked in my garage, hours after it was shut down and by all means should be cooled off by then. Grab the key, turn car on then off, fans shut off. Go about evening thinking it was just an odd gremlin.

Sunday morning: Wake up before show and overhear fans running from the garage again. Open driver's door to find battery is drained and lights are dim. Were these running most of the night? Start panic searching Rennlist threads and find this one, amongst others. Start running tests to get fans to turn off:
1. Opened hood, fan did not turn off (as they should due to the hood sensor for safety)
2. Pulled plug on intake manifold sensor, fan did not turn off
3. Pulled plug on fan amplifier/final stage unit, this got the fan to turn off.
4. Charged up battery and plugged fan amp in to drive to car show. Unplugged fan amp at show and then back in to drive home. Unplugged fan amp again once home and car cooled off, fans stayed off that evening.
5. Read a few more threads and decided to order a new intake sensor.

Monday: Ordered new intake sensor

Tuesday (tonight): Remembered I have a spare intake (from a fellow Rennlister for an upcoming refresh project) that has a used intake sensor attached. Decide to test this one while waiting on the replacement new part. Results after installing the part, plugging the fan amp back in, and turning the engine on for a moment, and back off (did not drive it):
1. With replacement used intake sensor part installed and fan amp plugged in, fans stayed off (it seemed)!
2. About an hour later, I hear the dreaded fans turn back on in the garage (engine is not warm)
3. This time, when I opened the hood - the fans turned off (weird, this didn't happen on Sunday morning)
4. Depressed the hood sensor, and fans came back on.
5. Unplugged intake sensor while hood sensor depressed, fans stayed on (grr...)
6. Noted the fan amp was making a "ticking" sound while plugged in but the hood sensor was not depressed (keeping the fans off)
7. Unplugged fan amp, fans turned off. Noted fan amp was warm to the touch.
8. Noted flaps are in the open position

While I realize I haven't done scientific multimeter testing to this point, does it seem likely I have a bad fan amp like the OP of this thread? (could it have gotten wet somehow on Saturday?). Hopefully someone else has had a similar flow of this weird behavior.

FredR

The fan amp is a slave device responding to inputs from the fan controller unit that does "the thinking". The fan amp connector has live voltage input from the battery and controlled PWM voltage output that drives the fans at the required load plus the control signal.

If the hood switch is not responding [fans stop if the bonnet is opened] and/or the inlet manifold switch is not responding then my initial suspicion would be the fan controller unit may have taken a dump. Historically these things appear to be very reliable but with some such models now 35 years old since they were made more and failures are inevitable. The fan amps themselves are also a known failure point nowadays- unfortunately I do not know much about how they fail - I suspect that they can fail to a "power on" condition - hopefully you will get further inputs in that regard - for sure if you pull the plug the fans will not work.

Alan

I agree with what Fred said here - the internal controller is a more likely culprit for this behavior. The controller is supposed to stay active for a limited amount of time (<<1 hour) for the behavior you note that cannot be working.

They do seem very reliable - but this seems like a failure you'd get from it. Given the reliability a used one would be a reasonable replacement choice.

An option - unplug the controller and leave the final stage connected - does it still happen? if its a failure in the final stage it should be independent from the controller.

Alan

GP White 928 S4

Great tips - thanks to both of you. I will test when I get back into town in a few days!