David R. Hendrickson wrote to the list:

[rule #1  :)  88 S4 AT - 106k miles]

on another note, in the two months that I've had the car, the fans don't run after shutting off the engine unless I turn the key back on to power them.  they'd run for a minute or two then shut off.  the last couple days the fans have been running for about 5+ minutes in a fast/slow cycle.  the tech at the shop today noticed that the driver side fan is not running at all now (I know that it was working previously).  the A/C pump is seized, so I'd guess there's zero Freon pressure. now Wally told me which sensors the fan system looks at (inlet air temp, water temp and Freon press.), but would a sensor reading or the fan brain (r/h side of hood latch or under the pass seat side box?) kill one fan?  (controlled individually?) or is the fan dead?  the wire plug feels firmly in place.  I suspect the longer running time and fast/slow running of the single fan is it trying to make up for the lost additional cooling ability of the dead one.  I'm probably forgetting something here...

Start off with the simple stuff on the fan motor.  There are individual fuses for the fan motors, so check both to make sure they are OK before you dig any deeper.

If both fuses are OK, try swapping the wire leads at the motors and see if the previously dead fan springs to life.  If it doesn't, it's probably the
fan motor.  If it does, go back to the controller output unit on the front apron.

That output unit receives a speed signal from the controller, both output transistors receive the same signal.  Therefore, if one fan is running and the other is off, you can eliminate any upstream controls and focus on the output unit itself and the wiring between it and the fan motor.

The output unit features a pair of FET-type transistors that switch the DC power on and off for the fans.  Speed is varied by switching them on and off quickly, with slow speeds having less "on" time compared to faster speeds, and finally high speed that is "on" all the time.  A capable lister in Europe (Theo) wrote about dissecting the controller and replacing the FET stages successfully; the components are marked with industry numbers and may be available locally to you.  Alternatively, replacements are available from 928 International, 928 Specialists, and Devek in both new and used flavors.

As far as the fan(s) running after engine shutoff--

Only on the hottest days after a hard uphill pull have the fans stayed on in my '89 after key off.  "Normal" operation has the fans off a second or two after key off.

So, look at what has changed to help diagnose the change in operation of the fans.  Your shop did a clean-and wash of the top section of the engine where the temp sensor-II is located.  That's the sensor that manages fan run after key off.  They also did a coolant replace, which may or may not have included an adequate 'burp' to get any remaining air out of that high spot in the intake manifold where the sensor sits.

The sensor is a reverse-resistance type, where the resistance of the sensor actually rises as the temp goes up.  Start off by removing the two
electrical connector at the sensor, and cleaning the contacts on the end of the cable as well as the tabs on the sensor itself.  Replace the connector on the sensor and give it a try.

Burping the coolant system is a task that involves bleeding air out of the high spots.  I won't go into details, but will state that both the dealer and the doctor have 'burped' my car after coolant services, and both times the reservoir bottle needed a pint or so of additional coolant after a week of driving.  I thought I had a fool-proof plan when I did it, and still it had a bit of air in it I guess.  Anyway, look to see that the reservoir has sufficient coolant in it.  My low-level light gave a warning after the dealer did it at purchase;  when I did it, it wasn't low enough to turn the light on but still took a little to top it up.

Hope this helps!  With luck, a fuse will get replaced and all will be well once again!

dr bob

on the fans continuing to run after the car is shut down:

The S4 and later cars use coolant temp to determine fan operation when the engine is running.  When off, temp sensor II on top of the intake will run the fans as necessary based on the amount of heat that convects up through the coolant passages to the top of that intake section.

Since temp sensor II is a reverse thermocouple type of sensor (somebody gave the correct technical name a wile back...) it reads higher
temperatures as the resistance goes up.   Errrr... The resistance goes up as the temperature goes up.  Anyway, that means that a poor connection, lifted or broken connection, fractured wire, whatever, will cause that sensor to read high and the fans will stay on.

So, the first diagnostic (after the battery is recharged with the cables disconnected from the car) will be to check/verify that the wires on that sensor are connected correctly.  There are two sensors in the one housing, and each should be read separately to ground with a good ohm meter, and they should be about the same.  If they are, inspect the wire connectors carefully, then the wire itself as it goes from the connector.  You'll want the schematics to follow the circuit much beyond there.
Wally's recommendation on the final stages is a good one.  Are both fans running together?  If so, consider that it's unlikely that both stages
failed together.  With the schematics, you can look at the signal from the control unit to the final stages and see if the problem is upstream,
downstream, or within that module.  The final output stages are in the black finned housing on the front apron, on the passenger side.  Round flat connector with a thin metals screw down strap retainer.  The module can be repaired, as the FET elements are industry devices.  Theo J published a note on the repair procedure including part no's in the last year or two, so a rennlist search on the keyword "FET" should net you the write-up.

The fan speeds are controlled in a module in the passengers door area under the cover with the rear lid release switch.  I think...

Good luck, and let me know offline if I can offer any further diagnostic help.

dr bob

masquerading as a computer control system engineer for a few months.
Subject: Re: help please, cooling fans staying on (and on and-  on) after car is off.
From: Wally Plumley
Date: Sat, 03 Feb 2001 23:13:35 -0500
X-Message-Number: 80

At 02:23 PM 2/3/01, Bobarino91@aol.com wrote:
I know they are supposed to stay on for a little while, but mine stayed on or 8 hours yesterday and killed my battery.  I opened the hood and learned something new.  the fan shuts off when the hood light comes on via the pin-switch. (for safety I'm guessing).  so with the hood open, push the pin switch, which turns off the hood light and the fan starts running again. (only the drivers side fan is staying on.)  so I'm guessing that the thermostatic switch is stuck in the on position. so can someone tell me where it is?  in the radiator I'm guessing.  does my diagnosis sound right?  if not, any other ideas?  seems to be the fan switch stuck "on".  thanks in advance.  you guys (and gals) are the best.

I don't think that a stuck switch would have only one fan running. Sounds more like a problem in the fan control final stage to me.

Wally Plumley
928 Specialists

At 07:41 AM 7/13/01, kaberez@mindspring.com wrote:
>Car in question: 1986.5 928S Automatic. (For Wally)
>Should my electric cooling fan come on as soon as I turn my AC on?

The answer would have been different for an '87, so the year model was important.

Should it? Yes.
Does it? No, that's not how Porsche designed it.

The '86 and earlier electric fans are triggered by one of three things:
1) Coolant temp - switch on the face of the radiator.
2) Freon temp - switch on the vertical stem of the receiver/dryer. The fan doesn't come on until the Freon gets hot.
3) Inlet air temp - switch on the top of the intake manifold. Mostly used for run-on after shut-down. This is the only fan sensor that is active with the ignition turned off.

I was tempted to put a relay on my '86 that ran the fan anytime that the A/C compressor was on.

Wally Plumley