New member here and could use some advice on a big problem I am having with my 90 928 S4. It seems the cooling fans are only operating intermittently. With the ignition on and turning on the AC both fans operate however as the car heats up while driving the fans stop and the car starts to over heat. It also seems that if I just let the car idle the fans continue to operate but if I take it for a drive, after awhile the fans stop and the temp starts to rise. If I posted this incorrectly I apologize so any guidance ce appreciated. BTW my fans do not run after the engine is shut off but the engine is hot

 

Please define what you mean by the term "overheat"

 

Someone should probably explain the temperature controlled fan circuit operation. I know the S3, but not the S4.

 

Mike,

There is potentially rather a lot to cover with this system and our new friend's post tells one a fair bit but the first thing to establish is whether the motor is in fact overheating or whether it is behaving as expected. In this regard the another thing to establish is whether the owner is new to the 928 in question or whether he is an owner with some experience seeing something he has not seen previously and is thus concerned.

That both fans are running is the first positive, that both fans run when the ac is switched on is another positive. That the fans are not running when the motor is switched off does not tell anything as that can happen as the fan run on is determined by what input the controller gets from the temperature switch located on top of and to the rear of the inlet manifold.

At the moment it is not clear whether the testing described was done with the ac running or with it switched off

 

first thing to check in fan operation is to check both of the fan fuses ,
and usually a replacement along with D100 and investigating if the CE panel sockets have enough grip on the fuse blades.
If the fuses are loose or appear to have melted then its a sure bet the fuse blades are not getting a good connection,
try adding a thin coating of solder to the fuse blades.
Clean the grounding points above the CE panel

Then check the battery positive terminal to see if both of the 2 red wires bolted to the terminal are tight,
Then a 14 pin connector inspection and hot post cleaning of the wires attached to it.
Verify that the sensor on the top of the intake is open unless the intake is hot.

 

Thanks guys for the quick response By over heat I mean in the red and the red warning light on just as I was able to clear the road and shut it off. My mechanic here turned me onto Roger at 928s rus regarding a new power output stage ,that finned device I am sure you all know about just before the radiator on pass side inside the front of the car. (Presently Waiting to hear back from him )I just returned from a drive where I had to shut it down again. After letting it cool down, I drove it home for about an hour with absolutely no problems. The temp gauge stayed about three needle widths before the last white line before the red zone. The voltage test that I was able to decipher from the ops Manual checked out. Now please realize I feel lucky if I can put batteries in a flashlight correctly so that shows my electrical competence. I did notice that when the engine stays relatively cool the battery gauge shows about 11.5 volts. When it over heats it reads about 13 so I am assuming the fans on are resulting in the 11.5 reading and the fans not on is the 13. Hope this doesn’t muddy the waters too much. Thanks again for the responses

 

A bit more info. The test was done with the engine off only the ignition on the ac on with blower on any setting as per the manual. All fuses checked and relays and all seem to fit tight and test ok

 

The fans are controlled by PWM [pulse width modulation]. This is a soid state control unit that instead of using a resistance to control fan speed switches the power on and off rapidly to simulate the effect of a lower voltage and thus is more efficient. These units are very reliable but nowadays they are falling over. These units can be rebuilt albeit I am not sure if that is what Roger does. Sounds as though there is a good chance the unit is failing given the erratic running. The alternator generally provides a good power feed above about 2500 rpms and you should see 13.5 volts even with the fans running. At idle the voltage will drop some

You also need to check that the slats in the nose are opening and staying open. Best thing to do with that system is to isolate it and wire the things open . Better still in a warm climate is to remove the slats altogether as that improves cooling air flow a bit.

What happens with the engine shutdown is largely irrelevant. For starters anything electrical running is at battery voltage which should be 12.5 volts and less if a heavy load such as the fans [about 40 amps] are running.

 

Thanks FredR. Appreciate the advice Being in hot Southern Florida I like the suggestion of removing the slats all together even though I believe I correctly manually placed them in the constant open position. I will keep an eye on that electrical issue

 

Remove the fuse for the flaps if they are installed

 

Thanks Mrmerlin will do. I just replaced the front power amp for the fans and although the fans appear to be cycling as they should and we’re not before, the car is still over heating( needle heading into red zone). My next plan of attack is to replace the battery and chassis to engine block grounding straps . Maybe there is a voltage issue. I read in hear that old fans tend to draw a lot of power and this could cause a power drain contributing to erratic fan operation. Not smart enough to tell the difference between a bad power draw or malfunctioning component and really don’t want to get into the part replacing game without good evidence to replace. Any suggestions greatly appreciated

 

You may also need to evaluate the coolant system, flush, fill with good coolant, test the cap seal and possibly investigate the thermostat behavior. If the fans are working correctly yet its still overheating there has to be more going on.

Alan

 

Thanks for the advice and will give that a try. I understand from advice given to some in this forum that it is recommended to stay with the higher temp thermostat( 80+ degree) instead of going to the lower temp(70+ degree) thermostat. Correct?

 

Yes stay stock on the thermostat, a lower temp thermostat doesn't actually help - may seem to be unintuitive but true. The engine runs best at the design temperature and if the thermostat works properly it should cool the engine fine as long as the rest of the system is operating correctly. You could also look at suitable water wetter additives for slightly better heat conductivity between water and the radiator (a small benefit).

Alan

 

Since you live in a warm climate you can also reduce the amount of anitfreeze in your engine from 50/50 to 25/75.

Water conducts heat better, has a higher boiling point, and a 25% mix still has more than enough lubricant and anti corrosion chemicals in it to do the job perfectly well.

It's what I have ran in mine for years now without any issue.