car is a 87 a/t

Neither of the two radiator fans are running. My hot temp warning came on.
They aren't running at the hot temp, or with the AC button in. I have voltage at the fuse panel. AC does work.

Several months ago I replaced one of the fans and have had no problems until now. I don't think they would both go at once. Before I go through the wiring diagrams I wanted to see if anyone had some quick advice.
John Pirtle
87 a/t 225k

The fan fuses 28 and 29 take power to the amplifier which is at the front of the radiator. Those fuses are good and have voltage.

I have also checked for voltage at the "coolant fan and flap unit" which is between the passenger seat (US) and door sill. On that unit, if I read the electrical diagram correctly, there are three wires going to the amplifier.

Connector I
Wire 6 blue
Wire 8 green

Connector II
Wire 7 grey

With ignition on and AC on, I have no voltage on blue and green. Labeling on the unit shows these are Sig 1 and Sig 2 (I assume "signal".)

Grey has voltage. The unit labeling calls this U Power.

So I believe the unit is not providing a signal to the amplifier.


Ok, power goes from fuse 28 and 29 to the power stage. This one is ok if you measure on the power amp. Ground is from the fan motors to MP2, so directly connected to the ground point.

I took a look at the documentation again... :) There's a good explanation in the service info manual of the 1987 model, page 93 (9-09). The grey wire is the management power of the power amp, called Ux. It doesn't say but it is pin 7 on the plug II, which powers the electronics in the power amp. The other two (connector I, pin 6 and 8) are control signals, [Sign] or in German: [Tastv/Rückm], steering the power, telling the controller that there's a problem and take desired actions (like fan blocked, a short circuit or a motor seized). It is in fact a bit more complicated. The controller sends a PWM signal to the power stage and modulates the PWM, but uses the same line to sense the output of the power stage. Based on this sense, it concludes if the second fan is running properly or not and changes the modulation of the other fan to compensate.

I'm not 100% sure anymore, but if 6 and 8 on P1 do not carry voltage and pin 7 on P2 does have power, the power stage could be defective. Can you see the flaps in your car operate? That will tell you that the cooling controller seems to work. It should become active when you switch ignition on, and push the AC button. The pin7 (grey) should also respond.
Hope that helps?

Oh, one more thing: the fan control module (the box besides the passenger seat) needs a switched 12v power (bus 15) on pin 4 on connector I (1). This power is taken via fuse #36. If the #36 fuse blows the fan system will never run. And to be able to run when ignition is off but intake temperature is too high (cooling the engine after it was shut down) it needs permanent 12v (bus 30) on pin 2.

1992 928gts Midnight Blue
1988 928s4 Cherry Red (Sold in 2006)
The Netherlands

I found the troubleshooting section in the workshop manuals - pg 19-22 through 26.

I worked through the tests and everything passes (including the flaps) until pg 19-25 part 4. This is the test of the control unit pin 6 and 8. I should receive 7V on each, but I only get a very, very low reading.

The manual seems to say that if all the tests pass but the fans still refuse to operate then the output stage is defective.

The manual doesn't say anything specifically about what indicates the control unit is defective.

So it seems to me that my test failure on pins 6 and 8 may mean the controller is bad. If these pins are feedback signals from the output stage, the controller must be involved because the test is with the plugs connected
to the control unit but the plug cover removed. ?

I certainly appreciate the help!

John Pirtle
87 a/t 225k
Hi John,
Good that you found the troubleshooting section. I now remember that I have seen that before.

Yes, I think the next step would be swapping the power stage with a known good one. You should measure with the plug connected, so the plug cover removed. It says so in the manual and seems logical as it is a feedback signal. I believe every 20sec the status is processed by the control unit.

If it turns out to be the power amp, you could consider replacing the power transistors (FET BUZ11a) and see if that helps. I repaired mine years ago. Here's a quick link:

1992 928gts Midnight Blue
1988 928s4 Cherry Red (Sold in 2006)
The Netherlands


Fuse, fan, or fan amplifier.

First check the fuses. Hook 12 volts up directly to the fan and see if it runs. Disconnect and spray the connector into the fan amplifier with contact cleaner. If the fan and the fuse is good, I can send you my "test" fan amplifier to plug in. The logic circuit in the fan amplifier is very clever, but can "fool" you very easily. If the problem is a fuse or a fan, you can "fix" the problem and not have both fans run, immediately. If the "logic circuit" has shut off the current to one fan, because the fan is not working, it will not re-energize that circuit immediately.

The "circuit" checks the "bad fan" every couple of minutes, to "see" if it is now functional and ready to run. You can actually hear this "checking of the circuit", when one fan is running at high speed. Every couple of minutes, that fan will slow down, while the logic circuit checks the other fan. If the other fan is then ready to work, it will "fire" both of them up, at normal speed. If the other fan is still bad, the circuit will then speed that one good fan back up.

What does all this mean? If the fuse or the fan is bad, it can take a couple of minutes for the fan amplifier to know this and resume "normal" operation. You have to be patient. Give things a few minutes.

greg brown


Starting the 1992 model year the fan controller unit was modified. In prior versions the fans would run full in a cyclic manner when full fan operation was required.

Starting 1992 the fans run in continuous full power when full operation is required. I think the controller box is a different although the part numbers do not show this.

Regards, Theo

1992 928GTS