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.
Thanks!
John Pirtle
87 a/t 225k
<http://members.rennlist.com/pirtle>
======
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.
John,
============
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.
And yet one more: I've seen a broken X-relay in a S4 causing the radiator fans and interior fan not to work.
Regards,
Theo
1992 928gts Midnight Blue
1988 928s4 Cherry Red (Sold in 2006)
The Netherlands
http://jenniskens.livedsl.nl
http://928gts.jenniskens.eu
=====
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 and see if that helps. I repaired mine years ago.
Here's a quick link:
http://jenniskens.livedsl.nl/Technical/Tips/0/MyTip011.htm
Theo
1992 928gts Midnight Blue
1988 928s4 Cherry Red (Sold in 2006)
The Netherlands
http://jenniskens.livedsl.nl
http://928gts.jenniskens.eu
=====
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. The controller operates with a microprocessor which is factory programmed. Maybe the code was altered at some stage.
Regards, Theo
1992 928GTS
_________________
A customer of mine had some erratic behavior of the fans, sometimes the ran, sometimes not at all. Even bypassing the power stage (bridge 12v to fan) did not make it run. So there was either a problem at GPII or a problem at the connector that hooks up the fan terminals. The latter was causing the problems. Oxidation caused bad contacts and confuses the fan control system.
regards Theo
1992 928GTS
the Netherlands