At 12:46 PM 7/31/02, wrote:

Just a couple days before the big drive down to DEVEK Days (from Portland OR), and the cooling fans on my 88S4 start acting up (or at least different).

The "normal" operational behavior of the cooling fans is somewhat of a mystery to me, so I'm not super clear on what's right or wrong now, or how to go about diagnosing a problem. Any help would be appreciated. BTW, already skimmed Rennlist archives and most popular tips pages.

The symptoms/observations:

 * driver-side fan cycles every 10-15 seconds, full-on,   full-off, full-on, ..etc.   (this is main problem -- and its very noisy at stop lights!)

 * AC switch to [on] just makes the cycling start sooner  (before reaching normal operating temp)

 * passenger-side fan does not run under any circumstances   that I can create through normal driving   (not sure if pair supposed to turn on in tandem, or in sequence)

 * opening hood has no apparent effect to fan cycling

 * operating temp stays normal, but its not very hot around here

The system checks for faults, and if it finds a fault, will switch the bad fan off and switch the good fan to high. This cycle occurs every twenty seconds.

Try unplugging the passenger-side fan. (While you are down there, check the fan for binding.) If the driver-side fan now runs at high speed, there may be a problem with the passenger-side fan. Try switching the electrical plugs on the fans, and see if the problem changes at all. If the passenger-side now runs, the problem is in the control circuit, if the driver-side still runs, the problem is in the passenger-side fan.

BTW - each fan has its own fuse - (28 and 29). Check them also.

Wally Plumley
928 Specialists


Porsche engineers had way too much fun designing the fan control system for the S-4 .Very simplified..... The "logic" is built into the computer next to the passenger seat . The finned amplifier looking "control unit" is mounted on the hood front latch panel right side . The sensor in the radiator no longer simply turns on the fans but talks to the computer .
Based on temperature sensors , radiator intake it decides what it should do ...turn on one fan two fans as well as variable speeds . 1987-89 it also plays with the variable air flaps in front of the radiator . When the hood is open a safety switch should prevent the fans from running to help keep them from consuming fingers , the switch can fail and prevent the fans from running . If one fan motor fails , the second is supposed to run full speed to help compensate .

Jim Bailey


At 06:22 PM 2/5/03, Martin Turner wrote:
>I Have an 89 gt. The top radiator hose split and been replaced.
>Now the fans will not switch on ??
>Fuses okay. Shorted out the Temperature switch but they did not come on..
>Have been told it may be the fan switch ..
>Where is it and how do I check it ? Any ideas please..

Check the hood switch.
Check for power at the fan motors.
Check fuses #28 and 29.
Check the plugs on the fan final stage control on the panel in front of the radiator.
Check the plugs on the fan controller under the cover on the passenger door sill.

Wally Plumley
928 Specialists


The cooling fans should switch on whenever the engine temperature requires this. Even when the engine is turned off.

The after-cooling is switched on when ignition is off, hood closed (switch), and intake temperature is over 87,5C (sensor-switch on top of the intake closes) or... if the coolant temperature is over 85C (NTC resistor based temp sensor) at the bottom of the radiator. The cooling fans should engage at approx. 6,5volts and the flaps opened completely. This is managed by the temperature control unit inside the pass.side cover next to the seat, and the power stage in front of the engine (with the fins, just before the AC dryer).

74C - 2300 ohm
77C - 2066 ohm
79C - 1931 ohm
85C - 1582 ohm
90C - 1330 ohm
95C - 1128 ohm

At 79C both Fans would run 50% capacity, at approx 7,5volt. When temperature rises the voltage increases to 9,5 volt and at 95C the controller lets the fan run at 12v