tbo12

My 1995 993 engine fan is acting up - moving the car cold (from one garage bay to the other with a run time of a minute or less), the engine fan comes immediately on when the engine is stopped. It then cycles on and off repeatedly - sometimes staying on for 20 seconds before shutting off, sometimes a matter of seconds before shutting off, but always starting right up again. then off, etc., etc. Anyway, this will go on for about 15 minutes before the engine fans stops this on/off cycling. Also during this period, the dash heat/ac control unit is humming even though the key is off and out of th car. Thoughts would be appreciated as to the most likely cause or component failure. Thank you.

ToreB

The Climate Control Unit is designed to stay on for up to 20 minutes after ignition off. During this time, it will measure the rear air duct temperature, and run the rear fan if cooling is needed.
If the rear air duct temp sensor is defective, the CCU will run the fan at high speed as a safety measure. This will also happen if the rear fan low speed resistor fails, the thermofuse in the reistor is known to fail, preventing low fan speed.
Use a Porsche compatible dianostic tool to read out any fault codes in the CCU, and test the two speeds of the rear fan, explained here: https://www.bergvillfx.com/categorie...test-procedure
Cheers,
Tore

tbo12

Tore - thank you for the prompt and very helpful reply. Since the motor is clearly still cold after running it for just a minute or less to move it, and the auxillary fan immediately starts when the key is off and then runs in this haphazard mode (meaning the length of time it runs before shutting off and then on again is from a second or two up to 15 or 20 second, it would indeed seem that the CCU (?) is getting a faulty signal that the engine needs to be cooled and the fan turned on. Based on my symptoms, would it be a reasonable step to simply replace the blower tube resistor (physically an easy item to get to and perhaps not that expensive). My car is an OBD I car, and I only have an OBD II diagnostic tool. Thank you.

IainM

Or remove them and test them with an ohmmeter. Search for threads on how to do this.

ToreB

I would not start throwing new parts into a problem without proper fault finding.

A Porsche compatible OBD tool is not expensive, a Windows PC and the T-OBD interface would do very well.
I would start with testing the fans as earlier described.

Measure the rear duct sensor with an ohmmeter like this:
Without CCU connected, and ignition off
Connect ohmmeter negative lead to G18 (Sensor GND - Brown/Blue wire)
Connect ohmmeter positive lead to G10 (Blue/Green) NTC Rear heater fan Sensor
Reading should be about 12,5 kiloohm at 20 degrees C ambient temperature.
Cheers,
Tore

pp000830

The cabin/engine off fan speed resistor in the engine compartment ductwork is an exposed resistance coil and an electro-mechanical bi-metal thermal switch in one. If you pull it out of the duct below the cabin blower to the left of the engine it is easy to see if its coil or switch is broken (stuck open), clean and test for continuity.
Andy