johnfm

I've noticed that the small electric cooling fan in my C2 comes on after turning the engine off even if it has run for less than a minute. I know that its supposed to come on if the fan temp is higher than 95 deg C. I doubt the temp has reached this level in this time, so is it likely the thermo sensor is buggered?

Any clues?

MikeF

John,

The oil temperature sensor is easy to check/replace. Remove the passenger front wheel...then remove the 12 or so screws that hold the mud guard in place. Once removed you will see the oil cooler and the sensor is located in the upper corner nearest to you. There is a wire harness attached to it. You can test it my checking the resistance between the two connector leads when unplugged. Should be above 17 ohms when cold. As the engine warms, the resistance drops until it is low enough to turn on the fan...typically about 100c. Sounds like yours is stuck at low resistance...0.5 ohms or below likely.

If that is the case, unscrew the old one when the engine is cold. Screw in a new one with a new sealing washer. The sensor is inexpensive, about $20 USD. If this is not the problem, you may have a bad relay. Try swapping it with the identical relay located right next to it in your fuse box (this one is for the AC condensor fan). If this fixes the problem, you have a bad relay...also reasonably cheap.

You might want to check with Randall Granaas for more troubleshooting tips if this doesn't solve the problem. Do a search. He helped me with a valuable document on solving these types of problems.

Hope this helps,

Adrian

Dear John,
Which cooling fan are you actually talking about.
I suspect what you are hearing is the temp sensor fan in the aircon/heating control unit) if the noise is coming from in front of you and this is quite normal or if the noise is behind you, the additional blower or rear blower fan activating. At this time of year this is also quite normal. However please advise us of the actual fan you think you hear running. There are many of them and many sensors,
Ciao,
Adrian
911C4

johnfm

Excuse my vagueness!! Its the electric cooling fan in the engine bay, top left as you look from the rear of the car. There is a black plastic duct leading from the pulley operated engine cooling fan. This duct sweeps up to the left to a small electric fan in a housing. The fan has a wire connected to a probe/sensor which is put into a hole in the duct. I expect this is to aid cooling the engine after stopping.

I think the manual says thet the temp is monitored and the fan comes on with a cooling fan temp > than 95 deg C.

Von

John, I have the same problem with my heater/engine cooling aux fan. I have had to at times unplug it to stop the thing from running all day. My fan is inconsistent at best, sometimes turning on after the engine has been off for several minutes. I have been told by my tech that it could be the sensor (a plastic plug in the top of the ductwork - not the thermal switch to the side) but I have not sprung for the fix until I know for sure it's the problem. But for US$36 from the dealer, it might be a cheap test. My only problem is that it happens so inconsistently - it may be a few months before happening again. If you find the solution, please post.
Von 89C4

Adrian

Guys,
If your rear blower fan is playing up like this, it means the "Blower Resistor" is shorting out and needs to be replaced. This is a fire hazard and needs to be fixed. Yes it is well documented.
The rear blower can start up after shut down of the engine if the engine is "REALLY" hot. In all my years of 964 ownership, this has never happened to me.
The following is the text to rectify this problem,
Ciao,
Adrian
911C4

Heater Blower Resistor.
AFTER THE ENGINE HAS BEEN TURNED OFF, THE REAR HEATER WILL AUTOMATICALLY OPERATE IN ORDER TO DISSIPATE HEAT IN THE ENGINE HEATING SYSTEM. DURING THIS OPERATION, THE RESISTOR FOR THE REAR HEATER BLOWER CAN BECOME OVERLOADED DUE TO A BINDING OR SEIZED REAR HEATER BLOWER. THE OVERLOADED RESISTOR CAN CREATE HEAT SUFFICIENT TO IGNITE THE PLASTIC AIR PIPE TO WHICH IT IS CONNECTED, RESULTING IN AN ENGINE COMPARTMENT FIRE. PORSCHE DEALERS WILL INSTALL A NEW RESISTOR CONTAINING A CIRCUIT BREAKER.
1989 PORSCHE 911 CARRERA 4
1990 PORSCHE 911 CARRERA 2/4
1991 PORSCHE 911 CARRERA 2, 2TL, RS/4
The actual repair can be found in Porsche Technical Service Bulletin REAR HEATER BLOWER RESISTOR No 9302. Under warranty under a voluntary recall program the Old Blower Resistor is replaced with New Blower Resistor part number 964 616 550 02.

johnfm

Okay
1. My C2 is an Aug 92 build UK car. Will the resistor upgrade already have taken place in factory? I expect it has.Even so, its now 9 years old. Can I easily checked if it is shagged?

2. My fan comes on if I run the engine for a minute or so. It stays on for 15min+. I think it is helping to drain my battery! Do I need to replace the resistor bit, the temp sensor in the duct or should I do both? The temp sensor is only £15, getting a price today on the resistor bit, but they're probably cheaper than a £70 battery!!

Adrian

Dear John,
Please do not get them mixed up. There is a Blower Resistor and there is a NTC temp sensor in the pipes. You need to replace the blower resistor not the temp sensor. The Blower Resistor is best described as a current limiter and Temp Switch.
The Blower fan at full speed can often come on after a start especially. If you have heat selected it will come on all the time at low speed as soon as you turn the ignition on.
Again this is a 2 speed fan.
Low Speed when heat selected and in winter weather this means anything out of the blue dot position.
High Speed for engine cooling.
If the engine is hot and you start up it is very common for the blower fan to crank up to full speed for a short time. No longer than 60 seconds though. The blower fan does not directly cool the engine, it dumps the hot air in the pipes around the motor.
Good trick for summer. If your engine is getting hot, wind down the windows, turn the heat to full red dot position. Cools the engine nicely and quickly.
I am really surprised at how many problems that this system is causing based on the posts on this board.
Ciao,
Adrian
911C4

PS: Just for info. Here are all the fans installed in a 964 (normally)
LH Blower Fan (speed controlled)
RH Blower Fan (defrost one speed fan)
Rear Blower Fan (2 speed fan)
Oil Cooler Fan ( 2 speed fan)
Condenser Blower Fan (2 speed fan)
Temp Sensor Fan (runs all the time at rear of controller. Can run for up to 20 minutes after turning engine off).
All above fans are controlled or partially controlled by the aircon/heating control unit.

johnfm

Is the temp sensor fan which can run for 20 minutes inthe cabin/trunk or in the engine bay?

Now I know there are 2 bits to the system, ie blower resistor and temp sensor, can I assume it is the resistor OR can I easily check that it is performing its function correctly.

Adrian

The temp sensor fan is mounted on the back of the aircon/heating control unit and has nothing to do with the temp sensor for the rear fan by the way. This is the funny noise you hear behind the dash that drives every owner crazy sometimes.
It is very difficult to determine the serviceability of the blower resistor. If you have the old part number installed replace with new part number which has a built in cct breaker,
Ciao,
Adrian
911C4

Michael Witz

What's the part number on the Temp sensor fan (or do I replace the whole module?) for an 89 C4.

If anyone has a link to where I can purchase online that would be appreciated also.

Mine was squealing something awful till I unplugged it. But Adrian mentioned this probably isn't a good long term fix.

So I'd like to replace it and this seems like a job I can do myself

Adrian

Dear Michael,
Now you have a problem. You need to remove the controller and remove the sensor/fan assembly from the rear and try and fix it. Why?, you cannot get these things anymore. If you try and purchase a new one, you will get a complete 993 unit offered. This requires a wiring mod to be carried out. I would try and do something DIY. Or check out the wreckers,
Ciao,
Adrian
911C4

johnfm

Adrian

its Monday am in the UK. I pulled the resistor coil/bimetallic strip thinhg out yesterday to have a looked and noticed a couple of things.

I started engine for 10 seconds and the rear blower was on. This is my problem I think.

So, I pulled out the resitor assembly and noticed a number of things:

a. Ouch - it was very hot. When the bimetallic litle swith is on the fan operates and current flows thru the switch & therefore the coil of wire. It gets hot & burns fingers!. My guess is that the coil heats the bimetallic switch, and the whole thing is designed so thet when the air blowing over it thru the duct is cool enough, the bi metallic switch opens (even though the coil is supplying some heat) & shuts off the fan.

I noticed that the 'nipple' on the switch was a little corroded and I gave the strip a bit of a clean, fddled it a wee bit a sprayed a bit of WD40 on it.

I now notice the fan doesn't come on at all (however, I haven't had the engine really hot since.

Have I damaged the coil or will the fan only come on when engine is quite hot??

Adrian

Dear John,
Select heat to say 30 and the rear blower fan should come on at low speed.
The rear blower fan "RARELY" if ever at this time of year crank up to high speed.
As long as it runs at low speed I am sure it will work at high speed when needed. If it is not working at low speed with heat selected then you have a problem,
Ciao,
Adrian
911C4

PS: I do not recommend the use of WD-40 on electrical components.

johnfm

O, so the fan will come on at low speed with heat selected. What determines high speed operation? Is it the resistor (I don't think so, FWIW) OR the temp sensor or something else entirely (like the engine oil temp sensor)?