SeriousMonkey

I have a 1993 Audi 100s w/175K miles, auto trans.

I just replaced the heater core and radiator. The radiator fans come on when the air conditioning is on, but won't engage when a/c is off and the temp exceeds normal levels. I changed out the thermostat sensor in the radiator. And checked fuses and relays, which seem to be working correctly. Running out of ideas.

I have searched this forum and can't find the right solution yet.

ncoupe

I had (and resolved) a similar issue recently, so I would suggest that you make sure that the ground connection in the current path through the thermoswitch is good. To do that, disconnect the connector from the thermoswitch and and short the pin (on the connector side, not the switch side) that correponds to the brown wire (this is ground) with any of the other two pins (again on the connector side). You should hear one of the two relays in the relay/circuit breaker box near the wiper motor click and the fans should come on, either at low or high speed, depending on which pin you connected to ground.

Also let me know if the fans ALWAYS come on when the A/C compressor works. If not, my experience with the problem I had may prove useful to you.

SeriousMonkey

I pulled the connector off the radiator thermostat and jumped the connectors brown wire with the brown/blue, and next the brown/white pins. The fans came on in both instances, running at two speeds as they should. So, from this I know the relays and fuses are working properly and the ground wire is good. Also, as I've said before the fans always come on when the air conditioner is on, and now I know the fans come on when they are jumper-ed.

The only other thing I can think of is the thermostat to the motor is bad, staying closed so the water does not circulate to the radiator but stays in the motor thus over heating. This would explain why the radiator thermostatic sensor is not getting hot enough to throw the fans on, i.e., not reaching the 198 - 207 degrees it needs to engage. I have the capability to test the temperature of the radiator with my multimeter. I will do this next to test out my theory.

I will hook up the themostat sensor on my multimeter to the exterior of the radiator and let it idle. As the temp on the gauge get to the point the fans should kick on I will measure the approximate temp at the meter and see what it is. If its not even close to being at temp, (198 F) then we'll have a pretty good idea the thermostat on the motor is bad.

upallnight

Did you replace the radiator with a brand new one or one found at the junk yard?

If the sensor is located at the bottom of the radiator, you have a clogged radiator which prevents the sensor from reading the actual temp of the water.

Remove the sensor and see how much water flows out of the sensor opening.

Prospeeder

you should go get one of those cheap laser temp guns. Just had a car in the other day fans wouldnt come on, overheats, and the thermostat was stuck closed, so the radiator never actually got hot since nothing was circulating.

SeriousMonkey

Replaced Radiator with a brand new one. And replaced the thermostatic sensor that plugs into the radiator with a brand new sensor.

SeriousMonkey

This is what I was thinking too. I have purchased a new engine thermostat and will put it in this weekend. Will update you with the results.

SeriousMonkey

I have replaced the radiator, radiator thermostat sensor, engine thermostat, have jumpered the plug to the radiator thermostat and the fans engage running at both high and low speeds, so this means the relays and fuses are working, and the fans run when a/c is turned on.

But the fans still won't come on when the engine reaches temp under normal conditions. Stumped as to what to do next.

ncoupe

Well, I will share my experience with you. It may help. Have you checked the fuse holder for the fans? I had problems similar to yours (although my fans did not always come on when I shorted the connection to the sensor at the bottom of the radiator, they came on sometimes). The culprit was a bad fuse holder. I can only guess what caused it but I suspect that at least one of the fans had become reluctant to work over the years (sticky bearings?). As a result the current going through the circuit was very high (a motor draws a very large current before it starts turning, as the impedance is very low during start up). As a result of heat produced by the large current, part of the plastic of the fuse holder had become brittle and one of the connectors had oxidized. The contact had become bad, and as a result the voltage across the motors was barely and intermittently able to turn the motors, making the situation even worse. So check your fuse holder, and replace the motor(s). The reason the fans come on when you short the sensor is that you make the resistance just low enough to allow enough starting current through the bad connection.

The fuse in question is at the bottom of the A pillar in the driver footwell just forward of the front edge of the front door. You have to take out a plastic cover that is held on by a screw and unclip some tabs locking it underneath the door sill trim, therefore you have to at least loosen the screws for this piece of trim as well. These screws are hidden by a strip of trim on tp of the sill trim, which you can carefully snap out of position.

AUDI_100

Hi, Can anyone tell me where exactly this fuse is on a 1991 Audi 100 2.3 Auto? I have removed the a-pillar trim and underdash trim and I can see the relays, but no fuse. My Electric fan does not come on, I have tested the motor with a battery and that works, but there is no power supply to it. So I am assuming it's this fuse that I cannot find. Would really appreciate some help.