We are the proud grandparents of a 1992 CS100, our daughters car. The sensor switch that controls the fans on the radiator went out, causing it to overheat. She(daughter) ran the heater to cool down her car, and blew the heater core. Luckily, I found detailed instructions on how to replace it on this site. The new core is in, the fan switch is replaced, and worked for one day. Now, no fan. Replaced the switch again, no fan. Still overheating. Is there a thermostat, and where is it? Does anyone have any ideas? We are stumped. Please help!

 

First off, smart girl for knowing to run the heater when it's getting warm!

Second, the thermostat is located on the left side of the engine (facing it) under a couple covers. It's not easy to get to and may require removing the timing belt to get to. Although others have said it's possible to twist the tb in a way to get the thermostat housing off. I haven't done this while the engines in the car but did it to a junk engine on an engine stand and it was tough!

These thermostats rarely fail closed though. In most cases they fail to the open position.

You may want to be sure the radiator isn't clogged as well. This has been known to cause that switch to not operate correctly.

 

thanks so much! I follow the keep it simple stupid rule, so we will start with making sure the radiator isnt clogged. This is the current problem with this lovely pearl white beauty, but not the only one. I plan on being a regular here! This site, and all you folks, are very, very helpful. Again, thanks!

 

Do the air bleed proceedure:

Top up the coolant, remove that plastic panel in front of the windshield and loosen that fragile plastic bleed screw there in the heater core hose up by the heater valve.

Now fire up the car and run the engine at about 2,000rpm constant, heater set to "hi" with the fan speed on a middle setting.

Keep the rpm constant, or higher. Your goal is to heat things really fast, get that engine coolant thermostat to pop open, and then have a forcefull water pressure wave moving through the entire cooling system to force out all the air trapped inside.

You can pull on the black cruise control vacuum diaphram there in the engine compartment. It's there just in from the coolant tank, up top with a silver metal wire-type rod running into the top of the engine sideways (horizontally). Pull the rod/diaphram toward you for increased rpms.

So hold it at speed, top off the coolant as needed, and watch that plastic bleeder. When the plastic bleeder screw stops putting out foam, and air, and just coolant is flowing out, carefully close it.

Continue holding the rpms up. You're now waiting for hot air to be pumping out of the dash vents. When you get hot air, you're getting close.....

Continue holding the rpms up until you have radiator fans working and both sides of the radiator are the same (hot) tempurature to the touch of your hand.

Once the radiator fans are cycling, you're almost done. Shut her down, put the cap on and give it about 15+min. It'll cool, pop the radiator cap and suck down more coolant.

Top it off and go for a drive. At the end of the drive, let it cool for a bit and loosen the radiator cap to let the coolant burp again.

During this process the coolant will sometimes boil up. You can raise the rpm to keep this from getting out of hand, and/or put the coolant cap back on for a few minutes...and then remove it and continue.

Yes, the engine coolant thermostat can be removed with the timing belt still in place. You just loosen the timing belt tensioner pulley so that the belt can be slid forward (towards you and away from the engine) on the right camshaft sprocket. Once the belt is way forward, you have room to finesse the t-stat housing out from behind it, and then get the thermostat out. Only install and OE stat, bleeder valve at the 12 o'clock position.

Check your fuse(s) for the radiator fans. The fans get bad bronze bushings and can wobble. They can get so wobbly that I can see where they'd wipe out the electric motor, causing a short and blowing a fuse. Check yours.

You can jump power to them and see how they run.

 

I need to thank you for your awesome directions in the heater core area,,You saved us a bundle! We will try this, and get back to you on the results. We have fallen in love with this car, our first Audi, and when the daughter goes to college this fall, it becomes the wifes car. We really appreciate all you have helped us with so far, again, thanks!

 

do we need to use Audi coolant, or will regular peak antifreeze do fine?

 

Audi Pentosin coolant is recommended. I use Zerex G05 in all mine.

 

You need to use the Audi stuff, or at a minimum, phosphate free.

 

Are the radiator fans coming on? In the summer, if they're not, you can definitely overheat. Jump the terminals at the sensor on the bottom of the radiator to test the fans first, then test the sensor.