We have a 1995 a6. The heater core blew and we replaced it(was shocked how easy it actually was). Put everything back together and car keeps overheating. I have bled the system over and over...thought it was the fan switch in the rad. replaced it...but car is still over heating. We have heat but goes kinda cool when we are ideling....car is not over heating on the highway...just when its ideling. I have Jumped pins 1 and 2, also 1 and 3 and the fans are working....but they dont seem to be coming on when the car starts to get hot. Also have done a rad flush....dont know what to replace now...thermostat????....water pump???...the odd thing is it seems to work fine sometimes. HELP

 

When was the timing belt replaced? Was the thermostat and water pump replaced at the same time?

Did you turn up the heater when you were bleeding the system after the heater core replacement?

Only overheat when it is idling but is fine on the highway? Do you know if the waterpump has metal or plastic impellers?

If you don't know when the water pump and thermostat were replaced maybe now is a good time to do a TB replacement.

 

By jumping pins 1 & 3, or 2 & 3 the fan kicks in and ran. This indicates that the fan circuit is working. Now you will have to determine if the temperature sensor located at the bottom of the radiator is bad or the radiator is clogged. Easy way to check for a clogged radiator is to verify that top and bottom radiator hoses are hot (very hot). If the bottom hose is cold or only warm, then you have a clogged radiator. The clogged radiator is preventing the hot coolant from reaching the temperature sensor. By the time the coolant makes it way through the clogged radiator it has cooled to the point that the sensor doesn't see an overheat condition.

 

The temp sensor was just replaced....a rad flush was done.....and the car runs fine as long as the ac is on so the fans run....if u turn the ac off it over heats ideling

 

Check the top and bottom radiator hose. They should be very hot at operating temperature. Since you already changed temperature sensor/switch then I would still check to see if the radiator is still clogged.

 

They are both hot....but would the water temp sensor on the back of the engine cause this problem???

 

So I just noticed that the heater temp goes down when the car is just ideling...

 

Sorry to post so many in a row, is there a way to edit post to *** stuff. As I said before we took the car in for a radiator flush I'm thinking the issue with the heater has to do with air in the heater core, the heat gets hot when the car is revved. However the fans still are not kicking on unless the air conditioner is turned on so I'm not sure if its the coolant temp sensor is defective. I don't think its a thermostat issue because it seems to run fine with the ac on... aside from the fact the car is ridiculously hard to get all the air bled from the system and the water in the heater core is boiling. I spoke to someone at auto zone and had all the relays tested and everything seems to be in working order on that end... however, he said his oms meter was only getting 10v not 12v. he seems to think we might have a wire that isn't plugged in all the way or its shorting out... but i can turn the fan on manually so i don't get it. I am totally dumbfounded at this point and don't know what to do next. coolant temp sensor? tear apart the car looking to see if a wire is partially unplugged or is touching some random object? replace thermostat and water pump? I will be taking it back to the place we got the radiator flush tomorrow and asking them to top it off and re-bleed the lines if they would... if i can get it working correctly with the ac on then we know it isnt the water pump or thermostat right? ughh...

 

The following is a wiring diagram for a 1995 C4 fan system.



V7 are the two fans
J26 and J135 are the relays
J135 relay is for high speed as you can see that it bypass the fan controller/resistor
S42 is a 40 amp fuse
N39 is the fan controller/resistor
F16 is the temperature switch/sensor for Low speed connecting pins 1&3
F54 is the temperature switch/sensor for High speed connecting pins 2&3
Pin 3 is ground

We know that shorting pins 1&3 or 2&3 will cause the fan to come on. This indicates that the relays, fans, fan resistor, and fuse are OK. So the only thing that appears to be a problem is that either F16 is bad, or F16 is not seeing an overheat condition because of a clogged radiator. You already stated that you replaced F16 radiator temperature switch/sensor and still the fans does not come on. You also stated that you flushed the radiator, but the car still overheats. My experience is that unless you remove the radiator and take it to a radiator shop to get it "boil out" that flushing will not cure the problem. The other solution is replace the radiator with a new one. From the diagram F16 will complete the circuit for relay J26, by providing a ground.

BTW you can edit your post by using the edit button. It is located along with the Quote and Reply, Quick Reply button.

 

Would a stuck closed thermostat, cause a lack of coolant flow across the fan switch?

I am having this issue as well, just picked up the car, fans not working, even with A/C on but the system is likely out of refrigerant. They do spin when i jump the pins so i know all of that is good and i got a new fan switch to be safe.

I was wondering if a stuck closed thermostat would cause a lack of flow in the radiator and keep the hot coolant away from the fan switch.

Any help with this will be appreciated. Since it sat for 2 years I’m leaning toward doing the TB water pump and thermostat based on that fact alone.