cooling fan question 2003 jetta tdi

I had some overheating in my 2003 jetta TDI. mechanic thinks i need a cooling fan control module.

I have a few questions about the cooling fans. I've seen different things about them on the forums.

1. should the cooling fans run if the AC is off?

2. if the AC is turned on, should only one fan be running or both fans?

3. do the cooling fans have anything to do wth a car overheating? (AC was off when this happened)? or is it likely the water pump?

thanks.

overheat?

Take the small hose off res. tank. Have someone raise eng. speed to 1500-1800 rpm & see if any antifreeze flows(hold hose over tank opening. You may have to warm it up a while first. My guess is a water pump failure(plastic impeller). If you have no stream coming from hose, pump bad!! Radiator fans will not come on if it not circulating!! Both fans should run when ever they work.

1) The fans are called to run with the key off or AC off only if the thermoswitch in the radiator is getting too hot. They can run only in slow speed with the key off.
2)Both fans should always be off, or running at the same speed. They are never run singly or sequentially. Whenever the AC is on both fans should be running in slow, unless the ambient temperature is less than freezing, then the AC compressor and the fans could be off together. The light on the dash (AC) will still be lit.
3) Your car would not overheat this time of year if both fans and the fan control module were locked in your bedroom closet. something else is wrong, not fans, not fan control module. Not saying you don't have a bad fan, you probably do. Probably 1/4 of the A4's on the road have a bad fan or fan control module. But it cannot cause overheating this time of the year where you live.
4) The words on the water pump are right on. But let me add this. If your car is overheating and the cabin heater ever blows cold air, either you are out of coolant or your water pump is bad. If your car is overheating and hot air blows out of your cabin heater, then you have a bad thermostat.
5) If you run your heater on high speed fans, with the temperature set to full hot, it should keep your car from overheating, this time of year, even with a bad thermostat. I can pinch close my radiator hose - completely closed, no flow - and cool my car to 170 degrees at 70 on the interstate, simply by running the cabin heat at full capacity. This was with 50 degrees ambient temperature.
6) VW thermostats can and DO FAIL HOT. You will often hear that they can only fail open or cool. This is not the case. We had one two weeks ago where the car would run at 220 degrees and climbing. A new $5 thermostat, $10 worth of G-12 coolant, and problem fixed.

After you fix the cause of your overheating, you may want to test your coolant fans. In fact you should about next March or April in any case. It is pretty simple.

http://forums.tdiclub.com/showpost.php?p=2295312&postcount=21

And if you need to test your Air conditioning circuitry:

http://forums.tdiclub.com/showthread.php?p=2330429&highlight=fan+test+airconditioner#post2330429

But these will not be the cause of you running hot this time of year.

Dan

thank you

DanG144: thank you for all of the info. that was extremely helpful. car overheated when it was snowing outside. the heater also started putting out cold air. stopped and checked the coolant. overflow was nearly empty. we put coolant in it, let it sit overnight. coolant level remained the same (wasn't leaking out). started the car and drove about 10 miles. car started to overheat again. heater never came on (it was in 30s outside). coolant level was steady. should i start by replacing the thermostat or just figure it's the water pump? i don't know if the mechanic looked in the coolant overflow to see if it's circulating. he works on a ton of cars, but if that's VW specific maybe he didn't check.

W/P have a higher failure rate than the thermostat. I would check to be sure the pump works first. As this can be done as described above. Joe

jhogan said:

DanG144: thank you for all of the info. that was extremely helpful. car overheated when it was snowing outside. the heater also started putting out cold air. stopped and checked the coolant. overflow was nearly empty. we put coolant in it, let it sit overnight. coolant level remained the same (wasn't leaking out). started the car and drove about 10 miles. car started to overheat again. heater never came on (it was in 30s outside). coolant level was steady. should i start by replacing the thermostat or just figure it's the water pump? i don't know if the mechanic looked in the coolant overflow to see if it's circulating. he works on a ton of cars, but if that's VW specific maybe he didn't check.

Click to expand...

The symptoms you describe are a bad water pump.

Checking the overflow line for flow and finding none would put the nail in the water pump's coffin. It should be flowing, not spurting hard, but constantly flowing. As the coolant heats up a small amount of coolant, equal to the expansion will come out even if the pump is bad, don't let this mislead you. Constant flow of at least several ounces per minute.

Replacing the water pump is a timing belt intrusion job on these cars. It is vital to get it right, and you need special tools. Depending upon your TB's age, you might want to replace the TB, rollers, tensioners, bolts, etc. while in there.

I would find a trusted mechanic on this site to do the work. Post in your local regional forum and look in TDI 101 Trusted Mechanic thread. I would not let the mechanic who (mis) diagnosed this do the work. And the dealers would be my last choice.

Dan

thanks again

Dan: thank you again for your help. my car has 197,000 miles on it. my mechanic did the timing belt and water pump around 118,000, so i figured it was almost time again. i searched for a vw guru in my area last time, and the closest i could come up with was a 3 hour drive, which is nearly impossible for me with my schedule. and now that the water pump is dead, i don't have much choice. i've been procrastinating because my car is an automatic and the transmission is starting to have some issues. i was debating if it's time to just buy a new car. i love my VW but the hassels of finding a good mechanic almost make it too much of a headache.

You might post in the regional forum, and see if someone has a recommendation.

Dan