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radiator fan diagnostics - step by step

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I have noticed that my 2001 996 tip might have a fan problem. When I start it from cold with the a/c on only the right side fan comes on - not the left side and it seems to be at high speed. Since the weather has not been over @50 degrees I do not notice either of the low fans running at any time. The temp is very steady with the needle between the 8 and 0 both in-town and on the fwy.

How can I do a step by step diagnosis to see if the ballast resistor is bad, a relay is bad or ? If I turn off the a/c the right fan turns off - but other than that not sure if all is ok. Would like to take care of this before it gets hot again and it causes a greater problem.

Checked the forums with search and have not been able to locate a trail to follow instead of just throwing parts at it.


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With the AC on and the engine running - both high speed fans should be on.

There are two fuses (40A each) - C8 and C10. There are 4 relays - 1 for each side; 1 low speed and 1 high speed.

A ballast resistor produces low speed fan operation - one per side.

The easy way to test these is with the Durametric software, a Porsche PST2, or PIWIS tester.

If you don't have a tester you can check the fuses, then swap relays from working to non working sides or speeds. This should allow you to pinpoint the problem.

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...How can I do a step by step diagnosis to see if the ballast resistor is bad, a relay is bad or ?...

You can swap the relays and ballasts between sides. If the fans start working again, then you have isolated the relay or ballast causing the issue.

If swapping relays and ballasts does not solve the problem, then it is probably the fan motor or wiring, but those don't seem to be as common as a ballast or relay going out.

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Thanks everyone for your ideas. I first tested the fuses - ok. Then I tested the relays - both low clicked when I had a fingertip on them and turned on the A/C. Then jacked up the car to test the fans - both turn freely. I then started the car with the A/C on and the right fan turned on and the resistor got hot. Left side did not turn on and resistor was cold - looks like I found the problem. I will order the resistor harness and replace just the resistor as it seems easier than trying to install the whole harness.

Should I also test the high relays with the fingertip test - should they click as well if A/C is turned on - or they will only turn on when water temp/A/C pressure is high?

Thanks again!

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For the fingertip test:

Turning on the A/C to cause the low speed relays to click on and off will only happen when the coolant temperature is below 206 degrees Farenheit.

If the coolant temperature is above 206 degrees, but below 215, the low speed fans will turn on (and the low speed relay will click) without pressing the A/C button on.

Regards, Maurice.

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Now that I have the new harness can someone post the steps to replace the whole harness instead of just the resistor? Do I need to remove the wheel and inner fender liner and can do it from there or is it move involved? I cannot find this from the online manuals posted. Does it make sense just to replace the resistor only - or changeout the whole harness?

Thanks for the boards help on this.


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ballast replaced - harness would have required removing the radiator support and other hardware - so i just installed the ballast resistor. It now works correctly. Of course it is now snowing in Nashville, TN so I will have to wait for the weather to heat up to test it in hot weather but a nice garage project.

Of course while I was working on it there is always another project to think about...

Thanks for all your assistance.

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