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Randall C
Randall C, ASE Master Tech
Category: Chevy
Satisfied Customers: 12803
Experience:  ASE Master, L1, 32 years, Diagnostic / Electrical Specialist
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1998 Chevy Lumina: It appears that fan control is not working

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I have an over temp problem with my 1998 Chevy Lumina. Replaced ECT. It appears that fan control is not working. I put a voltmeter on the fan input supply and see 1 sec or less 4 volt cycles when the engine coolant warms up. This appears to be a fan controller problem. What part do I need to replace? I wanted to verify with a wiring diagram, but am not able to locate one. Any suggestions?
Hello, My name isXXXXX will do my best to assist you with your question

A couple questions:

Does the car run hot at hwy speeds?

Do fans come on with AC on?

Did you replace thermostat?

Are both raditior hoses hot when car starts to overheat?
Customer: replied 5 years ago.
Car runs hot after about 15-20 minutes in idle and at highway speeds (indicator on dashboard). The thermostat was replaced. Did not check fan operation with AC on, but am reasonably confident that AC is not issue. Problem has been persistent for about two weeks and during my troubleshooting, I did see radiator fans operate previously at very high temp, but operation appears inconsistent. Both radiator hoses were hot. I also bled air from radiator system.
Sorry, I was not stating AC problem but actually the way to confirm the fan circuit is ok is to run AC, If fans come on with AC, then the fan circuit is ok.

The fans are controlled by 3 fuses, 3 relays, 2 fans and the PCM (computer)

If fans had worked then not, then worked, i doubt fan motor problem, nor relays problem.Doubt 3 relays failed all at same time. You did ECT sensor.

This leads me to one of a few things. Either the water circulation is not good and the water at the base of the ECT just is not hot enough to turn fans on ( even if hoses are hot) OR you could have a failed PCM. What concerns me is running hot at hwy speeds. This would be the same with air through radiator just like fans on as fans usually do not run at hwy speeds.

I am not there nor running tests but leads me to more of a coolant circultion problem rather then lack of fans. You can also put a large home fan in front of car to see if still overheats OR you can hose down radiator and if it still runs hot, you could have partially blocked radiator, a headgasket beginning to go bad, defective T stat, etc. just giving ideas and info on what to try. here is the wiring diagram you requested


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