Skip the auto shop - Our mechanics make house calls

Q: Overheating

asked by on

Hi i have a 2007 trailblazer which is overheating on run and with ac Changed head gasket,water pump, thermostat, cleaned radiator .the only thing left behind is changing fan clutch. Before changing fan i want to make sure its faulty Things which i noticed is the fan gets engaged when i bypass fan relay fuse no: 45 under hood with a 10 amp fuse by 2 and 4 pins ..after this there is no more overheating and the car runs good with a high roaring sound because of fan But when i change the bypass from pin 2 and 4 and connecting the no 45 fuse back car overheats and no more fan is engaged. I checked the fuse 45 by swapping ,fuse is working Im now doubting of a electrical issue Please guide me in this How to check continuity from pcm to fan relay

My car has 100000 miles.
My car has an automatic transmission.

A: For the sake of clarity, it is important to...

For the sake of clarity, it is important to distinguish when the car is overheating. If it is overheating when the car is moving, then the fan clutch isn’t your only problem. If the car only overheats at a stop, then you are probably on the right track.

As for the electrical circuit testing, I wouldn’t jump to testing continuity at the PCM just yet. I would first confirm the correct operation of the ECT (engine coolant temperature) sensor. I would also suggest checking all of the fuses. You never know when other fuses supply power to another. My wiring diagram shows the cooling fan relay fuse to be number 20.

If all the fuses are good, then I would check the signals coming from the PCM. To most efficient way to check the ECT is with a scan tool. Look at the scan tool data to see if the temperature the ECT shows looks correct. In other words, if the car is cold, hasn’t been run yet, it should show a temperature close to the ambient air temperature. If it is warmed up with the thermostat open, it should be above 180 degree Fahrenheit. Many scan tools have the ability to turn on the cooling fan system. This is the single most effective method for diagnosing this system.

If you find you still need to check the signals coming from the PCM, I recommend getting the repair manual for your car and study the wiring diagram for the cooling fan. I am not going to attempt to describe which wire is which in this system. That is troublesome to do over the computer. In this process, I would again suspect the ECT and check the resistance in that circuit at the PCM connector with the connector disconnected. PCM’s are rare to fail, so I would only suspect it after everything else has been ruled out.

It wouldn’t hurt to replace the cooling fan relay as well. Sometimes you can swap the relay for another one under the hood and see what happens.

If you you need help with this, consider contacting a certified mechanic who can diagnose your overheating issue and fix it accordingly.

Good luck!

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Transmission won't go

Hello. If the transmission was leaking then the fluid needs to be checked first. If it got low enough then this can occur. If it is low then I would add some fluid and see if it helps. If not...

Q: Central locking is no longer working.

It sounds like your vehicle has a problem with the body control module, mainly because you said the power locks are also inoperative from the door panel. The first thing I would suggest is to check the fuses, then have...

Q: Engine light has come on, fans are coming on when vehicle has only been started for a few minuites and remain on

With the previous work that was done recently, it is very likely the problem is that coolant leak. If its not the leak repair that has failed, it may be an additional leak. Have you checked the level of the...

Related articles

How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
What are the Car Pool Rules in Hawaii?
Hawaii is widely regarded as a land of vacation and relaxation, and as such, its scenic roads and routes are far better known than the state’s freeways. But, as with all...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.