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Q: Overheating

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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.

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!

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