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2000 Mazda 626 2.0L cooling fans operating irregularly

Customer Question
causing occasional overheat. Jumping each...
2000 Mazda 626 2.0L cooling fans operating irregularly causing occasional overheat. Jumping each relay makes the corresponding fan run. The car can go days of normal operation with no problems but on occasion while idling for more than a few minutes the cooling fan fails to engage and the temp climbs. I just need to get the car moving to bring the temp down but if left unattended it will overheat and spit coolant out of the bottle. The a/c (fully charged) sometimes gets warm when the car is not moving and this corresponds to when the fans are not running. I'm not sure where to go next since I am unsure of exactly when and how the fans are intended to operate.
Submitted: 2 years ago.Category: Mazda
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Answered in 10 minutes by:
8/5/2015
Mazda Mechanic: Mike V., Technician replied 2 years ago
Mike V.
Mike V., Technician
Category: Mazda
Satisfied Customers: 25,889
Experience: 25 yrs. experience with all makes and models. Factory trained on Nissan and Subaru.
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Hi there,

It's the job of the PCM to send the ground signal to the fan relay's and turn them on when the car gets up to temp and they have to come on. It should also turn one on when you turn on the a/c and the compressor engages.

Since the issue is not consistent, I would first try disconnecting the PCM and re connecting, it's quite possible there is a bad connection at that point or a loose wire that is sometimes not making contact. Since the fans do work at times it seems the computer is turning them on when it's suppose to.

If this doesn't help, then it's likely that you will have to trace the wiring harness from the computer to the fan relays.

Please let me know if there is any more I can do to help you from here.

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Customer reply replied 2 years ago
Thanks for the prompt response. I will try your suggestion about the PCM harness. It sounds as though the PCM is a switched ground to the relays. Do you know if the fans are also switched ground or are they switched hot? Also from info found on the web, disconnecting the coolant temp sensor should engage both fans while engine is running, can you confirm? I am getting only the cooling fan on and not the condenser fan when CTS unplugged.
Mazda Mechanic: Mike V., Technician replied 2 years ago

The relays ground via the pcm and that sends power to the cooling fans.

I would be lying if I told you that both fans should come on, to be honest I don't know, but the condenser fan should cycle with the a/c unless the engine is very hot, then both fans would kick on. So, it seems likely they should both default to on when the coolant temp sensor is unplugged.

Also, Check the box itself for a bad connection. With the age it's possible there is some corrosion inside the box where the harness goes in. Maybe you have a loose wire in there causing some grief.

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Customer reply replied 2 years ago
I've got time to try your suggestions before work so I'll check back later with the results. I have only a basic wiring diagram but enough to know which PCM pins to ground to turn on the fans. I also plan on putting a test light across the low current relay pins to see if the relay is being signaled to energize at the proper times. But since it is intermittent and hard to duplicate it is going to take some patience.
Mazda Mechanic: Mike V., Technician replied 2 years ago

I agree, it's going to take some patience to locate the problem, but also seems to point to a poor contact. It could be a poor contact inside the computer, but honestly I would be thinking wiring first. Since you see the temp increase I would assume the ECT is working properly as well.

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Experience: 25 yrs. experience with all makes and models. Factory trained on Nissan and Subaru.

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