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Hey Guys,

2003 civic EX 1.7L automatic

This will be my first post here! I am trying to help my friend with a really weird cooling fan issue. The cooling fans were not coming on and while sitting in traffic the car would overheat. Temperature is normal while moving. My first thought was either relay or fan switch. I pulled the connector from the fan switch sensor and shorted it while the key was in the on position and this caused both fans to come on correctly, so I thought replacing the switch would fix the issue. Proceeded to go to an autozone and get a new switch, however while sitting parked and the temperature rising well above the middle, the fans still did not kick on. However, we found that with the cabin fan switch (blower) turned on to the lowest setting (or anything above off) WITHOUT the A/C on, the fan started working normally. (Turned on while heat was high, and cut off when heat went back down) When turning the A/C on, the fans continue to turn as they would while using A/C and A/C works fine. Also tested shorting the fan switch connection again, which turns the cooling fans on like before.

So, what is going on with only getting the automatic thermostatic fan behavior when the cabin fans are on? Because shorting the switch still works, it is possible that we got a bad replacement part from the auto parts store? Obviously he can just drive the car with the fan on the lowest setting for the time being, but it is really bugging me what the actual problem is.

Thanks for reading!

 
Rep Power: 517

My first thought would be checking that the radiator is completely full. These can have big problems with less than a quart of liquid missing from the radiator. (HEAD GASKET BLOWN???)


The temp switch on the thermostat housing controls the radiator fans independently of the AC system and PCM. You jumped it, fans ran, the only questionable part then is the temp switch operation. (AND coolant level, and circulation through the radiator, and heat transfer efficiency of the radiator, fin blockage, etc.)

I kinda think the heater controls may have been set on a defrost position, and AC turned on without the light. Or maybe it was just a coincidence.

 
Rep Power: 0

Hi ezone,

Thanks for the reply. I didn't check the radiator when beginning the repair however it was fully filled after the fan switch replacement was done. Also, when draining all of the old coolant, I looked for any oil from a possible headgasket problem (assuming that the previous overheat condition might have caused some damage) and there was not a trace. (I know how it looks from other cars I have worked on )

The defrost possibility is the only thing that makes and logical sense to me. However, if the AC was set to on, would the fan still cut in and out automatically? In other cars I have worked on it usually just stays on full time. In any case I will check if the defrost was on as soon as possible. I guess we should try another fan switch if that is the most likely culprit? Any brands you would recommend or should we just try another random auto parts stock?

Thanks for reading.

 
Rep Power: 517

And burped completely??


If you only look for the stuff everyone else expects, you'll never see when it blows a head gasket.

These engines push liquid out of the radiator-into the reservoir. If the reservoir was already full it may overflow. Then it overheats after some liquid goes missing from the radiator.
If you're in the cold, the heater output becomes cooler or nonexistent as liquid is lost.

A blue fluid block checker can't find most of these. There won't be steam in the exhaust, no fluids mixing, no chocolate milkshake in the oil. Not unless it's been ignored long enough to be a total disaster.

Search the forum for more. Head gaskets are soooo common-- but soooo hard for most people to figure out. .

Radiator fans cycle with the compressor. They do not stay on full time unless someone hacked it up.

You can use boiling water and ohm meter to test it.

Bad thermostat? Wrong stat?