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Old 01-17-2008, 07:08 AM
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N-body cooling fan

I apologize that this isn't really a rodding question, but I don't know where else to turn and am hoping someone could give some advice.

Dad's an old-school mechanic, although he's doing pretty well at keeping up with the new technology. Nephew has a '95 Skylark (N-body) with the 2.3 Quad4 engine. Its only worth a couple $hundred, but he needs it to get back and forth to high school.

Its been overheating. Dad went over the usual stuff like the radiator, hoses, tstat, etc. Then he noticed that the fan was never coming on.

He's been troubleshooting that and has reached an impasse.

On this particular vehicle, the cooling fan relay is always hot, and is grounded to activate by the ECM. To make a long story short, he's tested every part all the way back to pulling the plug off the ECM, and grounded the appropriate pin. The relay engergizes, the fan spins fine.

If allowed to operate normally, the fan won't come on until the engine temps are around 260 and things are starting to really overheat. He has changed the temp sensor as well.

One more thing...I did some reading on N-body boards last night. It appears that turning on the A/C is supposed to turn the cooling fans on. Dad tested that last night and it has no effect.

I can only think of three possibilities.

1) The temp sensor is either still malfunctioning, or the wiring is somehow faulty, and is reading low "tricking" the ECM into thinking it isn't time to energize the fan relays. This doesn't explain why the A/C doesn't turn on the cooling fan, unless this function has a lower temp threshhold under which it doesn't apply and the ECM thinks the engine is cold

2) The ECM is bad. Would explain a lot, except that everything else works fine.

3) Something else is sending a bad signal to the ECM so that its mapping is programmed it to let the engine get that hot under the (erroneous) situation it thinks its in.

A rebuilt ECM costs about as much as the car's worth and the current one is not throwing any codes.

If Dad manually operates the fan by grounding the relay, he can keep the temps at normal levels.

Not sure what else to do. We really don't want to manually ground the fan relay so it runs all the time, but other than swapping out ECMs, we don't have any idea where the problem could lie.

Any ideas?

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Old 01-17-2008, 07:39 AM
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Your right in assuming the ECM reads the AC on signal to energize the fan relay. If it were mine and I was trying to get by as cheap as possible I would just by-pass the ECM activation of the fan and hook it up via toggle switch operation.

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Old 01-17-2008, 07:54 AM
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Atoggle switch that turns the fan on and off and is shuts off when the keyswitch is turned off.

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Old 01-17-2008, 10:45 AM
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Aftermarket fans are controlled by a temperature activated switch..perhaps one of those can be obtained and installed..

I have tried most all of it and now do what is known to work..
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Old 01-17-2008, 11:54 AM
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The ECM uses individual switching transistors to control various functions like activating that relay. It's not uncommon for one of these transistors to go bad (could be a cold solder joint, for example) and have that one function fail while everything else works fine.
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Old 01-18-2008, 09:11 AM
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Thanks for all the suggestions. I forwarded them, and a few of my own, to Dad.

One other idea I had was to try and utilize some of the PC-based scan tool software. Looks like there are a couple of free versions out there that will read all the sensor inputs.

Waiting to hear back from Dad though on the type of data connector. Apparently, both OBD1 and OBD2-type connectors were used in 94/95 in anticipation for the 96 OBD-2 changeover. From what I've gathered, GM may have used an OBD-2 connector, but with OBD-1 data interchange.

Could be fun to play with...
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