dadgum computer cars...

[SRath]

My mom has a '98 Chevy Cavalier with an irritating computer/electrical problem. Her temp gauge quit working and so her ecm was posting a fault code. I replaced the sender and the gauge began working fine. Right up until the electric fan kicked on.



Then the gauge again went dead. As soon as the electric fan finally stops, the gauge works again. Every time the gauge goes dead (every time the fan runs) the fault code reappears. So the little computer brain that tells the fan relay to turn the fan on is also killing the temp gauge somehow when it does.



But there is an exception: If I turn the electric radiator cooling fan on by switching on the defrost or air conditioning, the fan turns on without killing the temp gauge. The fan only kills the gauge when it kicks on due to engine temperature, not a/c or defrost.



Any ideas?

 

[fest3er]

Bad ground? Poor power connection? Is the fan turning full speed?

It's possible that when the fan is powered up, a bad ground is causing a voltage rise that makes the gauge think it's ice cold. Or there's a short that's cross-feeding voltage to the sensor. Or when the fan is on, it's drawing too much past a bad connection, dropping the supply voltage and causing a loss of power to the sensor.

Try unplugging the fan when it's running without A/C and see if the gauge comes back. That may yield a decent clue.

 

[SRath]

I tried pulling the fan fuse and the fan relay thinking along the same lines, Neal. I don't have the car to try anything else right now since she is driving it. It comes down to whatever tells the fan to turn on based on engine temp alone (not the a/c or defrost), is also killing the power to the gauge, which doesn't just read cold then, it clicks off completely dead.

To me, that almost has to be the ecm.

 

[mwilson]

Did the parts store perhaps give you the wrong sender or one that was boxed wrong??

Almost sounds like it is working backwards, ie normally open instead of normally closed for example???

Especially where it is only on the block temp circuit and has not affected the A/C high pressure circuit.

Just a thought, that would freak things out for sure.



Mike.

 

[SRath]

It would, Mike. But the sender does seem to tell the fan to kick on at the appropriate time and to turn off when no longer needed, as well. Once it kicks off, the temp gauge swings back into action until the fan is cycled once again. So I think the sender is wroking fine and that the ecm is reading the signal correctly. It is the extracurricular signal the ecm is sending to "kill the temp gauge" that seems to be the only problem.

Temporarily having no temp gauge reading in itself isn't a huge problem. But during that "kill the temp gauge" cycle, the ecm is apparently also reading the engine as "cold", and that might adversely affect how the engine runs. Much like having your choke stuck on on a non-compuker vehicle. She says it hesitates and stumbles sometimes.

This gets confusing because if the ecm knows when to tell the fan to turn off, it MUST be reading the correct temp despite the gauge being dead.

 

[mwilson]

Wow, I didn't catch that the fan was indeed coming on, sorry.



Now it sounds more like a ground, low voltage or a downstream relay perhaps. Like the isn't enough ground or voltage to support both the fan and gauge together.



May well turn out to be the ECM.



That is a first class electrical gremlin you are pursuing... ... ...



Mike.