1993 Camaro Z28.
Neither fan comes on as the engine heats up.

The dash guage is definitely accurate. I road tested by trying to baby it back, causing some coolant to volcano out of the reservoir. Lots of ignition killing and coasting. Stupid, but effective.

Fans both come on if I jump the two pins on the DLC. One fan comes on if I turn on the A/C.

I've replaced the coolant temperature sensor under the water pump. I've reset the PCM by pulling the fuse for a few minutes. Just to be safe, I checked relays and fuses. I know the diag blinking/fan turning on trick cleared all that, but I wanted to be positive.

I guess that only leaves the two wires coming out of the sensor to troubleshoot? I've ordered a serial data cable and the TTS Datamaster software for reading the PCM, so that'll tell me what temperature it thinks it's seeing. I wanted to buy that crap for a while and this was an excuse. Lame, but still an excuse.

Have I missed anything? I've never done electronics diagnostics, so I hope I can stumble through Shoebox's writeup.

 

Did you test the fan relay?

 

If you're taling about the two relays under the hood, yes. I swapped them with the fog light relay. The fog lights came on with the fan relays in its place. Is it correct that all the relays, fuses, and wiring between PCM and fans is good if the diagnostic mode kicks the fans on? And that would only leave the temperature sensor, wiring from there to PCM, and slightly possibly the PCM itself?

 

Not so much just the relays themselves, but the trigger voltage.
Is the PCM getting a signal to the relays to engage the fans?

Can you apply voltage to the trigger circuit for the relay and cause it to engage the fans?
That's how I tracked down an issue with my EWP, the trigger circuit had an open.

 

i have the same exact problem, im interested in this topic to see how to fix it, right now i have a toggle switch.

 

Now here's an interesting plot twist. I jacked into the OBD port and saw that the computer is getting the correct temperature. I got up to 105 degrees Celsius/221 degrees Fahrenheit and neither fan had kicked on. The battery in the notebook died, so I didn't go further, but it does show that the temperature reading is correct. What else could it be? Is the trigger circuit a valid pursuit considering diagnostic mode turns the fans on?

 

What temperature do the fans come on with stock programming? If the AC is turning one fan on, then that fan is fine. Troubleshoot the other one. Did you try switching the relays and then testing with the AC?

 

226° for the first fan and 235° for the second one.

 

Jumping pins on the diagnostic port causes both fans to come on and turning the A/C on causes one fan to come on. This is normal behavior.

With the A/C off, one fan should come on at a certain temperature and the second should come on at a hotter temperature. Neither fan comes on automatically from the heat.

 

The "trigger" to the relays from the computer is a ground. Better not be putting any voltage on there.

 

LOL! Thanks Shoebox. Any ideas? I'm baffled and at the point where the only possible culprit is the computer itself unless I'm missing something.

 

Find C100 that has the blue and green fan relay control wires in it. Unplug it. Using the connector going toward the front of the car, jumper a ground to either of those two wires with the key ON ( don't touch any other wire lead with the ground). The relay should kick in and the respective fan should run. That will prove your wiring good from that point, through the relays and to the fans.