96 Achieva fan problem, sensors good

I bought this car with a split radiator. Found the cooling fan wasn't working... So... I've checked everything I know how to check and still can't get it running.
The relay is good, all the fuses are good. I checked the sensor and it was good... I replaced it with a new one despite. I even get the correct resistance at the ECM plug. All the ECM grounds are good, body grounds are clean. The dash temp gauge works. I can ground the fan at the ECM and it will fire up, I can even get it to come on if I un plug the temp sensor, so I know the ECM can control the fan.

My only guess is that the ECM was messed up, so I bought one used, but it hasn't been reflashed for the car. I don't tried the wait 10 minute thing and that didn't work, I read somewhere else they waited 20 and it reset and worked. I can try that. Anyone run into anything like this? Any more ideas of things I can look at?

Welcome to AF.

Do the cooling fans work with the A/C on? The fans normally do not turn on with A/C off until the engine temperatue reaches around 220 degrees (low speed), 230 degrees hi-speed.

I'm not sure, the AC was discharged, previous owner said it had a leak. That was next on the list to fix :grinyes:

I did get the new ECM to work after letting it sit. Same problem.
This time around I couldn't get the car to heat up much past the second mark on the gauge, so no where near the red. We did plug in the old sensor and put a torch to it, but during the process I think the connection broke and the SES light came on. I might try that again with some longer wire.
On the old ECM, we did get the thing to bury the temp gauge and it never came on.

I know there is a pressure switch in the AC line, if I jumper that can I bypass the pressure sensor or does it need a resistance.

Thanks

Yes there is a A/C lo/hi pressure switch but it need a specific resistance value to jumper. The resistance must be within the low and high threshold settings that the ECM//PCM will recognize.

What interesting is the fans will come on with the engine temp sensor (ECT) disconnected. That means the ECM sees little or no resistance at the input and therefore turns the fans on. Have you checked the ECT sensor with a digital multimeter to see if it is calibrated properly? When engine is cool the resistance will be high. As engine warms up the resistance is lower towards zero ohms.

For example if the engine temperature is around 212 degrees the resistance across the sensor contacts should show around 177 ohms.

Yeah, I've got the resistance chart. I've taken readings from the ECM plugs with the multimeter and get good readings (in line with the chart) and I see the resistance go down as the water warms. So the ECM *should* be getting the signal. I only see one temp sensor in the wiring diagram, I initially thought the dash was being driven off 1 and the ECM another, as it is with some other GM cars...
Its weird to me that when the car goes into limp mode that the fan kicks right on. That tells me that the ECM *can* definately control it, though.

Some brief history about the car... I got it on the cheap, the previous owner split the radiator from overheating it. They thought they also blew the head gasket. The radiator was easy, and it ended up not being a head gasket, just the o-ring in the outlet hose fitting... but its got some other problems like the A/C leak, and the TS/ABS is out on it. I'm decent on the mechanical end but I'm still coming up on electrical.

I'm going to try the new ECM again tomorrow (I'm storing the car somewhere else). I'll plug the old sensor in and hit it with a butane torch... if that doesn't work I suppose I can just wire the fan to come on when the car is running. I really don't know what else to look at.

one thought... if its looking for low resistance, if I bridge the 2 pins in the temp sensor plug (zero resistance) that should simulate the car overheating, shouldn't it? or does the ECM know if it sees zero there is a problem?