My Cooling Fans Will Not Turn On

I own a 91 Grand Prix SE 3.1 L (Vin T) and heres my problem. When i got my car it had about 140,000 miles on it. It had some new parts on it but i decided to do a whole engine tune up replacing all gaskets, No problem. I put all new parts with the help of my uncle who used to be a GM Mechanic 9 years ago. just about everything you can think of was replaced cleaned and sealed. The Problem is that before we started working on the engine the cooling fans would turn on at about 220 F, no problem. But since we replaced everything the cooling fans won't turn on when the temp even gets up to 250 F.

We even tried to see if we could get them to run by turning on the A/C, but that didn't work. If i ground out the A.L.D.L. connector when the car is in the run position with the engine off, both fans kick on. My Temp sensor is brand new. Can somebody help me determine what the problem is? Could it be a faulty temp sensor wire? Your Help would be appreciated.

Hello;

if you ground the diagnostic connector and the fans come on, you have verified the fans, relays and wiring to the ECM. You may have a problem on the "input" side of the ECM, as in the sensor or sensor wiring. A scan tool would be useful for diagnosis, but if you don't have one I would suggest carefully inspecting all the engine harness wiring, especially the coolant temperature sensor connector and harness, paying close attention to anything that could have been damaged during your repair work.

Thanks raffiican (http://www.automotiveforums.com/vbulletin/member.php?u=451803) i will try that and see how it works out. I also tried putting a paper clip in the harness that connects to the Temp sensor and grounding it out, cuz my other uncle who is also a gm mechanic said if you do that it will trick the computer into thinking its 300 degrees and the fans should have come on but they didn't. So maybe you might have some more tricks up your sleeve. Thanks for your input its very appreciated.

I agree with raffiican, it sounds like a wire is broken or corroded between the sensor and the ECM. MAke sure you don't use any thread sealant on the threads of that sensor (unless it's an electrically conductive type). That sensor needs to be grounded to work. Your engine has two temp sensors. One in the rear head (1 green wire w/ 8" long lead) that controls the temp guage on the dash. The other is in the intake near the t-stat housing (2 wire connector) and is the ECM and cooling fan switch. I mention this to be sure you're testing the correct one.

I just tried to use a test light on the on the temp sensor harness with the harness detached form the Temp sensor with the key in the run position and i didnt get the test light to come on, i tried the connector many times, So can any one help me to determine this problem. Thanks for all your help guys

Just to clarify, read my previous post. Are you testing the proper sensor?

like i said both my uncles are GM mechanics and i'm a 100 percent sure im testing the Right sensor. Thanks for your input. Keep um coming if you can.

That sensor has a resistor that changes with tempreature, and the ECM sends a reference voltage to it, then reads the return voltage drop to determine engine temp. If you have no voltage at either wire of the CTS connector with the key on, then either the ECM or the wiring between it and the connector is bad. I don't have my wiring diagram here at work, but I believe the green wire is the feed from the ECM and the green w/black stripe is the return to the ECM.

I just got my uncle over here with a scan tool and i found out that all fingers point to the ECM. The Temp sensor is telling the ECM that its too hot and need the ECM to turn on the Fans, But the ECM isn't telling the fans to turn on. So once again the ECM is bad so i will get a new one.Thank you guys for all your help. I hope we can help each other in the near future.