Yesterday I started to check into why my fans weren't working. Its a 91Z 305 TPI with the dual fans.

First: I jumpered the big wires in the relay plug. Fans worked.

Second: I plugged the relay in, stripped away some wire casing on the green/white wire and used a jumper to ground it. Fans Worked

Third: I started it made sure I was up to temp. No Fans

Fourth: While running I tried various A/C and defrost settings. No Fans

Fifth: I jumpered A and B on the ALDL port. Fans WORKED.

So after all that I know the wiring is good and the relay is good. I thought when the a/c or defrost are on it doesn't use the temp sensor so that would rule that out. Am I correct?

Does this narrow it down to the ECM or could it still be the temp sensor. Or what else could it be?

Thanks

 

Dual fans work independently of each other. One fan will kick in when the motor is to temp and the other is based off of a/c line pressure. So you may have two separate issues. There will be inlines fuses for the relays as well as fuel and ecm. They should be on the inside fender of the passanger side where the battery is. Check those.

On the passanger side frame rail, you will see the liquid line that goes from the evap to condensor. On the line you will see a pressure sensor with a blue/white or grey and a black wire. with the car running or in the On position, if you jump those wires I believe the passenger side fan will kick in. If it does its the pressure sensor.

The driver side is controlled by the ECM when it reaches temp. If you have the ability to put a scanner on the car, you will see if the temp sensor is working or not....

 

The primary fan is controls the fan relays ground side when the coolant temperature (cts) reaches 226F the ecm will ground the coil in fan relay.
The secondary fan is turned on then the coolant temp sensor if equipted is grounded or the a/c pressure switch is closed.

 

ECM itself must be fine because it would not run the fans when the ALDL is jumpered and it uses all the components you have tested

Looking more like a input problem to the ECM so it is not getting the trigger to operate the fans?

 

Its likely the sensor.

The other fan will come on when the AC is turned on AND BUILDS PRESSURE. In the case of my car, it never came on. This fan also turns on when temps get HOT, IMO too late to do any good, you've already noticed it was way too hot and have shut the car off waiting for it to cool down.

 

I changed the sensor today and still no fan. This is getting frustrating. Any other ideas?

 

Actually, this doesn't rule out the ECM completely (possibly dependent on the ecm PN). I had a problem with my fans on my EBL P4 ('730 ecm). The fans worked when the ALDL was jumpered, as they should, but wouldn't come on at any other time. When datalogging, the EBL's "fan on" flag was set, but the fans wouldn't come on. Let me see if I can find the PM's between me and RBob to explain what was going on.


To the OP, if you have the ability to datalog, check and see what the ECM is seeing for engine temps.

 

By RBob from the EBL thread:

 

Is it a custom tune? Is there ANY chance at all that the fan turn on temps were raised in the ecm?

 

I remember reading that the stock ecm was programed to kick the fan on at 238 degrees or something high like that. Are you checking the fan sensor on the passenger side head under the manifold?

 

A lot of attention has been put into keeping the car original. Even when new shocks were put on they were Ac Delco's so I doubt the tune has been messed with. I am checking the correct sensor. I replaced it today with an AC Delco one.

I just ordered another ECM off eBay to hopefully fix it.

Off topic question. My Speedo doesnt work either. Its not the VSS. I thought it was and replaced it and got back my odometer, but not speedometer. Could a bad ECM kill the speedo too or should I just concede the fact I need a cluster too.

 

On 90-92 TPI cars the speedo signal comes from the ECM