Ok, I found alot of threads about fan issues and got some usefull info to troubleshoot. But I figured Id run this up the flag pole anyway. Heres the scenario:
My car has been making a low growl sound when running the AC. So when I get home I decied to investigate. Turn on the car and run the AC, and look under hood. AC compressor seems fine. No growl, give it gas, compressor kicks off, let off gas compressor starts again, all normal so far. I decide to shut engine off. Then I hear it...the bubbling...the gurgling..the overflow filling. Car is overheating. Look at temp gauge and its in the red. Woah! Let the car cool down, check for voltage at fan with key in run. No voltage. I then check fuse...thats good. Hmm...car has cooled down. Start car (with AAC on) fan works :/ ok....turn off ac, fan still turning. hmm. turn off car. Start car...no fan....:/ turn on AC...no fan....wait till temp gets to red...no fan...shut off engine. Fan no work anymore. gets dark....go check 3rdgen boards.. Ok, so Im thinking temp switch....or relay. Temp sensor (or sending unit) obviously works as gauge is working. Its a '90 rs with a 3.1 Does temp switch seem like a good start? (once I get the fan working again, Ill tend to that growl) also car has never overheated the time Ive had it.(which is only about 3 wks) Sorry for the long post

 

I have the same problem as you. No voltage at the fan, when I hook it to power direct, it turns, so the motor is good.

I was just out there with the multimeter and I get power at the relay too... So... The sensor is what I'm thinking.

The car has TWO temp sensors on it. So just because your gauge works does NOT mean that's not what it is. They run off seperate sensors. I guess the one on the drivers side head is for the gauge and the one on the passenger side head is for the fan (or so I read somewhere else on this forum).

I'm going to change the one on the passenger side head in the next couple days and I'll let you know if it fixed it for me. If you do the same and it does/doesn't work, let me know.

 

The one on the passenger head is for a secondary fan. The driver side head is the guage. There is a sensor near the thermostat on the manifold that is the main fan switch, it is what would be wrong out of any of those switches. this is the one u can replace with a cooler one, or take Willie's advice and read his articles about the fans and hell explain a better way.

 

Thanks for the feedback, I did happen to check the Wille thread but didnt see anything related. Anyway, I noticed today that with the ac on if I push the fan to get it started it will spin on its own and shut off and back on. So I may have a bad fan motor. I also have not noticed a temp sensor on the Driverside head, so theres three of those bastards? I noticed one on the manifold, one on the pass. head but damn, how many sensors do they need? Anyways, Im gonna try replacing one or two of the sensors then the motor as it seems to be a combonation of the two. But Id like to sart out with the least expensive route first.

 

Ok, thought Id just give a quick update incase anyone else has the same issue. I happened to replace the Relay, hard to find that one, there are three mounted on the firewall. Having done that I verified that I finaly have 14V at the fan. I didnt have that before. However the fan will not start on its own. I had to push it then it would spin on its own until the temp came down and it would shut off automaticly, so I know the whole relay, sensor circuit is working. I guess Im looking at replacing the fan motor now. Im not sure which would have gone bad first, the relay or the fan motor, but one seems to have caused the other perhaps. Ill post an update afterwards.
Also I think its important to note that the sensor on the passenger side is located right above the starter. I would have swore that it did not exist. And you will have to go from underneath the car. Also the connector is very very brittle. Im sure it gets plenty hot located where it is. What the hell was GM thinking?

 

I dont mean to double post but I wanted to conclude this thread incase anyone else has the same senario. I ended up replacing the relay which indeed was an issue but I also had to replace the fan motor as it was very intermittant as to when it would come on, even though there was proper voltage at the connector. Sometimes I could get it to start if I spun it by hand. Later that didnt even work. Since replacing the relay and the motor Im running cool at all times

 

Was it the relay on the firewall ?

 

it sounds to me that some models had the fan relay on the firewall, but on my 86 iroc LB9 the fan relay is very close to the fan, mounted next to the battery which is on the pass. side. if yours is here it will be clear to tell that it runs to the fan.

i dont know why it would be on the firewall since that is far from the fan. i always thought the whole reason for a relay is so that the high current 30 Amp going to the fan only had to run a short distance, letting the lower current 20 Amp wires be run the longer distance to the computer. im no big electrical guy tho, so i may be wrong but it makes sence to me

 

Yes, mine is on the firewall near the Vacume Booster. I have three of them mounted there. One is for the fan, one for the fuel pump and Im not sure what the other may be for. In the pic below the fan relay is the shiny new grey one. Nevermind the big red circle and the wire, that was for another post. There is another relay to the right in the pic.