Problem with '91 Grand Prix

[Cheeser]

Hi guys -

OK, this one has me stumped, maybe one of you guys can help.

I have a 91 Pontiac Grand Prix LE, 3.1 V6 AT, PS, PB. I bought it this winter, and after checking it over I noticed that the twin electric fans weren't coming on. Well, it was winter and I don't think they come on until 210 degrees (or something like that) so I didn't worry too much about that, but I thought that they should kick on when I turn the A/C on max, correct? When I turned it to Max A/C, the compressor didn't engage, so I figured the system had a leak and I believe there is a pressure switch on the low side that won't engage the compressor unless there is 5-10 PSI in the system.

Anyways, I didn't worry too much about it, until now. On a whim, (my wife was complaining about it ) I threw my gauge on it today, and low and behold the system is good on R-12. However, the A/C compressor won't engauge, and now I suspect the fans aren't coming on either. I was stuck in traffic on 494 the other day and I noticed that the temp climbed all the way to 225 and I didn't hear the fans. I checked all of the fuses in the fuse panel, as well as the ones on both the left and right service panels. I also replaced the relays. I have power at the relay plug, and when I bypass the relay it will turn the fans on and it will also engage the A/C compressor. What am I missing? Help!

Mark

[Jeremy airjer W]

It is not uncommon for GM's to wait to turn the fan on until the temp reaches 220-230 degrees. It is important to use a scan tool to see what temps the are actually being read by the car.

What relay did you bypass?

[Cheeser]

There are three relays; two for the fans, and one for the A/C compressor. I checked all three, one at a time of course.

If your wondering about the two sensor problem that commonly occurs on the 3.1 V6, I have replaced the sensor below the thermostat with a 2-wire sensor and compared the gauge reading to the scan tool - they are now within 1 or 2 degrees of one another, so I'm pretty sure the gauge is fairly accurate. I know what your saying about the fan coming on at high temps; the last corvette I worked on didn't come on until 232. Wow.

[Jeremy airjer W]

I'm sure 4 will chime in sooner, but I would have to look at the diagram to give you some more pointers. If you bypassed the a/c relay than I would be concerned that the message is not getting to the relay. I'm not sure specifically how it is controlled but I might be suspicious of the control head. I won't be able to take a look until Monday but like I said when 4 sees this he will likely chime in with some specifics for you.

[Scott K]

Sorry I have been out all day, and I am on my laptop now, so I will check in the am.

[Scott K]

I would first start by checking to make sure you have power on 2 pins of each relay. Then jump a ground to the primary sides of the relays and see if the stuff works. In other words you want to do the ecm's job of grounding the relays. If it works then, you are down to a ecm input, a wire or a bad ecm.

Here are the diagrams.