The 3 **** switch heat/ ac controller is known for getting melted connections on the circuit board. If you are good with a soldering iron, most of them are repairable. And yes there is a heater fan resistor that provides the fan speeds (other than full speed) located behind the glove box. It's #8 on the second diagram.

 

Thanks for the quick reply. Do you have any insight into the "cold air" part of my question?
The AC is properly charged, I have power at the low pressure switch, and jumping that switch does not activate the compressor.
Sometimes the AC works and sometimes not. I suspect a bad connection due to the fact that it will kick on sometimes after hitting a big bump.
The post I initially responded to mentions a violet sensor on the condensor manifold. I believe I found the condensor, if it's the thing that looks exactly like a radiator, but I don't see a sensor wired to it. If there is a high pressure switch, where would it be? Is there a relay to look for? This is a 1990 740.

 

I don't have a wiring diagram for a 92 - a 94 940 will be similar. They use the same control unit. Power to the compressor goes from the control unit on the green with red stripe wire to the low pressure switch (7/53) on the accumulator. The will prevent compressor operation if the ac is very low or empty of freon. Power then goes to the high pressure switch located on the condenser (7/41) that shuts the compressor off if the pressure is too high - if perhaps the electric radiator cooling fan (if equipped) is not working. Power then goes to the compressor 8/3. Yes the condenser is in front of the radiator - take the grill out to see it, the switches are normally down on the right lower corner.

On newer cars compressor clutch gap is a problem - if the clutch gap gets too large due to normal wear and tear the magnetic field is not strong enough to close the clutch - so you may have power to the compressor but the gap is too large to close the clutch. Usually worse the hotter it is outside.

My bet is on a poor solder connection inside the control unit, take it out, pry the tabs to get the black plastic cover off, undo the 4 screws to flip the circuit board over - and find the bad solder joints that connect the mini relay onto the circuit board that powers the compressor. Have probably done that 100 times. 95 times it fixed the problem. The other 5 there was an electronic problem with the control unit.

 

I appreciate the information. This is a '90, not a '92, but I imagine the problem you describe is a known issue with the '90 or any other near year, assuming there's a circuit board (which I'm not certain of - mine has sliding controls for direction/ac and temperature).
I dug into the control unit earlier but wasn't sure what to look for. Now I have some direction. I'll let you know what I find. Thanks again!

 

is that a 240 or a 740 ? the HVAC systems are fairly different between a 240 and a 7/940. a 1990 740 was kind of a transition year, 91 was the first year 940, and the 91 740 was pretty similar, 1992 brought in more changes, and then most of the rest of the 940-only years had only minor incremental changes.

 

This is a '90 740.

 

On the 1990 the controller has a switch that gets a bad connection at it. #7 is the switch - and connectors melt. Still the same fan resistor.

 

I'm not sure this could explain intermittent problems, but I'll have a look at it.

 

So I took the control unit apart and removed the #7 switch, took it apart to it looks fine, definitely nothing melted. There is a lot of what I presume to be dielectric grease inside that switch. I did clean up the flat connectors that fit into the black plug at the back. Still no cold air. EDIT: I thought to jump the low pressure switch. No power to the compressor. So that indicates the control unit somehow? There's power to the low pressure switch, but even jumped, there's no power to the compressor. But it looks to me like the wire goes directly from the low pressure switch to the compressor, no?

 

I would clean the innards of the switch with DeoxIt D5 or similar, make sure all the contact points are clean (I use little slivers of soft pink pencil eraser with a bit of deoxit, held in the tips of angled needllenose to clean the harder to reach contacts, certainly nothing more abrasive), then dry it out and squirt it full of dielectric grease again, and reassemble.

 

So, one of the things I just discovered is inconsistent voltage at the low pressure switch connection. Sometimes 13+V, sometimes 5V or so, sometimes, between 2-3V. I finally thought to check out the wiring at the #7 switch of the controller. There are only two wires there; one is blue and green the other is all green. The blue/green wire is hot, so I concluded whatever the controller does, it has to involve voltage moving through the green wire eventually. So I put a jumper wire across that connection, checked to make sure I had voltage at both terminals, and then went to check at the low pressure switch connection. I do NOT have voltage there (at least when I checked). But again, I have previously measure inconsistent voltage there. I'm starting to suspect a bad ground somewhere. Does that make sense?

 

the wire to that low pressure switch thats on the side of the dryer/reciever can is the power that actuates the AC clutch. so if the control unit wants the AC off, then it should be basically 0V. if the control unit wants the AC on, it should be basically 12V or battery system voltage. the pressure switch itself is closed as long as there is adequate coolant pressure, its open if there isn't, so if there's no pressure, the compressor won't engage.

 

I should have been more precise in my description. When I mentioned inconsistent voltage at the low pressure switch, I meant the wire connector you plug into the switch. The part that, once removed, can be checked for voltage coming to thew switch. It is here that I'm getting inconsistent readings.

 

are you measuring those from one pin to ground, or across the two pins? across the two pins, you'll have the compressor in series with your reading.

 

one pin to ground

 

I said one pin to ground, but I'm not sure that is accurate. I did initially test each wire that way to see which one was hot. Now that I think of it, I may have checked afterwards with the jumper wire in place. But I also did some checking with the jumper in place and other green wire disconnected at another connection near the firewall and definitely got weird numbers.

 

ok, then if the car is switched on, and the heater control is set for AC, you should see the full battery voltage on one pin to ground, the other pin should be pretty much at ground, so 0V.

 

Right. So what does it mean if I'm getting a reading of sometimes ~3V, sometimes ~5-6V there instead of 12 or more? I think what I found out is that it isn't in the controller, because I ran a jumper across the blue/green (hot) wire at the controller to the green wire, and still getting sometimes ~3V, sometimes ~ 5-6V reading at the hot wire on the low pressure connector wire measured one pin to ground. This is with the car on.

 

what voltage do you get at that hot wire behind the switch ?

hmm, I just realized, even though this thread says 1992, you have a 1990 which I don't have schematics for. :-/

 

Sorry, I jumped in on this thread and it's probably a bad idea since it appears there is a big difference between the 90 and 92.