Hi All,

I need help on diagnosing the problem with the heater on my '91 740GL.
Both air conditioning and heater stopped working. I can live without the first but it is getting colder in colorado.

The people from the auto shop said that the compressor is not getting power.
It could be a problem with electrical wires b/n the stitch and the compressor or the control unit, they said.

I have worked somewhat on the car and love to fix things but have no idea at all how the air conditioning system works.

Any. Any help on how to track down the problem would be greatly appreciated!

Thanks
Stefan

I don't believe that a problem with the compressor would affect the heat side of the system. You may be having a problem within the control panel.

Are all the fuses OK?

First the AC stopped working - it used to blow fuses all the time while I was on a long drive. I was hell of a ride because if the fuse goes out apparently the power windows stop working too :) They installed a circuit breaker but now the compressor is actually not getting power ant more.
Fixing the heater now is most important to me. It stopped working sometime during the last few months - I do not know why.
I am not clear - if the compressor is not working would the heater not work as well? It is blowing, but no heat.
What should I check?
Is the control board under the dash involved in a case when the fan (or whatever blows the air from the heat core) is operating?

Thanks
Stefan

Ah-ha, the additional details say don't tear into the control panel quite yet....

First the A/C:
A common problem is that the windings inside the compressor clutch start shorting out with old age, so you start blowing fuses when you turn on the A/C. Don't forget that when you turn on the defroster, it also triggers the A/C compressor on, so - blown fuse again. I don't know about the whole circuit breaker thing, but it sounds like kind of a bandaid fix. You really need to see if the A/C clutch is drawing too many amps. If so, you may be able to pick up another clutch somewhere or replace the entire compressor (more $).

Now the heat:
Blowing cold air can be caused by a) low coolant level; b) a bad engine thermosadt, which usually shows up as the temp guage not climbing to its normal mid-point position after running the engine for a few minutes - if guage stays down near "cold" replace the thermostadt; c) a bad heater valve - it's located inline in one of the heater hose near the back end of the engine by the firewall and is operated by a small vacuum hose from the control panel.

There could be some other causes for the "no heat" but this is a good starting point. Hope this helps by providing a little food for thought.

Many thanks!

Regarding the low coolant air scenario - does that mean that coolant is circulating when heater is turned on? I such a case should I fix the compressor before looking at the problem with the heating?

Stefan

When I say "coolant" I mean ENGINE coolant (radiator water) - not A/C refrigerant. Check the water level in the radiator overflow bottle and make sure it is about 1/3 full with a cold engine. It it gets too low, then of course you might overheat the engine, but if it's just "sort of" low, there may be enough water to keep the engine from overheating, but not enough to supply the heater core with warm water.

Given the time of year, I think I'd attack the heat issue first. What does the temp guage read when the engine is running?

If you follow the power wire from the compressor, you should find a disconnect plug. You could disconnect it, that way you could run the defroster without blowing the A/C & window fuse. Then tackle the compressor problem when you get time. (Yes, you were right by the way - isn't it just dandy the way they put the compressor and the windows on the same fuse?)

thank you! You are helping me to to gradually turn my nose towards the problem :)

The coolant level in b/n min and max.
The temperature gauge gets to the middle after engine in warmed up.

Next thing you mentioned to check is the heater valve.
How do I check that?

Stefan

It looks like Robert Ludwig and I'm No Mechanic gave you some pretty good advice in your other thread. The advice to go ahead and replace the valve is probably not a bad idea. Even if it's not the reason for your no-heat, that valve is probably old and risky. (Another typical problem with those valves is just the opposite of your problem - they don't want to CLOSE completely so in the summer it seriously degrades the A/C performance.

One thing though about that valve - As I seem to recall (and I may be all wet here), but for some reason I'm thinking the valve works the opposite as you would think, meaning the vacuum pulls it SHUT, and limiting the vacuum allows it to OPEN. I can't say that with absolute certainty, but if you had it removed from the car you could suck on the nipple and verify how it works.