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

The vacuum supply hose to the heater controller may have come loose. On my '89 it is a small hard plastic hose connected to a rather large vacuum line that goes up to a vacuum resevoir located near the radiator. Don't know if the '91's have the same arrangement. You may have lost all your coolant and thay's why the AC doesn't work. With the engine running disconnect the pressure switch on the drier and jumper the plug pins. The compressor clutch should actuate. If it doesn't work check the compressor ground wire. It's usually connected to one of the rear mounting bolts.

I can perhaps help,I retired FROM THE volvo business inthe Denver area as well as the Colorado springs area the car has a relay that controlls the power given to the a/c compressor the relay is most likely problem the dash acontrol unit is next most likely problem but control unit can be fixed by a knowlegible technician and relay can be replaced

compressor not getting power, assuming the fuse is OK, look here http://www.brickboard.com/FAQ/700-900/HeatingAirConditioning.htm#MCCClimateControlACNotWorking
Resolder the relay on the control board, it happens to most of them eventually.
As far as the heater, does the blower not come on, or does it just not get hot?
--
-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '92 Ford F350 diesel dually

The blower comes on but there is no heat.
What do I need to check?
Is the compressor involved in this?

Thanks!
Stefan

If the blower is on but no heat, you need to see if the heater valve is opening.
It is vacuum operated. look at the position the arm is in with the car off, then fire it up and turn the heater on and see if it changes, pull the vacuum line off and feel for vacuum on it with the heater control in the high and off position, if no change vacuum leak or bad control, if change , bad heater valve.
--
-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '92 Ford F350 diesel dually

The heater are air conditioning are separate. Robert is right. Check the valve that opens and closes the heater core. Open the hood and look at the valve cover that says "Volvo" go to the back of it where it drops off and look for two black hoses that go through the fire wall and to the end of the engine block. Stand on the drivers side and look in sideways. One of the two hoses has a valve on it that is part metal and black plastic or all black plastic. That valve stuck in the close position will deny you the heat. With the motor idling, have a friend turn on the heat in the car and see if the valve works. It operates by a narrow air tube that vacuums the valve to open it. When opened, water from the engine cooling system goes to a little radiator ( heater core) inside your vent system. Check to make sure that you have plenty of coolant in the system so that air can't cut it off. You should not see any green coolant on the floor of your car behind the gas pedal ( a leaking heater core) or any dripping from the valve. You ac is too long to explain but if the gas (R12 refridgerant ) has leaked out of the system over the years, a pressure switch sensing the loss will shut the compressor off. R12 gas is pricy and restricted in favor of R 134 1993 and after.

many many many thanks!

I checked the heater valve. As you suggested it definitely does not open and close when heater is turned on and off. I took the vacuum tube out of the valve and felt no vacuum.

I pressed manually the lever on the valve with the heater blower turned on but still did not get heat.
What else could be the problem?

I took out also the two hoses that go into the firewall and stuck a garden hose into the openings to flush the heater core. So it should not be congested or leaking.

If the heater valve is all broken and even manually opening it would not work, could I just bypass the valve? In such a case would there be anything else that would prevent me from getting the precious heat inside?

On a separate note where does this vacuum get lost?

Stefan

"In such a case would there be anything else that would prevent me from getting the precious heat inside? On a separate note where does this vacuum get lost?"That is a key question here because if the vacuum is lost, the little doors inside your climate control system can't work either. When you choose heat or a/c , those little doors open and shut to form an instant duct from the heat or the a/c to you or to shunt air to the windshield instead of you. I don't know much about this but the vacuum runs off the engine and if you've never replaced the the lines that deliver this vacuum to the system, then maybe one of those hoses is cracked or leaking at the clamp-on. You could remove the heater valve and direct connect and you will get full time heat in theory. But you may not be able to direct it so if it only comes out of the windshield or does not respond to the blower motor or blows only when speed sends air into the system , then the doors have defaulted to a shut position maybe. This by the way would affect a/c delivery as well. I regret that I don't know where that hose is on your car
About that valve; it has a reputation for blowing when it gets old. If this happens at highway speed, I would guess that within a hundred seconds, you'll be on the side of the road blowing steam and with a leaky cylinder head gasket unglued from the warped cylinder head. If it's old, replace it for $14 at fcpgroton or any of the reputable internet parts depots.
Test again with the engine fully warmed with the valve held open to see what happens if you haven't yet. Post back if you find that major vacuum hose