Volvo 760GLE 1988 Heater intermittently goes to defrost

DeRail · Post by **DeRail** » 20 Feb 2009, 13:58

Hey all, I've got a 1988 760GLE with the automatic climate control system

the other day I was driving and I noticed that the heat was no longer blowing at me it was on defroster mode. I tried to cycle through the vent control controls, no dice. Usually I can hear the vacuum when I change vents, but I noticed this was not happening. I quickly checked the lines running through the firewall into the vacuum line and they seemed fine. Does this sound like a vacuum leak? If I let the car sit for about 20 minutes and fire it up again, it works and cycles through fine... anything I should look at first?

EDIT: I also noticed that I can hear a hissing from the vents when I accelerate, sometimes.

Thanks!
Darrell

Kmaniac in California USA · Post by **Kmaniac in California USA** » 20 Feb 2009, 21:32

I had a similar problem with my 740 when i bought it. Only mine was stuck in defroster mode no matter what vent choice was selected and I could not get any heat no matter what temperature setting I selected.

It turns out that I had a vacuum leak. The heater controls tap off of a large vacuum line that has one end connected to the intake manifold and the other end connected to a plastic vacuum reservoir, mounted under the engine between the wheels. It turns out my vacuum reservoir tank had a large crack on one end. Once I replaced the tank and the large hoses (which were old), my heater controls worked again.

Also, there is a check valve on the large vacuum line located between the intake manifold tap and the vacuum control tap. If this check valves stop functioning, you will lose vent control vacuum during heavy acceleration and the vents will change to defrost on their own until the vacuum builds back up again.

Note: With no vacuum available, the vents default to defroster mode.

Let us know what you find and what you do.

DeRail · Post by **DeRail** » 20 Feb 2009, 21:43

I'm guessing this is the same for the V-6 engine?

When I looked under the hood there was a T connector with a line running to the intake, and the top ports were connected to the clear tubes running from the firewall. One tube was white and the other was a yellow if I remember correctly (I'll double check tomorrow). But, the check valve I'm guessing is between where those two small (maybe 1/16"?) tubes go into the vacuum T and the T fitting itself?

Thanks for the help

Kmaniac in California USA · Post by **Kmaniac in California USA** » 20 Feb 2009, 22:42

The check valve is on the large vacuum line, between where it connects to the intake manifold and the T to the heater controls (about where my thumb is). The other end of the large vacuum line connects to the tank.

You should see two small tubes that pass through the fire wall. One connects to the T in the large vacuum line. The other connects to the heater control valve.

Large vacuum line with T connector for the heater controls

Vacuum reservoir under the car (It may be under the splash shield)

Heater control valve

I hope these pictures help you understand what I have been talking about.

Let us know what you find and what you do.

DeRail · Post by **DeRail** » 21 Feb 2009, 12:34

All right, thanks for the tips everyone. Got a chance to check the vacuum T fitting under the hood today.

On my engine it looks like this:
sorry for the paint diagram, I don't have access to a camera... on the left and right of the T are small translucent tubes running thru the firewall. They connect to the T thru a couple of check valves on each side. I tested these valves out by blowing through them, they were working fine. I didn't see any check valve between the intake and the bottom of the fitting, it just connected straight through.

DeRail · Post by **DeRail** » 22 Feb 2009, 22:08

Hi guys been thinking this problem over a little bit...

Is blowing through the valves an adequate test of their ability to keep vacuum flowing one way? Also, are there any other circumstances where the vents will default to defrost? Reason I ask is that I can always remember a slight hiss upon acceleration, but I've never had this problem. Also, when it is working, I'm able to cycle through all the vents and I can hear the vacuum kicking in to move the door. So is it possible that the actual control panel is faulty? And if so, can these be repaired or is it easier to pull one from the boneyard?

EDIT: Was on the highway last night and everything and it was blowing through the vents a'ok!

Thanks!

DeRail · Post by **DeRail** » 06 Aug 2009, 00:57

Well, it was scorching out the other day and I turned the fan on just to get a little circulation going with the windows down and lo and behold we were back to defrost again

Seen as how if I let the car off for awhile and then turn it back on it works, I was almost thinking a controller problem. The Swedishbricks volvo faq mentions something about an electric vacuum valve behind the glovebox that can suffer from a cold solder joint. I was wondering if anyone knew about anything like this. Also, when I turn my car off, and the vents go back into position, there's a faint whine as they return to the closed position, could this be part of the problem?

Thanks

Post by **billofdurham** » 06 Aug 2009, 15:40

I haven't come across the problem of the cold solder joint but, like you, had read of it on Swedishbricks.

There is usually a whine when the vents close on switch off. If you leave the engine running and close the vents with the switch you will hear them if you listen carefully.

Bill.