87 760 Turbo.. stuck in defrost mode?
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shadowrides
- Posts: 7
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87 760 Turbo.. stuck in defrost mode?
Was wondering if anyone can help me out with a problem I'm having... A/c and everything works.. fans work.. behind the glove box there's a "vacuum" bag, not sure what it's called.. It's always sucked in causing hot air to come out and keeping the two main vents above a/c controls closed... If I take the hose that goes to that bag off it quickly lets off and opens the vents and I get cold air from vents like it should be.. Not sure if it could be a succion leak cause there's suction at this hose.. it just doesn't let the air escape when i turn the controls to a/c.. this hose goes to some kind of valve.. maybe diaphram valve.. small black round..and has 2 hose fittings on top and 2 on bottom..maybe this is clogged or has stopped working properly? there's another hose that leaves this valve and goes to some kind of filter that's pretty dirty.. tried taking filter off but no luck.. Any ideas? any help would be appreciated... Thanks
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Kmaniac in California USA
- Posts: 301
- Joined: 15 January 2005
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- Location: Concord, California USA
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I'm not sure how much help I can be. If you haven't seem my post of 10/1/2004 regarding disfunctional A/C vents, I will repeat it here for clarification.
October 1 2005
I don't need help with this one because I figured it out myself. However, others may learn from my experience.
Shortly after I purchased my car, I discovered that the A/C vent controls did not function. The fan switch worked well, but I would only get air flow through the Defroster vents no matter what vent position I selected. Also, air would be slightly warm no matter the position of temperature control. I also discovered the A/C belt removed from the compressor and the A/C clutch wiring purposely disconnected.
The previous owner had the car serviced at an independent Volvo garage in the "People's Republic of Berkeley", California. Two years ago, the shop discovered that the controls did not work and quoted $250 to repair, though they didn't state why they would not function. On the same repair bill, they charged a fee to remove the A/C belt and to replace the temperature control valve. Then, ten months ago, they replaced the temperature control valve again, for whatever reason.
I quickly discovered that these controls required engine vacuum to operate. I found two vacuum tubes that passed through the firewall. One went to the heater control valve. The other to a tee in a large diameter vacuum line. One end of the large line attached to the intake manifold. The other end went somewhere under the left side of the car. I looked under the car and found the line attached to a long, cylindrically shaped, plastic tank, mounted forward of the front crossmember.
I disconnected the main vacuum line from the intake manifold and plugged the tap. Then I disconnected the vent control vacuum line from the tee, attached a hand-held vacuum pump, and pumped down a vacuum in the vent controls. I quickly found that the vent controls operated normally as long as I kept a vacuum on the controls. This told me that I had a vacuum leak somewhere on the main vacuum line.
I went back under the car and unbolted this plastic tank from the undercarriage. It is held on by three bolts. Once I got it out from under the car, I easily saw the large crack that traveled half way around the far end of the tank. No wonder the vent controls did not work, there was no vacuum in the line. Not only that, the car had a major intake vacuum leak that went undiagnosed for over two years, and the independent mechanic that had serviced the car all that time, saw the clues, but was too blind to figure this out!!
This tank is positioned in such a manner under the car, that it is the first thing to be hit by large debris on the roadway. It appeared that the crack was cause by blunt force trauma from road debris.
The purchased a new vacuum tank from my local Volvo dealer for $47. For good measure, I replaced the vacuum line between the intake manifold and the vacuum tank. Once installed, the vent controls worked like new. I then replaced the A/C compressor belt, reconnected the clutch wiring, and now the A/C blows 40 F air in the coldest position. I don't think I would recommend that independent Volvo garage to anyone I know.
So, if your vent controls stop working, be sure to check for vacuum and the condition of this tank before trying to take apart your dashboard controls.
Now that this is out there, I do know that the vent controls default to defroster mode with a loss of engine vacuum to the system. Be sure to verify the integrity of the vacuum hoses and reservoir tank before taking apart your dashboard controls. If your vacuum source is good and your controls bad, then I won't be much help.
October 1 2005
I don't need help with this one because I figured it out myself. However, others may learn from my experience.
Shortly after I purchased my car, I discovered that the A/C vent controls did not function. The fan switch worked well, but I would only get air flow through the Defroster vents no matter what vent position I selected. Also, air would be slightly warm no matter the position of temperature control. I also discovered the A/C belt removed from the compressor and the A/C clutch wiring purposely disconnected.
The previous owner had the car serviced at an independent Volvo garage in the "People's Republic of Berkeley", California. Two years ago, the shop discovered that the controls did not work and quoted $250 to repair, though they didn't state why they would not function. On the same repair bill, they charged a fee to remove the A/C belt and to replace the temperature control valve. Then, ten months ago, they replaced the temperature control valve again, for whatever reason.
I quickly discovered that these controls required engine vacuum to operate. I found two vacuum tubes that passed through the firewall. One went to the heater control valve. The other to a tee in a large diameter vacuum line. One end of the large line attached to the intake manifold. The other end went somewhere under the left side of the car. I looked under the car and found the line attached to a long, cylindrically shaped, plastic tank, mounted forward of the front crossmember.
I disconnected the main vacuum line from the intake manifold and plugged the tap. Then I disconnected the vent control vacuum line from the tee, attached a hand-held vacuum pump, and pumped down a vacuum in the vent controls. I quickly found that the vent controls operated normally as long as I kept a vacuum on the controls. This told me that I had a vacuum leak somewhere on the main vacuum line.
I went back under the car and unbolted this plastic tank from the undercarriage. It is held on by three bolts. Once I got it out from under the car, I easily saw the large crack that traveled half way around the far end of the tank. No wonder the vent controls did not work, there was no vacuum in the line. Not only that, the car had a major intake vacuum leak that went undiagnosed for over two years, and the independent mechanic that had serviced the car all that time, saw the clues, but was too blind to figure this out!!
This tank is positioned in such a manner under the car, that it is the first thing to be hit by large debris on the roadway. It appeared that the crack was cause by blunt force trauma from road debris.
The purchased a new vacuum tank from my local Volvo dealer for $47. For good measure, I replaced the vacuum line between the intake manifold and the vacuum tank. Once installed, the vent controls worked like new. I then replaced the A/C compressor belt, reconnected the clutch wiring, and now the A/C blows 40 F air in the coldest position. I don't think I would recommend that independent Volvo garage to anyone I know.
So, if your vent controls stop working, be sure to check for vacuum and the condition of this tank before trying to take apart your dashboard controls.
Now that this is out there, I do know that the vent controls default to defroster mode with a loss of engine vacuum to the system. Be sure to verify the integrity of the vacuum hoses and reservoir tank before taking apart your dashboard controls. If your vacuum source is good and your controls bad, then I won't be much help.
Chris the "K MANIAC"
1986 740 GLE
(5) 1964 Chrysler 300-K's
1986 740 GLE
(5) 1964 Chrysler 300-K's
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shadowrides
- Posts: 7
- Joined: 7 August 2005
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can you tell me more about this vacuum tank.. Can't find it on my 87 760 turbo and none of the vacuum hoses off manifold or T's under dash seem to go to one... On my volvo I have vacuum at that hose that controls defrost mode.. vacuum pulls the valve to defrost mode.. if i take out the hose going to that valve releasing the vacuum it goes back to a/c / fan mode... Where's this temperature control valve located and what does it look like?
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Kmaniac in California USA
- Posts: 301
- Joined: 15 January 2005
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- Location: Concord, California USA
- Been thanked: 1 time
The vacuum tank on my car is mounted to the front side of the crossmember, under the engine and between the two front tires. Poke your head under the front license plate, looking strraight back, and you should see it right there, centered between the tires. At least that is where it is on my 1986 740 GLE 2.3 four non-turbo.
All the vacuum connections I talked about were in the engine compartment of my Volvo. The tee connection for the vent controls on my car is located right behind the alternator.
The temperature control valve on my car is located in the engine compartment, between the back of the engine and the firewall. It is attached to the heater core coolant line and has a vacuum diapham on one end.
The vacuum tap off the intake for the vent controls on my car is 3/8 inch nominial diameter. Look for a similar sized vacuum tap on your car and follow it to find your vent control lines. These lines are small diameter from the tee on your main vacuum line to where they pass through the firewall. I have taken digital pictures of these components in my car, but do not know how to post them to this site. I could send them to you separately.
I would also suggest you buy a service manual for your car. I have the Haynes manual for 1982 through 1988 740 & 760, #97040(1550). This one covers the basics but is not as comprehensive as a factory manual. A picture of the vacuum tank can be seen on page 88, Chapter 2, picture 17.2.
Anyway, good luck with the troubleshooting.
All the vacuum connections I talked about were in the engine compartment of my Volvo. The tee connection for the vent controls on my car is located right behind the alternator.
The temperature control valve on my car is located in the engine compartment, between the back of the engine and the firewall. It is attached to the heater core coolant line and has a vacuum diapham on one end.
The vacuum tap off the intake for the vent controls on my car is 3/8 inch nominial diameter. Look for a similar sized vacuum tap on your car and follow it to find your vent control lines. These lines are small diameter from the tee on your main vacuum line to where they pass through the firewall. I have taken digital pictures of these components in my car, but do not know how to post them to this site. I could send them to you separately.
I would also suggest you buy a service manual for your car. I have the Haynes manual for 1982 through 1988 740 & 760, #97040(1550). This one covers the basics but is not as comprehensive as a factory manual. A picture of the vacuum tank can be seen on page 88, Chapter 2, picture 17.2.
Anyway, good luck with the troubleshooting.
Chris the "K MANIAC"
1986 740 GLE
(5) 1964 Chrysler 300-K's
1986 740 GLE
(5) 1964 Chrysler 300-K's
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shadowrides
- Posts: 7
- Joined: 7 August 2005
- Year and Model:
- Location:
thanks.. would appreciate it if you could send me those pictures you took with your camera to [email protected]
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