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I now realize what activates the flow of hot air. A door behind the glove box.
I recently swapped out the heater hose valve as it had been leaking for over a year. I removed the throttle body and bellows for access and clamped the hose and did a quick R&R.
Also the Climate control unit had been sitting on my desk all summer asking me to try to get the AC Going again. I never did it. I have repaired and replaced this unit in my 940s a number of times.
When I went to put everything back, the heat would not blow hot. That's how I came to realize the heat door is not working. I have a working example(940B) in the driveway, so I have tested three control units. All of them activate the door in in 940B.
I did a little diagnosing of the wires at the motor and found no differences between the two cars.
I find it hard to believe the motor that activates the door just stopped working. I must be missing something. Any thoughts on this?
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89 240 wagon, 94 940, 300K, 94 940, 141K
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Thanks for the thoughtful responses to my original post. Please note the title is heat door not working, rather than anything to do with the various vents.
The amount of hot air seems to be regulated by a motor attached to a rod which opens and closes a door in the airway, located behind the glove box. As I said the motor seems to have failed and the heat door is closed.
The solution I found was to dis-attach the rod from the motor and open the door. I secured the rod so the door doesn't close and that allows heat to blow and thankfully all my vents work fine
Not the most elegant solution as I will not be able to regulate the amount of heat, but that is better than no heat. I'll probably close the door when the summer returns.
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89 240 wagon, 94 940, 300K, 94 940, 141K
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Thanks for the thoughtful responses to my original post. Please note the title is heat door not working, rather than anything to do with the various vents.
The amount of hot air seems to be regulated by a motor attached to a rod which opens and closes a door in the airway, located behind the glove box. As I said the motor seems to have failed and the heat door is closed.
The solution I found was to dis-attach the rod from the motor and open the door. I secured the rod so the door doesn't close and that allows heat to blow and thankfully all my vents work fine
Not the most elegant solution as I will not be able to regulate the amount of heat, but that is better than no heat. I'll probably close the door when the summer returns.
--
89 240 wagon, 94 940, 300K, 94 940, 141K
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Welcome to the club of frustrated 700/900 owners with impaired air vents.
There are doors (flaps) inside the center console air distribution box that control air flow to the defrost, face and floor vent ducts as well as the cabin air recirculation flap (cuts off 80% of fresh air intake from the cowling vents below the windshield).
They're operated by three spring loaded vacuum servos (some call them actuators or vacuum motors) connected by colored pneumatic tubes to the heater control console (dash control knob). The servos are round plastic disk chambers in a cluster on the left side of the air distribution box. The rubber diaphragm in the middle operates a lever rod that's connected to the spring loaded door vent. The rubber gets ripe with age and eventually starts to split and leak. Initially the air flaps won't stay open without strong engine vacuum (such as when vacuum drops off during acceleration or going up a hill). Eventually they won't operate at all.
When one leaks it affects vacuum pressure in the whole system, so affects the other vents as well. All the flaps may only be partially open. An initial bypass is to identify the leaking servos, disconnect the hose and plug both the hose and the nipple on the leaking servo so that the other servos can now operate normally. First to go is usually the middle one that controls the floor vents. It's actually a compound actuator with two pneumatic lines on it.
To see what I'm talking about, refer to the exploded schematics of the various air distribution systems at the end of my heater core replacement article in the 700/900 FAQ here
https://www.brickboard.com/FAQ/700-900/HeaterCoreReplacement.htm#Diagrams
The servos are unfortunately not repairable and the parts have long been unavailable from Volvo. They’re also a pain to remove, basically having to cut them out from the side of the box. If you do a bit of research there was a similar, but not identical, GM Delphi vacuum motor that could be retrofit to the 7090/900 air distribution box. I'm not sure if it's still available. It's long been on my do-list to try finding them again as both my 940s have failed servos and they're getting worse this past year.
In the past I've seen a number of good articles on dealing with, bypassing and trying to replace these servos. For starters, there's a feature article in the 700/900 FAQ here.
https://www.brickboard.com/FAQ/700-900/HeatingVacuumServoReplacement.htm#google_vignette
Others can be found elsewhere on line.
Please post back to let us know how you make out.
If I trip over more articles in the next while, I'll try to remember to come back and post link in this thread.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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Found a little more info. The GM replacement vacuum servos I was trying to remember are AC Delco Remy parts and among other fitments were apparently used in the likes of 1990s vintage Chevy S10 pickups. It appears they're also difficult to find now, also not cheap.
The part numbers to go looking for are:
o The single port vacuum servo is Delco p/n 15-71503 (GM p/n 01996701), a fairly straightforward replacement for the Volvo servo p/n 9463044 (superseded factory p/n 3522973)
o The two port compound servo (for floor/defrost) is Delco 15-71438 (GHM p/n 1996732), a more complicated replacement for the Volvo floor/defrost servo p/n 1388206 as the mounting is a bit different as well as the orientation of the servo connector.
Now here's something interesting I wasn't aware of before. I see in another Volvo forum where the leak may sometimes be the rubber seal on the servo shaft rather than a leaking diaphragm. It's a special shaped rubber o-ring seal and may not be easy to find or fashion a replacement. You still need to remove the servo to get at it, which is a rather involved process. My heater core article in the FAQ explains how to properly open up the center console dash area to remove the front of the air distribution box so you can get at the back of the servos to unbolt the threaded mounting studs and disconnect the servo rods. It involves many hours of work. To save time and frustration, others have simply cut the servos out of the side of the air box and mended the box later with something like JB Weld.
Refer to this article on OzVolvo for more info and pics
https://ozvolvo.org/d/5921-5921
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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Dear j.pelchat,
Hope you're well and stay so. When the climate control system is opened, air enters the system. That can produce an "air lock", a bubble that blocks the flow of coolant through the heater core.
Before you start swapping parts, do the following. Park the car on an incline (a hill), with the hood pointing up-hill. Loosen the cap on the coolant overflow tank (in the engine compartment). Bring the engine to operating temperature. Allow the engine to run for 10 minutes. Then, reverse the car's direction - nose facing down-hill - and do the same. This should allow any trapped air to move to the overflow tank, from which it exits the system.
See if this restores the flow of warm air.
Hope this helps.
Yours faithfully,
Spook
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