Wow…
For months now, my defroster vents on my ’89 Silver 244 (320k miles) were not working. It’s been frustrating and dangerous since I’ve had times where the inside moisture was obstructing my front view. So, I read a bunch of the Brickboard discussions of the cause and potential solutions. They all seemed to involve a lot of work to remove various things like instrument panel, glove compartment, side panels. In any event, one thing that stood out was the possibility that the vacuum was not getting to the push-button control unit.

So, today I opened the hood and saw that the thin rubber pipe leading to the vacuum nozzle on top of the engine was completely split. I was then able to re-connect it using some odds & end rubber pieces. Lo and behold the defroster was working perfectly. See photo of repaired line below…


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-Cool Volvo- 1989 240 5 Spd w/ 310K, 1989 240 5 Spd w/ 231k. Past proud owner of 1966 122, 1968 144, 1970 145, 1972 144, 1980 245, 1982 240 4 Spd OD

When this happens the dead giveaway is no defrost but nonstop floor heating regardless of how they are switched.

And, as in your case, the problem is usually under the hood and easily remedied.

Dash vents are always active in any case.


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'80 DL 2 door, '89 DL Wagon, '15 XC70 T6

In my case, the floor ducts were also not opening.
The vacuum line seems to control all of them.
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-Cool Volvo- 1989 240 5 Spd w/ 310K, 1989 240 5 Spd w/ 231k. Past proud owner of 1966 122, 1968 144, 1970 145, 1972 144, 1980 245, 1982 240 4 Spd OD

Yes. The vacuum lines control all of them. I am not disputing that.

But, given no other problems, when the vacuum line under the hood is disconnected, the defroster vents fail closed and the front floor vents fail open.

In normal operation when the engine is running and the floor vents button is not depressed vacuum is present at the floor vent actuators keeping the flaps closed. When the floor vents button is pressed, vacuum is cut off and the flaps open.

Control of the defrost vents is the opposite. When the engine is running and the defrost vents button is not depressed no vacuum is present at the defroster vent actuators and they are closed. When the button is pressed, vacuum is introduced and the flaps open.

So when no vacuum is present because the supply is disconnected under the hood, the front floor vents are open, the defrost vents are closed and the buttons at the control unit have no effect one way or the other.


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'80 DL 2 door, '89 DL Wagon, '15 XC70 T6

That's a great explanation.
Thanks Don.
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-Cool Volvo- 1989 240 5 Spd w/ 310K, 1989 240 5 Spd w/ 231k. Past proud owner of 1966 122, 1968 144, 1970 145, 1972 144, 1980 245, 1982 240 4 Spd OD