In my 1987 740 stationwagon, after I turn the fan on, pretty much all air comes up through the windshield defrost vents.
A little leaks out elsewhere, but no matter what position I switch the control to, it will only come out toward the windshield.

I assume it has something to do with the vacuum system, but I have no experience with that. Any suggestions?

 

If it matters, a couple of years ago the A/C died and instead of fixing it, we decided to just leave it and remove the belt going to the compressor.

 

Not related to lack of A/C, but yes, flow through the defrost only, means that the system defaulted to that position due to vacuum failure or a controller failure. Check the vacuum lines in the engine bay, the small one way valve form the intake manifold first... It may be a simple fix or it could be complicated if the leak is somewhere under the dash. Hissing sounds, air escaping, is what you are looking for.

 

Alright, I will check those things tomorrow.

 

Okay, this afternoon I checked around by the engine while it was idling. I found the vacuum tubing, and felt around but didn't feel anything specific. I didn't hear any hissing over the sound of the engine + radiator fan, however when I was going to listen under the dash, I noticed that it sounded like a hissing underneath the vehicle. If I understand correctly, there is supposed to be a vacuum reservoir somewhere, but I don't know if that is under the vehicle.

Would that vacuum reservoir be somewhere kind of under the driver's seat area? If so, that is probably where the leak is.

 

No, it should be under the front area, close to the radiator--a black plastic tank.

 

I'm not familiar with the layout of the 740, for the 240 the vacuum reservoir is in the center console. I did a quick google search and found this post that seems to indicate that at least for the 960, the tank is under the car:

http://www.matthewsvolvosite.com/for...33347&p=267699

 

I found one important looking tube that ran down through the T in the line, then got lost sort of into the front bumper, and I couldn't see where it went.

@lev I did find a black plastic tank by the radiator (see picture), but it didn't seem to be attached to the vacuum system, unless I am just bad at tracing tubes. I didn't see anything visually wrong with that tank, and didn't hear any hissing when the vehicle was running.

@act I couldn't find the reservoir pictured in that post, though I didn't actually crawl completely underneath the vehicle.

I did find something that attached to one of the vacuum tubes that looked important, but I don't know what it is (the other picture). Is that related to finding the leak?

 

First is the heater valve, i guess you have no heater?
Second pic is the charcoal canister, not related to vacuum either.
The thing we want is under the front bumper, it's pictured on the mathew's site act provided.

 

Maybe I just didn't look close enough under the bumper. I thought that was the right tube...tomorrow when it is dry (it was rainy today, so I didn't look to carefully) I will find the correct thing.

My heating works fine...when I turn the heat up and fan on, it gets nice and toasty in the car--just all through defrost. (Actually, I just talked to my dad, and he said a number of years ago there had been some sort of leak, and on someone's advice he just replaced that valve with a straight pipe.)

 

FIXED!

I found the vacuum reservoir, and when I was wiggling all the plastic around the area trying to get it loose, I realised a rubber cap had come off an unused hose port. The cap had a major crack in it. Replacing the cap sealed the reservoir, and it works wonderfully now.

Thanks guys