I recently began to hear a loud vacuum hiss in the dash area upon engine start and when selecting the vent position with AC on. Its not the usual momentary hiss heard when changing air source positions but a very loud constant hiss. Also when under boost the air comes out of the defrosters and floor regardless of control setting. It appears to be coming from a double acting servo mounted on the left side of the plenum. Anyone have any insight on this? Thanks very much!

David

Thanks for all the help. Today I checked the infamous ATE check valve and with slight vacuum it leaked. But the double acting servo seems to be weak. I must have missed the FAQ concerning this problem.

David M

I have the same problem, but only under load. After reading the FAQ's I was tempted to go under the dash and tear everything apart. However, I talked to the guy at the volvo dealership and he recommended a "less then perfect" solution. This valve that is going bad is used to control the recirculating air function of the volvo. If you simply plug that one vaccum line, the recirculating function will not work, but the rest of the system will work fine. He asked me how many times a year I actually use the recirculate button, and if it was worth 4-6 hours to tear up my dash.

He made a good point, so consider this as another option to actually "fixing" the problem.

I'm no volvo expert but it does seem the "double acting servo" is the culprit.
not sure if regular climate control and "automatic" are same but mine did same
thing ('91 945 SE).was driving on interstate at 75mph with full load, was
passing under full boost and heard loud pop and then hissing under dash. I did
no diagnosis, just bought double acting servo and put it in. car now works better than ever, completely fixed. seems like a recurring post theme, probably
since A/C is so improtant this time of year. If your moderately skilled and
have basic tools the "shortcut" procedure in FAQ is very helpful, took me
about 1 1/2 hours.

Check the FAQ. Mine wasn't hissing but had the same symptoms as yours under boost. I plugged the yellow hose and changed out the one way valve under the hood and all is well now.

--
Bob W.
15 Volvos ('58-'91)
445-544-122-240-740