Hello,

Just returned from vacation with my mint 88 760 t-wagon with 96K. Great hi-way car.

Before I left my a/c was not blowing out of the vents upon acceleration. I poured over the FAQ on this and even spoke to a dealer mechanic. The auto fan ran great, the auto temp ran great, and at first, the auto position selecter.

FAQ's and the dealer mech told me to pinch off the yellow wire on the near most dual wired vacuum pot on the back of the control box...the dish on the end with the blue and yellow wire. There was also a more elaborate "hacksaw" repair to replace the dishpot if desired.

I doubled over the yellow wire and pinched it off with a black spring loaded paper clip...should have easily closed vacuum. Next morning I took off for St. Louis. Pooo...did the same damn thing...as soon as I pulled Turbo boost, no air from the vents, as soon as I was out of boost air came flowing back through.

Acceleration without Turbo did not affect it. I had replaced one of the check valves and all the hoses at the vacuum T under the hood. Used a new dealer check valve on the white line...older valve on the black line. Lines looked great to the firewall.

Was running good for a long time with all 3 wheels on "auto" losing vents at turbo boost. Which of course was any noticeable power increase especially on cruise.

After stopping at a rest stop and heading out, I noticed the vents wouldn't open at all with the vent indicator wheel at "auto" and without turbo boost...they were opening fine all along on "auto" prior to boost.

I nearly panicked trying to get the obvious cold air I could hear in the dash to come out some where...manual "face" setting did this...Thank God.

I was too scared to try "auto" again on the trip and left it in manual "face" selection, of course vents closing whenever boost was applied. I had pretty good air on the trip except on the hills...it was very hot in Kansas and Missouri.

I unhooked the spring loaded paper clip from the yellow line in St. Louis, this made no difference on the way back. I am sure that double bend with the clip applied shut off vacuum, I know this is a vacuum problem somewhere...but where?

I can supply you with a map of the vacuum system on your car if you want to trouble shoot it yourself.
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john

Yes John...that might be very helpful.

Could you give me a phone number to reach you at? I would consider it an honor to talk to you about this, I have seen your posts before!

The information is on it's way to you.
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john

You will need to find the "where". No one on the brickboard can where your leak is from your description,

Both Dick Reiss (Dick often posts here as "dick") have had trouble with our cars (he has a 1991 940SE, which is a combination of a 1988+ 760T and a 940) with the sealing at the vacuuum check valves on top of the engine.

Below is a picture taken of the engine on our 1988 760T. It shows the vacuum check valves and the worm drive clamps which cured the vacuum leak(s).


Your problem could be there, or it could be inside the car. Try the small worm drive clamps, and then check the vacuum servos inside the car. There is lots of good information in the 700/900 FAQ. Check the FAQ for sure. Vacuum leaks can be difficult to find, but they can be found if you are willing to work hard instead of paying the repair shop.
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john

Yes...great shot John!

I have strong suspicions about this area. I replaced the rubber on the "T" and then I applied the plastic lock ties that you pull tight, and put them on the old 88 vintage boots going from the check valve to the vacuum line. I left the new hoses on the T alone as I thought them tight enough.

I didn't check the T for possible cracks other than visual, but I have a burning question....is the vacuum on this T and hose situation much heavier during turbo boost than the vacuum on a non-turbo car? I would think it is, but a mechanic at a foriegn car shop told me it was the same.

During boost conditions, the check valves are supposed to be doing their best to preserve the vacuum in the climate control system.

One check valve is for the air temperature sniffer in the overhead light assembly. The other is for the ACC.

You might want ot try new rubber parts where the check valves are. The old rubber gets pretty hard and will be more difficult to seal.
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john

John, Thanks a million for that pic. I have been troubleshooting my vac problem for months now (same problem with a/c). After looking at your pic, I realize I have replaced (probably) the wrong check valve (the one on the left in your pic). Maybe one day I'll eventually get it!! Thanks!!

Matt
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--- '89 740 Turbo

The ECC in your 760 has a built-in diagnostic, have you tried it? It might give you a hint of what's wrong with the vents. Details of how to use the diagnostic can be found in the 700/900 FAQ.
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Will
'90 Volvo 744 GLT (B230F) - daily driver
'73 Volvo 164E - awaiting repair & restoration

Thanks V-Jet!

I was reading about the diagnostic check but I was under the impression that it started blinking on it's own whenever it found something wrong.