This 1988 760t has been plagued for a few years with the "no air on boost" senario. But now, on a warm day, after driving for a little while, air stops coming out the louvers and there is no response when turning the selector that directs the air to heat or defrost. This is new. I have read the FAQs on this topic and am not sure I can pull the unit out to look for bad solder joints. I will try to look at the vacume motors. I want to know... can I manually apply vacume to the various motors if the CCU is unfixable? I few aquarium valves, some tubing, and I should be able to make a mini-manifold and connect to vacume and the motors I want to actuate. I don't know though, if I can even find the bellows/motors. My car has 271,000 miles, original tranny, headliner sagging badly, and with the way gas prices are going... I just want to keep this running a bit longer until I can find a car that gets over 30mpg. Has anyone done what I am thinking here? to manually control the vacume motors? Thanks, Glenn

my wifes 89 760 has a vac line comming off of the air manifold, tech it comes off a T vac line where as there are others connected as well, this hose pulled off one day and i have NO vac pressure at all to the vent system, i couldnt get anything out of it, after hooking this very small yellow vac line back up and pressure to the vac system returned, maybe you also have a similar setup

I wish my problem was this simple... a vacume line on the engine. But I see that I will have to dig in to the dash............

Glenn,

Where is the air coming out when it does this? The system is designed to fail-safe to defrost if a loss of vacuum occurs. The most common explanation for your symptoms is a pair of faulty one-way valves in the vacuum supply hoses to the climate control. They are black and white and are right on top of the engine (large vacuum lines run to them). If you have never replaced these, definitely try it first. I had to replace mine two or three times in the 9 years I have had my 760 ('88 turbo, as you have; best car I have ever owned). These will cause the mysterious mode changes on uphill acceleration, as well as any other times (the climate control will have a mind of it's own). You could have broken solder joints, as I had to resolder mine when I bought the car in 1996. However, I would replace these first and go from there. Let me know if you need anything else...

Joe S. - Volvo Master Technician

1988 760 Turbo w/280k miles, 1981 242 Turbo w/243k miles

Yes.. I have replaced the b/w check valves a few times since 1996 when I bought the car. Usually in boost the air stops coming out the air vents facing me. It is not the check valves. Vacume is leaking somewhere else. But recently it has changed to .... DEAD. It is like a switch is on or off. The Climate Control either is working, albeit not so good in possitve manifold pressure, or not working and the air is coming out the side vents for the door windows and a little defrost and some at the heat vents on the floor. AS the FAQ's state it, all actuator motors in a no-vacume position. The first time this "new" senerio happened was a few weeks ago and I quickly noticed it. You see... I live on a 1.5 mile dirt road and when it is dusty in the summer, I have to have the windows up and the air on 4 or 5 so no dust will creep in thru the flapper air passages in the spare tire area, or is it called a tool compartment. So with a 3000 mile trip coming up the end of September I need to get something done with this. My last trip to Seattle with the windows down loosened up the rest of the head liner that had not yet fallen. Great. The AC is out of service since I hit a deer last year. I need a new condenser and fan. So.... life is fun. Just getting the air to come out where it should at this point would be a major plus. One thing I just remembered that is peculiar, is that last year towards the end of the summer, when I had been driving for hours, and the AC had gotten everything quite cool, I noticed that on up hills that the AC kept blowing out the front vents. This was nice... but short lived upon using the AC the next day, but would repeat itself once everything was cooled down or the day got cooler. So I thought, what is causing the vacume to not get leaked? It led me to think there was something intermitant that was heat sensitive. Not knowing this system I could only guess. I am realy greatful for your help.

Glenn

On my '89 740 there's a rather large vacuum line that goes up to a resevoir in the front of the car. With the engine off I can move the vent control lever and hear a faint hissing sound as the vacuum reserve operates the heater/ac/vent valves. Does yours do the same? If not the resevoir may be cracked and leaking.

The vacume line goes thru the firewall to the back of the dash. No leaks on the engine side. But this latest non-operation of the selector makes me think it is a solder joint because the unit is totally dead instead of going up hills in boost. I think the 760 is different than the 740.

The 760 is very different - electronically controlled rather than a lever that operates a valve. There's not much to removing the controller and opening it - maybe 10 minutes, tops. Don't be afraid of that job. But it does sound suspiciously like a vacuum problem somewhere. Have you verified that you have vacuum to the unit? I'm afraid I can't help in describing which hose, but you're looking for one that runs through the firewall and has constant vacuum, and I believe it'll be directly hooked to one side of that vacuum reservoir up front. Seems like less trouble to try a bit of troubleshooting before replumbing the whole works... I wouldn't be surprised if it's just a check valve that's completely packed in - either plugged up or broken open.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina

I have been replacing check valves every few years and am used to that. This latest episode is different. It is like the control unit went south. But after being parked a while and starting it it will work for a short time. I remember the first time I noticed it. Usually you can hear a difference when a louver moves to a different position. But this unit is either on or off now. A cool morning and it works fine. Let the sun heat up the inside of the car and after just a few minutes of driving and all control is gone. So a bad solder joint sounds good. 10 minutes to get the control unit out? Are we talking about the unit that has the 3 knobs on it... fan speed... selector... and temp? Or are you talking about the small unit that has the vacume lines hooked to it? I was given a few picture schematics over on TurboBricks and have a little better picture of where things are. But AI thought I had to take the whole front of the dash apart to get to anything. When you have never tackled a job... the first time always seems bigger than it could really be. I can rebuild a motor, but dashes and trim peices brake in my hands! :) Any pointers are more than welcome.

Glenn

The most likely failure is in the solder joints of the control unit (the 3 knobs), and extracting that means carefully prying and pulling the trim that runs around the face of the unit and across the dash to the ignition key hole. Then it's a matter of removing the phillips screws securing the controller in place, sliding it out, unplugging the connectors, and removing the small screws holding the cover on. Then look over the solder joints with a magnifying glass and a light for the ones that are cracking, resolder, and give it a try.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina