Volvo Maintenance Hints for 7xx/9xx - Bill Garland's Nuclear ...

hose = floor) and has a boot on the floor side that retains vac on that side of diaphragm. The
boot fails and you lose vac. Blocking yellow hose fixes it simply with compromised floor air
volume (floor would be floor/defrost). All of this aforementioned stuff is usually the case if
symptom is loss on acceleration.
740 or 760 Tips. The 760 has an electric vacuum valve set to left behind glove box and a cold
soldered joint on it at one of the end pins will cause no response in any position which defaults
to floor/defrost. The 740 has a rotary vacuum switch linked to slide lever for vent selection and
the hoses behind panel (white is source) can be crushed or the feed hose (white) above
accelerator pedal at firewall can be crushed. The 760T with ECC has a floor/defrost servo
sprung to return to center when no vac is applied. This will direct air reaching this part of the air
distribution housing to the defrost and to the floor outlets. The yellow hose applies vacuum to
the side of diaphragm that will extend the servo rod to push the door up blocking most of the
defrost vent air and directing 90% +/- to floor. The seal that rides the servo rod can dislodge or
just split and you have the result you observed: air stops coming out of any vents during turbo
boost (manifold pressure instead of vacuum), indicating a leaky push-pull vacuum servo.
Disconnecting the yellow hose will not allow you to get full floor air. When the floor air position is
selected (by you or by the ECC logic) the air will be split between floor and defrost. This is likely
not a problem unless you have really poor leg circulation that gives you a severe tendency to
cold feet long after the rest of your body is warm. The logic in the ECC always has the floor
position selected on this servo when it is directing air to the dash face vents so if it leaks you
lose the vac for entire system and all the doors go their respective sprung positions.
960/90 With ECC Tips. [Tip from Steve Long for 960 ECC] If you have done the tests above and
you are still having a problem while in recirculation mode, try to pinpoint the cause to a specific
vacuum motor. With the recirculation on and the controls set to max fan speed and dash vents,
accelerate until the air no longer flows from the vents and then put your hand down under the
dash to see if air is flowing from the floor vent. If this is the case, then the recirculation vacuum
motor is likely leaking. [Tip] In a 960, you can pop off the panels on either side and put a clamp
on the recirculating bellows (orange hose)and drive the car and see if the problem is gone or put
a clamp on the floor side of the bellows (Yellow hose IIRC), or both (as in our case), and see if it
goes away. If it does, the respective solenoid is at fault. The vacuum reservoir located behind
the glovebox may also have a leak (common problem on this model). Unfortunately, in their
infinite wisdom, Volvo opted to make the canister part of the evaporator case (VERY
expensive). I replaced mine with a separate and independent canister. Any good A/C shop can
do this. You can also just permanently cap off the offending vacuum line (probably orange or
yellow), although you will lose some functionality. Remove the glovebox to locate the vacuum
junction box with the lines installed.
Replacing the Vacuum Motors. Tom Irwin and Dick Riess developed a shortcut method
requiring cutting the driver's side inner kick panel. Another technique is listed from Beka at
Brickboard. Both are in the FAQ reference document.
Punting on the Fix. [Tip from GregV] 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. The servo valve that is going bad is used to control