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

Vacuum Check Valve Failures. [Inquiry] I have a
1990 760T and the H/AC is losing vacuum during
low vacuum and during boost from the turbo.
There is a tee above the intake that has two black
& white fitting that connect to two smaller hoses
that go to the fire wall. I suspect these supply
Climate Control Check Valves
vaccum to the H/AC but does anyone know if
these are actual check valves to prevent loose of
vacuum or boost from entering the system?
[Response: RL] There are numerous sources of
potential vacuum leak (including some of the
vacuum motors (bellows) on the actual heat AC
system) but those two valves are a common
problem. They are check valves to prevent lose of vacuum and if you bench-test them
individually, you may find that they perform well under a fair amount of vacuum but that as soon
as you try minimal vacuum, the valves will give up. You may try cleaning them out in WD40 or
some rubber-softening substance but failing that, your best bet is probably an OEM
replacement. [Tip: Chris de Courcy-Bower ] If the air conditioning misbehaves and hisses when
accelerating, check the two "one-way" valves that connect the climate control system vacuum
pipes to the inlet manifold. If they let ANY air through in the "from manifold" direction when you
gently blow into them then replace them. (The good news is they are cheap.) [John Sargent]
The two black and white check valves are shown in the picture to the right. The car is a 1988
760T but most are similar. One hose is for the ACC, and the other is for the temperature sniffer
in the dome light (if so equipped). I found that the hoses leaked at the check valves and using
the small worm drive hose clamps took care of the leaks. Prior to that the panel vents would
snap shut as soon as you depressed the gas pedal
Application of Vacuum to Water Control Valve. [Inquiry] My heater won't shut off. When is the
hot water control valve in the engine compartment shut off? [Response] Vacuum to close the
water valve is only applied in max AC mode or when the MCC temperature selector is fully
"cold". This varies according to the type of control system in the car (MCC, ACC, ECC) and the
Volvo OEM service manual should be consulted for details of vacuum signals.
Heater Problems.
Heater Does Not Heat:
[Inquiry:] The heater in my 745TD still thinks it's summer... The engine itself heats up in a
normal period of time, coolant temperature is at a normal level. The coolant system builds up
pressure in the expansion tank. This morning I put a piece of cardboard in front of the radiator. I
still got cold feet, so the thermostat is OK, right? The hoses that go towards the heating
element are fairly warm. The air from the top dash vents is not very warm and even on position
4 the flow is rather weak. The middle vents are warmer and the flowrate is good. The flowrate
below is nearly neglible and the temperature is just above 0 C.
[Response: Dennis Hamblet] Several suggestions: