Vacuum Pump on 1985 Volvo 760 Turbo

I was given a 1985 760 Turbo with a problem I am trying to diagnose.
The climate control is stuck on heat. From previous post I see it could
be many things such as the sensors or a vacuum leak, but before I go
down that road I have a suspicion it is the vacuum pump. It stays on
continually and is incredibly loud. Can anyone tell me if the failure of
this pump would keep the auto climate control stuck on heat? Thanks.

Robert

 

If it's loud then there's something wrong, try a new one from a junkyard,
should be pretty cheap. You should be able to test it for function easily by
pulling the hose off and use your finger to feel for suction.

 

The heater system "relaxes" to full heat with the defrost vents open if
there's no vacuum present. If there's a big vacuum leak, then you'll
full heat, and lots of air out the defroster vents. This will also
cause the vacuum pump to run all the time. The noise means it's
probably going to die soon. However, as these wear out, they often run
all the time because they can't pull a deep enough vacuum. Then, for
your full heat problem, you need to look at the blend door (that
bypasses air around the heater core) and the heater valve itself. The
system applies vacuum to the heater valve when the system determines
heat is not necessary, often these valves don't close properly as they
age. However, this is not the biggest problem, the blend door should
divert all the air around the heater core when you have it set to full
cold. There is also an expensive component called the ACC programmer
that often fails with unpredictable results.

--
Mike F.
Thornhill (near Toronto), Ont.

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