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

could have happened, most likely because the dealer who put in the new trans broke a vacuum
line or crimped one somewhere...if you had heat before the trans was put in! I think there's a
vacuum reservoir underneath the car, passenger side, up front...it's a plastic reservoir, should
have a line leading to engine and one leading to passenger compartment. Check for vacuum at
the engine line and at the line leading to the passenger compartment. Fix as necessary. You
might try putting straight engine vacuum or vacuum pump to line going to passenger
compartment (there are check valves somewhere too, it IS a turbo engine!) and see if that
actuates the heater valve/flappers. The actual vacuum switch is pretty reliable, but I've had to
replace the climate controller/computer on my 87 764T. If the vacuum terminal plugs leak, see
this tip.
Air Conditioning Failure. Note that the ACC system may also suffer from the same PCB solder
joint failures as the MCC unit above.
ACC Climate Control Instability. [Inquiry] Recently I have noticed that my climate control has
a new personality; in fact, several of them. Our early Spring days in Charlotte sometimes
require heat, and sometimes a/c. My trouble is that the hvac system delivers heat, then
coolness, then nothing, then more unpredictability. This happens when set to a temp or total
heat or total cool. What's more, the system's ability to select the right venting for defrost, heat, a/
c, etc. has also gone mad. Anyone had this problem, or recognize the symptoms? [Response:
Abe Crombie] The manual on this version ECC contains no troubleshooting charts. It only has
the list of fault codes. The fault code list contains no fault tracing either. Any fault codes that
would impact temp regulation would make the A/C button flash on start up and this was not
mentioned.
Does it not provide heat if you go to absolute last stop HOT and cold air if you go to the absolute
last stop COLD? These end points override the temp sensors. The temp knob being set to
either full end point should make system default to the respective mode. If this isn't happening
then the servo that moves temp door needs inspection. If the temp goes to the proper mode at
end points but not in any other temp setting (normal operation as opposed to end point defaults)
then the ambient sensor next to blower in blower case would bear inspection.
ACC Temperature Sensor Not Operating.
Blower Motor Ambient Temperature Sensor:
[Inquiry:] When its cold out, usually around 50 degrees, the air conditioning works OK. When its
warm out, it doesn't work at all. No fan, No a.c. I assume its one of the sensors, probably the
one on the dash. Its a 3 wire affair with a small lens in front. The other one is in the ceiling light,
but I don't think that's the one giving me a problem. Any suggestions? [Response: Abe
Crombie] The solar sensor is not going to cause anything to fail to function on that system. It
only makes the system go slightly colder when it's sunny. It sends no signal anytime it's dark
outside. The sensor next to blower motor may be at fault. Turn the air distribution knob to face
vent, the temp to full cold, fan in AUT, a/c switch off, and recirc on. Now punch the a/c switch on
and then off and count the flash code, punch a/c switch again and repeat reading two more
times. This will give any fault codes that are present in system. The sensor can be bad and give
no code as the system doesn't know it is defective as long as it gives a reading that is within -
50F to 180F. If it says it is 60 deg out when it is 96 the control unit doesn't know it is a bad