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

diagnosed with an A/C gauge set. Poor cooling can also be caused by a lot of things other than
a bad compressor, so don’t replace the compressor until you’ve ruled out other possibilities such
as a low refrigerant charge, too much oil in the system, air contamination, a clogged condenser,
plugged orifice tube, inoperative electric cooling fan, etc.
Compressor Clutch Failure.
General Notes. [by Larry Carley, Import Car Magazine, May 03] If the compressor isn’t turning,
make sure the magnetic clutch engages when energized. Underlying problems here may
include a bad relay, fuse, wiring problem or a defective clutch. If the clutch fails to cycle on and
off when the A/C is turned on, jumping the clutch lead with a jumper wire from the battery will
show if the problem is in the clutch or elsewhere. If the clutch engages, the problem is the clutch
power supply (relay, fuse, wiring, switch or control module). Refer to a wiring diagram and work
backward toward the battery to find out why the voltage isn’t getting through. Many A/C systems
have a low-pressure cutout switch that prevents the compressor clutch from engaging if system
pressure (the refrigerant charge) is too low. This is designed to protect the compressor from
damage in the event of a leak. So if the clutch isn’t engaging, check the refrigerant charge and
the cutout switch.
Diagnostic Notes. [Diagnostics] Where can I hook up a jumper wire to see if the AC compressor
clutch is still good? [Herb Goltz] In 740 cars, remove the connector at the low pressure switch
on the accumulator/dryer (the aluminum cylinder at the firewall). Jumper across the two
terminals inside the connector (a paperclip works fine). With the AC set to on and the engine
running, the compressor should engage IF the terminal inside the connector is receiving +12V
power via the 10-second delay relay behind the glove box. In 940 cars, remove the same lowpressure
switch connector at the receiver-dryer and jumper it: current comes directly from
terminal 10 of the MCC control unit with no relay.[Larry Spooner] If there is a circuit problem
after the above test and you want to test the compressor directly, unplug the connector in the
black wire going into the clutch assy. If you have an ohm meter there should be a low resistance
3-5 ohms measured at the compressor side of the connector to the compressor body or engine
block. You should be able to jumper this directly to the pos battery terminal or the B+ terminal
on the back of the alternator and see the clutch activate. The engine does not have to be
running. Because of the low resistance of the clutch coil there's going to be a little bit of a spark.
[Symptom:] My '86 760T is losing its compressor clutch. What does it take to change this
clutch? [Diagnosis:] First is it really the clutch?1) If the compressor is starting to seize internally
that will cause the clutch to slip and burn but the fix is a new compressor. 2) Is it the pulley
bearing making noise? That requires a special tool to hold the front plate while you unscrew the
big nut on the input shaft. The clutch and pulley then press off. You need to remove the freon
since you're working at the front seal. 3) If the clutch is really worn out get the special tool and
press it off. However compressor rebuilts are not that expensive and once you're at a u-pull-it
yard the cost of a clutch vs a compressor is about the same. So I'd say replace compressor,
either used or rebuilt.
Apparent Clutch Failure Caused by HVAC Control Unit. [Jim Holst] My '93 945 AC compressor
would not come on but I measured nearly 12 v from compressor lead to ground. I was puzzled