Last year my air worked fine, this year after only using vehicle 2
times the heater air control inside car blinks sometimes then goes back
to normal operation. The problem is the compressor for the air won't
engage. The unit is charged, heater works. Why won't the air compressor
engage? Checked the fuse inside vehicle thats ok, are there any other
Sounds like you've got an electronic control head (RPO C68) rather than
manual (mechanical -- C60) control. Initial hunchs would be a loose
connection between the head and the compressor clutch, a problem in the
control head or an interior temperature sensor (location varies with
Confirm body style and type of control and we might come up with some
Just got back (the C5 saw its first rain since I bought it) and checked
the shop manual. The compressor control circuit is entirely different
between the C60 and C68 options. The compressor is 'hot' all the time
and is supplied from the A/C fuse. Control is in the ground circuit,
turned off-on by the Blower Control Module (BCM). That module is behind
the instrument panel on the left-hand side of the evaporator housing.
Two pressure switches, a high limit switch and a pressure cycling switch
(comes into play if the evap ices up) signal the BCM. The A/C & Heater
control head gets signals from the inside air temp sensor (over the
cargo deck) and an outside air temp sensor.
The ECM doesn't control the compressor. A signal from the A/C to the
ECM causes the idle speed to increase and the ECM turns on the coolant
fan. As I read the diagrams, the only ECM problem that could affect the
A/C compressor would be a short at its input. That would probably smoke
the ECM and blow the A/C fuse. So, I'd forget the ECM for now.
Sounds like you've got A/C - Htr. control head problems or temp sensor
problems. The troubleshooting tree for the C60 option is pretty easy
but the tree for the C68 option is too lengthy to play with here.
The shop manual has a good troubleshooting tree and schematics. If you
can't get a copy (sections 8A-62 and onward.) I can burn the pages and
mail them to you. Drop me a snailmail address at my email address.
Please confirm though that you've got the C68 option rather than the C60
I had a similar problem last year with my 89. I had GM verify it and 1 hour
later they told me what my problem was. It turned out to be my ECM was shot.
It was engaging the compressor and disengaging it, so i was going from
freezing to heat. They replaced my ECM and everything has been working fine
Couldn't tell from the other post if this was an electronic controlled
unit rather than manual. If electronic it could be the inside temp
sensor or wiring -- up over the cargo deck (manual control doesn't have
Other problem with the '89 electronic controls was corrosion on the
connectors to the control head. That was more of a problem in
high-humidity locations where condensation in the cockpit was a problem.
Gotta run to make. When I get back tonight I'll pull the shop manual
and get some more info.
I want to thank you for your help. I jumped the switch and
the compressor came on, I then put the gauge back on with the
compressor running and I needed Freon. I put 1 can in and the
compressor now engages. The air is cold but I need about 1 more can of
freon. Thanks again for all the help now it will be a cold summer for
my Baby (1989 Vette).
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