Re: 94 Concorde A/C electrical problem - RESOLVED

'94 Concord. The air conditioning quit suddenly and solidly.

> Compressor was not engaging. > > Took it to an A/C place, assuming refrigerant had leaked out. They > hooked up a scan tool and were able to fire up the A/C using it; no > problem with refrigerant, mechanics, compressor clutch or relays. They > concluded the problem was in the control head ($327 plus labor) on the > dash. > > The control head has its own microprocessor and it passes its internal > diagnostic tests, but these don't seem to test the signal from the > control head to the body computer, a single wire called "A/C damped > pressure switch". After getting the car back I did some measurements on > that wire, which shows +5 VDC unless the A/C is switched on, which drops > it to + .4 VDC (that's POINT 4 volts). That seems reasonable to me (TTL > switching level?), but the A/C doesn't engage. > > With the control head disconnected, the wire has +5 VDC on it, which > must come from the body computer, thus indicating that the wire and > connectors are OK. > > BUT I don't know what voltage this line SHOULD be (the shop manual is > useless at this level of detail). If the voltage is wrong, the

control

head is probably at fault. If the voltage is correct, the body

computer

is suspect, since it should turn that voltage into a command to turn on > the A/C. > > I opened up the control head to look for visible damage, but found none. > > Can't get an appointment with the local Chrysler dealer for a week, and > it's hot here. Any suggestions?

The dealer replaced an evaporator temperature sensor ($27 part) and the A/C works fine.

The only explanation I can come up with is that the scan tool 'diagnostic' that the A/C shop ran was actually a brute-force turn-on of the compressor, which led them to believe that everything downstream from the control head was OK. Instead, I suspect the scan tool request was bypassing several safety sensors, such as the evap temp. The shop manual info on the scan tests is brief and somewhat cryptic, so it would be easy to make this mistake if you didn't have experience with this model.

Net for anyone with this symptom in the future: the control head voltages noted in my original post apparently represent correct operation. Failure of the system to respond to this voltage can be the result of out-of-range safety sensors.

Chris Beall

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Chris Beall
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