re:quirky A/C

[pjrose@servtech.com (Phil and Judy Rose)]

>>My Audi mechanic thinks the AC ('89 100) probably just needs some more
>>freon, but much of the time it behaves perfectly OK--as long as it's not
>>too hot outside. The problem is that the AC fails to go on when the
>>outside temperature is near 80 deg F, or higher!!! I know the system is
>>supposed to keep the AC from working when the outside temp is too low,
>>but can the temp be too HIGH??? Is this actually a symptom of low freon
>>pressure?

BTW, when the AC turns on (temp below, say, 75 F), it appears to keep going
even when the outside temp rises to that magic (higher) level. And it cools
very well, IMO. But if the ignition is switched off when temp gets to 80 or
above, the AC will probably fail to work after a restart. So is more freon
gonna eliminate this symptom, or is the controller or some sensor screwed
up?>>
=============

Update:
The vehicle (OK to refer to Audis as "vehicles"?) spent a few hours at a
local audi (non-dealer) shop during which--you guessed it--the AC
functioned perfectly. The mechanic said the AC air output was so cold that
there was no question in his mind that the freon was OK. His diagnosis of
the problem was that the ECU was intermittantly receiving a bogus "engine
overheat" signal from the engine temperature sensor, which causes the AC
compressor to be shut off.

Remedy:
Replace the multifunction temperature sensor (I think that's what he called
it--  it's the 4-wire sensor located where the radiator hose connects to
the head.) He suggested I wait to get the AC problem to recur; then pull
the sensor plug and verify that the compressor comes on. If so, then that's
the source of problem. BTW, there aren't any other symptoms, e.g., the
temperature gauge works fine all the time.

So far: $48 for diagnosis with about $80 expected for eventual replacement
of the sensor.

Phil Rose
'89 100 semi-cool

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