I'm having an issue on the '91 with airflow from the interior vents
when the A/C is on. After 5 to 10 minutes of running the A/C the air
flow from the vents drops to a barely-feelable trickle of cold air. If
I turn off the A/C the air flow will, over a period of 5 to 10 minutes
gradually return to normal. Then, when I turn the A/C back on rinse
lather repeat. Note that during periods of low flow the temperature of
the air does not drop to ambient temperature; it remains colder than
Is the above symptom consistent with a clogged heater core drain not allowing condensate to drain at a fast enough rate thereby leading to the excess condensate freezing and reducing the volume through which air may flow?
The next question would be how to gain access to the drain hose from below? I've looked at the shop manuals and the method by which one may gain access from the interior is clear - and a bit involved to say the least. On other cars I own and/or have owned the above symptom has been cured by 'burping' the heater core and it has been possible to access the drain from below and unclog it with thick fishing line etc.
I've looked under the '89 - since it's on stands and the '91 isn't - and can't find any sign of the drain.
i had a similar problem on the 89 and it turned out to be a bad freeze switch. it's a temp probe that goes into the hvac air plenum under the cowl to measure the temp of the heater core and cycles the compressor off when temp gets close to freezing. you'll see a kind of silver wire covered by a clear plastic tube which runs to the switch that is bolted to the plenum mid-cowl. If it's bad the core will freeze up and gradually reduce your air flow to barely anything. you can test them (not very scientifically) by immersing the probe tip in a glass of ice water. the switch should make (audible click) after being in there for a few seconds, then open after being out for a few seconds. the switch is adjustable by means of a set screw, but i forget which way to turn it for higher temp (trial and error will work here). also make sure the probe is advanced far enough to make contact with the heater core so it's getting an accurate measurement.
As far as the drain hose, it's a real b*tch to get at from underneath. Need
to remove the cats/test pipe and the heat shield above that. then it's kind
of tucked above and to the drivers side of the torque tube. be careful if
you run something up in there as if you tear it you will have to remove the
whole dash/pod/center console assembly to replace it.
89 s4 shaaark
Your AC is freezing up. The low flow is from ice blocking the air. When
you turn it off for a few min. the ice melts and then you get your air
Freeze switch. I need to replace mine also :).
Actually, the opposite. The switch should show continuity at temps above freezing, and be open below freezing, so as to cut the power to the compressor.
Some switches are adjustable, most are not (no adjustment screw is the clue).