Michael Benno

On my recent roadtrip in my 88 S4 with rear AC from Portland āSpokane āPortland I noticed some inconsistent cooling in my air conditioning system. The symptoms are it will blow very cold for a while and then blow cool to warm and then cold. This cycle will take about 10min.

If I turn off the AC and turn it on after a few minutes, I get cold again. The problem seems to get worse at the outside temp goes up. A couple other observations: the problem is worse if I don’t use the rear AC. I get really cold air if the car has not been driven for a while. I get more consistent cooling not in direct sunlight.

Any thoughts?
- My initial thought was the evaporator is freezing/thawing because of the cold/warm cycling. But I am not familiar with the symptoms.
- When I had the system serviced, I don’t think the tech turned on the rear AC (opened the solenoid) when they ran the service, could that have affected the evac/charge?

Background on tests and recent work:

• April 15: AC service vacuum and Pressure tested and fresh 134a refrigerant and cold temp verified at 35-degress F. from center vent.

• April 15: AC refresh: Replaced ALL seals, Rebuilt compressor seals, new barrier hoses, new expansion valves, and receiver/drier.
• April 25: Successfully tested heather valve (all metal), vacuum actuators and solenoids, setting flaps and motor.
• May 14: Verified the heater valve is closed when the AC is on at the lowest temp. Verified the setting motor is not moving while the AC cooling temp is changing.
• PO did the HVAC Compressor relay mod

Any thoughts or tests appreciated.

Speedtoys

Icing of the evaporator,

35c vent air _IS_ admirable, but in high humidity environments you will develop ice more than in lower ones.

My car will ice up on a drive thru SanFran in evenings in the foggy districts...but when I climb above the fog, its gone.

You can...adjust this, shoot for a few more degrees at the vent..35d vent suggests a 20d evaporator.

FredR

Relay in the head unit may be going south- most of them are past their sell by date now. Mine was doing something similar albeit somewhat hotter over here.

If that is the case the possible solutions are a rebuilt unit, change out the relay yourself or possibly use the signal to trigger an extra relay with a fused power supply from the hot post to the compressor solenoid.

Michael Benno

Michael Benno

Clarification, I am using a laser temp probe and I am able to hit the metal on the evaporator through the center vent. So the 35-degrees F is the surface temp of the evaporator.

Earl Gillstrom

That problem is a bad antifreeze switch. You can adjust them but it is usually futile.
Replace the anti freeze switch.

j.kenzie@sbcglobal.net

Yep. I've had the same thing. If you park it after it's been poorly functioning, you should find a big puddle beneath the car as the evaporator defrosts.
Good luck,
Dave

Speedtoys

that could just be a good working AC system.

Totally normal in many conditions,

j.kenzie@sbcglobal.net

That's true, but if his was not working well, ie. poor air flow and mediocre vent temps, then when shut off, drained a lot of melt water, it might indicate evaporator freeze up.
Dave

worf928

Mrmerlin

the freeze switch failed in my 88 S4,
the same symptoms as what the OP reported were also going on in my car,
a new freeze switch and its working fine now.
The WSM says they can be adjusted ,
but IMHO its better to just swap in a new part

Michael Benno

Earl Gillstrom

Put an ohmmeter on the contacts. It should read 0 ohms. Put the sensor in ice with a little water. It should read infinity in less than a minute.
Remove from water and it should go back to 0 ohms in a few seconds.

Michael Benno

Michael Benno