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Inconsistent air conditioning after refresh

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Old 05-16-2018, 03:37 PM
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Michael Benno
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Default Inconsistent air conditioning after refresh

On my recent roadtrip in my 88 S4 with rear AC from Portland àSpokane àPortland 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.

Last edited by Michael Benno; 05-16-2018 at 04:05 PM.
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Old 05-16-2018, 03:43 PM
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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.
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Old 05-16-2018, 03:47 PM
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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.
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Old 05-16-2018, 04:08 PM
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Michael Benno
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Originally Posted by FredR View Post
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.
The relay on the head unit was replaced with the external oversized relay (AKA Compressor Relay Mod). I have been able to confirm the compressor engages/disengages when I press the AC button and when I move the temp slider.

Last edited by Michael Benno; 05-18-2018 at 11:11 AM.
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Old 05-16-2018, 04:09 PM
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Originally Posted by Speedtoys View Post
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.
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.
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Old 05-17-2018, 12:54 PM
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Earl Gillstrom
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That problem is a bad antifreeze switch. You can adjust them but it is usually futile.
Replace the anti freeze switch.
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Old 05-18-2018, 09:23 AM
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Originally Posted by Earl Gillstrom View Post
That problem is a bad antifreeze switch. You can adjust them but it is usually futile.
Replace the anti freeze switch.
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
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Old 05-18-2018, 10:59 AM
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Originally Posted by [email protected] View Post
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
that could just be a good working AC system.

Totally normal in many conditions,
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Old 05-18-2018, 07:26 PM
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Originally Posted by Speedtoys View Post
that could just be a good working AC system.

Totally normal in many conditions,
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.
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Old 05-19-2018, 12:11 AM
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Originally Posted by Earl Gillstrom View Post
That problem is a bad antifreeze switch. You can adjust them but it is usually futile.
Replace the anti freeze switch.
This-^
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Old 05-19-2018, 01:46 PM
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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
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Old 05-20-2018, 12:19 AM
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Is there any way to bench rest the temp switch?
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Old 05-20-2018, 12:29 AM
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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.
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Old 05-20-2018, 01:54 AM
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Michael Benno
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Exellent thanks👍
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