Yes, another AC thread. - Mercedes-Benz Forum
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post #1 of 75 (permalink) Old 06-23-2019, 04:26 PM Thread Starter
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Wink Yes, another AC thread.

But don't panic - I've taken all the readings, checked the error codes (no surprises, the normal sun sensor fault), etc.

Basically, the AC on the driver side sometimes only supply lukewarm air instead of cold air like the passenger side. Other times it cools perfectly. Refrigerant pressure (reading 07) varies between 10 and 16 depending on speed. All other readings seem to be normal. The interesting thing is that when the problem occurs, the left heater core temperature is always about 17 degrees higher then right core temperature, even if I set both sides to LO.

My feeling is that the duo valve for the left side is sticking or not closing properly. Any ideas on my diagnosis or any other way I can test my hypothesis before I disassemble the duo valve? I think that if I do disassemble the duo valve, I'll just order this from Amazon for $41.99 - "Heater Valve Repair Kit Compatible with Mercedes-Benz Cars - Set of Heater Repair Parts for Self-Repair - W210, W220, W124, W202". I think if I open it up, I may just as well replace all the rubbers, seals, poppet valves, etc while I have it open.

Your thoughts?


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post #2 of 75 (permalink) Old 06-23-2019, 04:53 PM
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Sounds like the correct diagnosis. It you clamp the hose to the aux pump, you should have the same readings as you are effectively stopping the hot coolant circulation.
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post #3 of 75 (permalink) Old 06-23-2019, 05:00 PM
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Clamp off hose from duo valve first. See if that changes readings. Otherwise your refrigerant is low.

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post #4 of 75 (permalink) Old 06-23-2019, 06:41 PM Thread Starter
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Thanks, I'll do the clamping part first. Should I disconnect power to the aux pump to reduce the strain on it with the water circulation blocked?


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post #5 of 75 (permalink) Old 06-23-2019, 06:50 PM
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Unless you live past north circle. And you are not. Aux pump should not even be on.

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post #6 of 75 (permalink) Old 06-23-2019, 06:50 PM
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No need. When you go LO LO, the aux pump should not be running, and the duo valves should be completely off anyway.
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post #7 of 75 (permalink) Old 06-23-2019, 08:11 PM Thread Starter
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Thanks. And temperatures in Texas are currently more than 100F higher than the North Pole.


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post #8 of 75 (permalink) Old 06-24-2019, 05:30 AM
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In Texas summers, your high side pressures should be north of 18 bar, so don’t rule out a refrigerant or compressor issue.

R-134a System Pressure Chart - AC Pro<?php/* bloginfo('name'); */?>
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post #9 of 75 (permalink) Old 06-24-2019, 08:43 AM Thread Starter
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Originally Posted by Hirnbeiss View Post
In Texas summers, your high side pressures should be north of 18 bar, so donít rule out a refrigerant or compressor issue.

R-134a System Pressure Chart - AC Pro<?php/* bloginfo('name'); */?>
Thanks, something else to consider. I'll first try the duovalve test to eliminate possible problems there.


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post #10 of 75 (permalink) Old 06-24-2019, 10:37 PM
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When MB never reports recommended high pressures, the high limit is 32 bars, just for referral.
yours max is 16.
Do the math.


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