AC won't work in idle - valve? - Mercedes-Benz Forum
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post #1 of 24 (permalink) Old 04-05-2019, 04:40 PM Thread Starter
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AC won't work in idle - valve?

Hi,

Not sure if I diagnosed enough for a conclusion - but here it is.

If the car's been parked for a while and I start it and just sit in idle, it won't cool at all.
After driving for a few minutes in more than stop&go traffic, the AC comes on and works fine.
Sitting in dense traffic or at a drive-through etc, it kicks out, gets warm, windows fog up etc.
If I restart the car when it's recently been driven, it sometimes works fine, even sitting in idle for a little bit until it kicks out.

So.. I hooked up STAR and a low pressure gauge - outside temp around 76F, Temp set to "Blue" Cold Auto:

Test1 after recently driving:
- Engine off: >4 bar / 11 bar | 60 / 160 psi
- Idle: 2.7 bar / 13 bar (cools fine) | 39 / 188 psi
- After idling 5 min: 2.3 bar / 13 bar (cools fine) | 34 / 188 psi

Test2 - Engine off / on:
- Idle: >4 bar / 10 bar | 60 / 145 psi
- After idling 5 min: same - as if something just doesn't open, the low pressure gauge stays in the red
- Driving around the block / or just revving for a while: AC kicks in and values are similar to Test1.

Test3 - AC Off button:
- See Test2

Is that the compressor itself? The expansion valve? It may be slightly low on freon, but that doesn't seem to be the cause?!
In either condition the compressor seems to be engaged.. I can hear it yowl and STAR shows it at 100% (50% and dropping with AC OFF)

Thanks!!
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post #2 of 24 (permalink) Old 04-05-2019, 07:21 PM
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Since you own more then one car specifically state which car your query applies to.

Values you have provided don't make any sense.


Typical values for good cooling a/c are 200/30 psi.

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post #3 of 24 (permalink) Old 04-06-2019, 10:14 AM Thread Starter
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Quote:
Originally Posted by Witek_M View Post
Since you own more then one car specifically state which car your query applies to.

Values you have provided don't make any sense.


Typical values for good cooling a/c are 200/30 psi.
Thanks!
It's the ML as one would hope in this section
So that means that WHEN mine is working fine, the 34 / 188 psi aren't too far off an ideal world.

Any pointers as to what could be at fault if / when the low pressure values remain in "ac off" territory when they should not?
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post #4 of 24 (permalink) Old 04-06-2019, 12:14 PM
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With vehicle after overnight rest check low and high pressures and ambient temperature. Pressures should be equal and match temperature chart.

https://www.forane.com/export/shared...ture-chart.pdf

At the same time check temperature sensors for sanity. They should be same (+/- 2 C) and jive with ambient.

https://www.benzworld.org/forums/w16...l#post17516805


What is compressor demand when it is not cooling? When was the last time system was professionally recovered, evacuated and recharged? Sounds like you are using suicide kit can with gauge.

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post #5 of 24 (permalink) Old 04-12-2019, 08:44 AM Thread Starter
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Quote:
Originally Posted by Witek_M View Post
With vehicle after overnight rest check low and high pressures and ambient temperature. Pressures should be equal and match temperature chart.

https://www.forane.com/export/shared...ture-chart.pdf

At the same time check temperature sensors for sanity. They should be same (+/- 2 C) and jive with ambient.

https://www.benzworld.org/forums/w16...l#post17516805


What is compressor demand when it is not cooling? When was the last time system was professionally recovered, evacuated and recharged? Sounds like you are using suicide kit can with gauge.

Thanks for your response Witek!
Just checked pressures after having the car sit overnight:
LP: >60 psi (the gauge isn't linear after 60 - so not sure what pressure exactly)
HP: 6 bar => 87 psi

Indicated ambient temp in my garage is 68F right now, so the HP seems slightly on the high side.

B10/6 Evaporator Temp Sensor is 22C
B12/2 Refrigerant Pressure and Temp Sensor is 21C
B10/7 Rear Outlet Air Temp Sensor is 20C
B10/25 Front Footwell Outlet Air Temp Sensor is 20C
B10/24 Center Vent Outlet Air Temp Sensor is 21C
So they all seem to be right on the money and equal the indicated outside temp at 68F.

Compressor demand with AC on whether working or not is always 100%.

Since my engine was replaced and the AC system opened it has not been professionally evacuated at all, the shop just filled it with one of those big freon jugs. So maybe that is just all it takes and I should take it to a shop that has the right tools to evacuate etc.
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post #6 of 24 (permalink) Old 04-12-2019, 10:07 AM
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Mine did the same thing...when parked or in idle, no cold air. Mine was a simple fuse that's blown. Lots of discussions about this on this board.
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post #7 of 24 (permalink) Old 04-12-2019, 10:49 AM Thread Starter
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Originally Posted by demichae View Post
Mine did the same thing...when parked or in idle, no cold air. Mine was a simple fuse that's blown. Lots of discussions about this on this board.
Mine is a face lift and the fans work fine.
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post #8 of 24 (permalink) Old 04-13-2019, 07:12 AM
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Quote:
Originally Posted by DarkRothaus View Post
Thanks for your response Witek!
Just checked pressures after having the car sit overnight:
LP: >60 psi (the gauge isn't linear after 60 - so not sure what pressure exactly)
HP: 6 bar => 87 psi

Indicated ambient temp in my garage is 68F right now, so the HP seems slightly on the high side.

B10/6 Evaporator Temp Sensor is 22C
B12/2 Refrigerant Pressure and Temp Sensor is 21C
B10/7 Rear Outlet Air Temp Sensor is 20C
B10/25 Front Footwell Outlet Air Temp Sensor is 20C
B10/24 Center Vent Outlet Air Temp Sensor is 21C
So they all seem to be right on the money and equal the indicated outside temp at 68F.

Compressor demand with AC on whether working or not is always 100%.

Since my engine was replaced and the AC system opened it has not been professionally evacuated at all, the shop just filled it with one of those big freon jugs. So maybe that is just all it takes and I should take it to a shop that has the right tools to evacuate etc.
Higher then normal static pressure may be result of contamination with non condensable gas like air.

Another problem is presence of water vapor in the system. It will freeze in expansion valve and restrict flow.
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post #9 of 24 (permalink) Old 05-16-2019, 02:10 PM Thread Starter
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Update:
Had an A/C refill done. Full evac, everything checked out normal, no leaks, refilled => same issue
Shop concluded that likely pressure valve has an issue and sometimes just doesn't open - also they mentioned that the low pressure was a little on the high side (I believe it was too high).
Pressure switch replaced, values apparently perfect now - but - still same issue.

Their assumption is now that the compressor has issues engaging and revving it helps it doing so which is why it likely fixes the issue.

Symptoms are still the same:
Driving / revving => A/C is completely fine and cold
Sitting for a while, cycling AC on / off or restarting the car most of the time results in the A/C not working as in the low pressure stays way high.

Are those clutches in the compressor known to have issues? Does it makes sense to throw more parts / a compressor at it?

My thoughts are that revving does two things
- increase RPMs on the compressor, so if there is a clutch issue that may help
- increase voltage - could a bad connection or something electrical be responsible for this also?

Thanks,
Clueless Patrick
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post #10 of 24 (permalink) Old 05-16-2019, 03:05 PM
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In 02 was around the time the clutches on the compressors were no more. Could of been 03 not 100% on that. They changed to the variable output type. No clutch.
You need to check to see if your fans are coming on with the A/C. Thatís the first systems of A/C not working when idling or sitting in traffic.


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