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2002 Mercedes G400 AC sometimes not cooling

gclass on Sun June 29, 2014 2:41 PM User is offline

Year: 2002
Make: Mercedes Benz
Model: G400
Engine Size: 4000
Ambient Temp: 32
Country of Origin: Israel

Hi,

I am having this problem for over a year now and will be glad to have some help.

the car - Mercedes G400 CDI 2002 LWB.
the problem - A/C stops cooling once in a while (more often on days over 30^C). the timing it stops has no logic at all, can be while cruising on a highway,while standing, at stops after long drive (very popular), high RPM, idle RPM, etc..
when the A/C stops cooling (but keep blowing air normally) it will get back to itself after random time (usually 10-30 mins or after a long rest).

to overcome it, I unplug the coolant temp sensor, therefore the system goes into "emergency mode", the front electric fan spins at highest speed and my A/C is working perfect with no stops at all even at ambient 37^C..

I replaced the dryer, and washed the A/C system, vacuumed and refilled with new refrigerant.

I tested :
1. the fan control module and it is working fine using a Mercedes star diagnose system (except if I unplug the coolant temp sensor).
2. the A/C pressure sensor next to the dryer is reading fine with Mercedes star diagnose.

testing the A/C compressor using diagnose system return 70% max capacity (which is weird as if I go into "emergency mode" by disconnecting the engine temp sensor the A/C is cooling fantastic).

Anyhow, I will be glad to have your opinion about it,

thx

bohica2xo on Sun June 29, 2014 9:44 PM User is offline

Well, since it looks like a condensor airflow issue start with the most basic step. Clean the air path through the condensor.

We don't see that model of MB here in the US, so I don't know what the primary engine cooling fan is. Do you have a mechanical fan behind the radiator?
.

To properly clean the air side of a modern condensor, you will probably need to remove the grille, and any fan(s) in front of the condensor. Start with a cold condensor.

Use a strong surfactant type cleaner. In the US we recommend things like Simple Green. Commercial floor stripper is great for this job, but it can de-gloss paint, so be careful with it.

Use whatever cleaner you have full strength. Spray it from a spray bottle deep in to the fins. Soak the condensor from top to bottom. Let it stand for a few minutes, but do not let the cleaner dry. Spray it again if it is drying.

To rinse, use a garden hose with a spray nozzle - do NOT use a high pressure washer, as this can bend or flatten the fins on the condensor. Plenty of flow @ 30psi is better than 2500psi.

Driving a car here in the dry Nevada desert I still find I need to clean the condensor every few years. Small insects, dust, pollen, etc stacks up in the fins below the front surface and the build-up reduces both airflow and heat transfer. You will be surprised at how much dirt rinses out of the average condensor after 5 years on the road.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

gclass on Mon June 30, 2014 5:11 AM User is offline

standing in front of the car, there is the A/C condensor (no fans in front of it), behind it there is the Gearbox oil cooling radiator, behind it the engine coolant radiator and a huge electric fan behind it all,
sucking air inside.

I washed those few months ago and the problem still exist but I will try to find a strong cleaner and clean it again.

thx

bohica2xo on Mon June 30, 2014 10:49 AM User is offline

Ok, I was not sure what was "washed". Small insects can be hard to dissolve with mild soaps, so cleaning the air path again will not hurt.

It does sound like the fan speed is not up to par. Single Electric fans with PCM control can be frustrating sometimes. It sounds like the compressor is getting shut down from a high pressure event.

I would check the coolant temperature sensor, since it seems to be the primary input for the PCM decisions.

The pressure sensor is harder to test. The MB system can test for continuity, but has no reference to check accuracy. I hate to just throw parts at a system, but the easy way to test one is to replace it.

.
I have had a sensor with an intermittent fault drive me crazy. Does the coolant temperature sensor drive the dash gauge as well as the PCM?

.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

gclass on Mon June 30, 2014 12:19 PM User is offline

I will try to do the cleaning asap as it involve a lot of work taking the 2 radiators out (otherwise it is really hard to clean it).

Yes - the coolant temp sensor also drive the dash reading as well.
the A/C sensor I have is reading temp and pressure and should drive the fan PCM as well but reading it using star diagnose system seemed fine.

I will read again for fault codes to see it some of them reappear.

thx

bohica2xo on Mon June 30, 2014 1:37 PM User is offline

You can usually clean the heat exchanger stack while it is assembled. There are drain points to accommodate driving in heavy rain.

If the dash gauge is stable, then the sensor & connections are sound. The sensor could be out of range, but that is an easy test. Just put your thermometer near the sensor overnight, and check the sensor resistance in the morning - then compare the resistance at a known temperature to the chart in the factory manual.

It is looking more like the random fault is a pressure sensor acting up. Some of those are piezoelectric - try tapping on it (With a metal object like the edge of a coin or a screwdriver blade) while running the on-board diagnostics.

.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

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