I have a 1992 Mercedes 400E with approx. 156K miles. We are getting ready for a long drive through Las Vegas to St.George, UT so I really need the AC working!!

Here are the symptoms that I have noticed:

Start the car cool, turn on Max. AC and fan and allow the car to run at idle. The compressor clutch is engaged and the compressor is turning.
Compressor seems to be slightly noisy (rattly) when it is engaged. It will run for approximately 15 minutes like this (ambient temp about 85F). Then it will start cycling and cycle fairly frequently (off about every minute).

The air blowing in the cabin at the beginning is only slightly cool, and the temperature does not drop much. The large diameter AC line under the hood is slightly cool to the touch, but definitely not cold.

After driving for a while, and checking again, I noticed that the compressor is off and not cycling. If I touch the large diameter AC line now, it is hot.

I checked the site glass, but it is hard to see anything clearly. It seems largely reflective. I will try cleaning it more.

The tap in the cool, large diameter line under the hood I believe is the low pressure line. Does that sound correct? (There is a larger diameter tap near the condenser at the front of the car).

I have a cheap pressure gauge that I got as part of a refrigerant refil kit. When I attached it to the low pressure side and tested it early after running the compressor, the lowest pressure I saw was about 45psi. When the compressor turned on, I could see this pressure go up quite a bit >60psi. From what I have been able to find, this sounds like too high a pressure in the system. The system has not been refilled for at least 2 years and maybe longer. Is there something else that can make the pressure too high?

I see the 2 auxilliary cooling fans in front of the radiator cycle on and run when the coolant temp reaches about 100C. They appear to be running on high at this point. I read some mention of a low speed fan run due to AC demands? Is this a major issue in terms of my lack of cooling? Particularly given that cooling is low even at highway speeds and low coolant temperature.

Thanks for your help with my AC problems. Think COOL!

Kim

rev the engine to 1500 rpms// then check low pressure gauge... if you can't get it in the 30 range you are a bit low on refrigerant

low fan should be coming on... if it doesn't come on within a couple of minutes of the a/c being on, it may be disengaging the compressor.

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1989 300ce 129k
( facelifted front,updated tail lights, lowered suspension,bilstein sports, lorinser front spoiler, MOMO steering wheel, remus exhaust,stainless steel brake lines). (Gone)

1997 s320 154k (what a ride). Sold with 179k miles. Replaced with Hyundai Equus

1994 e320 Cabriolet 108k



1972 280se 4.5 153k Owned for 12 yrs, sorry I sold it


[/SIGPIC]

My 45 psi pressure reading was at idle. Are you saying that by revving to 1500 RPM, the pressure should drop from 45psi to about 30psi? If it is still higher than 30psi at 1500 RPM, I should ADD refrigerant? Add until what measurement?
Thanks,
Kim

With the compressor running and the engine speed up to 1500 RPM or so, the pressure dropped to about 25 psi. I did add almost 13oz of R134a to the system. The AC system seemed to perform better, the compressor ran a lot longer (almost continuously) and the air temperature was cooler. I started to see some condensate dripping, so that is a good sign.

When the compressor cycled off, the engine RPM increased by about
400 RPM. Is that excessive?

If the system is low on R134a, can the pressure still be at the expected level or will it be low?
Likewise, if there is too much R134a in the system, will the pressure register too high?

Thanks for the help!

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1992 400E (W124) - 177K Miles - fun car!!

Your first test should be low fan circuit test. If that passes and the low fan is still not coming on with hot ambient, you are most likely still low of refrig.
If the high pressure sw cuts-out the compressor while low fan is operating, you may be slightly overcharged. Buit if low fan is not working, the comp can cut-out b/c the high pressure is not being checked by low fan..[ that is what the low fan does]
..so, you can see where low fan circuit integrity test has priority over all other test..That is the first test to be done b/c it effects all high side pressure conditions more than any other faulty condition.
Changing a system w/o having low fan circuit in operating condition is a waste of time and refrigerant.

These are just quick check diy things to look for for basic diagnosis..actual charge shoud be done by weight or, in the case af topping, with gauges in comparison to ambient temps.
Test the low fan circuit first.

.

Thanks for the info Arthur. I searching for further information on the Low Fan circuit and what to test. Please let me know if you have further pointers.
Thanks,
Kim

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1992 400E (W124) - 177K Miles - fun car!!

Ok
I though you had mentioned seeing the test procedure.

It is quite simple.
At the drier/reciever, there are a couple of pressure switches..the one that has 2 pigtail wires coming out of it is the low fan switch. At the end of the pigtail, you can just peel back the connector covers and jumper right there with a screwdriver . The low fan should start right up [ key On , of course]
This is a test that takes 2 secs to do and is the first test I always do on a/c systems..I can not stress the importance the low fan has on both the effeciency of the a/c system and the engine coolant temps when ac is on.
If the fan does not pass this simple test, you have a fan circuit problem and it should be addressed before any other work is done/started.
We see so many a/c repairs get wacked out with all kinds of craziness when all that was wrong right from the get-go was a simple fuse for the aux fan............

A refrigeration system comparison to stress this point for all:
I used to do Domestic/Commercial refrigeration years ago, and by far, one of the largest complaints we would get on Service Calls was Poor Cooling ...
[ be it a refrigerator/freezer/whatever].. well, that complaint could be caused
by many things [low refrig/leak..thermostat..relay..etc] ..but the winner by far was bad airflow across the condenser..either b/c the cond. fan crapped out , the blades were broken, or the cond. fins were so plugged up with dust that the fan could not draw air through . Same deal with your car..pay attention to the condensers airflow..it's a must. Specially at stationary condition.
The Condenser is as important to your systems cooling ability as is the Evaporator..maybe even more...........