The air conditioner on my 1998 ML320 works great when the compressor clutch engages. The engagement is becoming a rare event and now I will get a week of no A/c before it works again for one hour. When this problem started we took the truck into the dealer twice. The first time they replaced the fan wiring harness and upped fuse #44 to 40 amps. The second time they could not find anything and were very rude. We have replaced the AC Module (also called amplifier assembly 163-820-01-97) and I just changed out the pressure sender unit (163-542-04-18) and lost most of my 134A. I cannot get the refrigerant to charge back in because the compressor clutch will not engage.
Can you help identify the terminals I can jump to force the compressor to run so I can charge the system? Any thoughts on the root cause of the problem? The fans do run when the compressor is working and the belt seems fine.

I can not diagnosis your problem but I can suggest you post your questions on the A/C website whose link I have below. A few weeks ago I replaced compressor, expansion valve and reciever/dryer on my 92 300E and they were extremely helpful.

As far as getting your system filled again you will need to evacuate the system with a vaccum pump to remove moisture. Since you may need to take something apart again I would not immediatley put in a new receiver/dryer. You will need a set of hoses with manifold gauges and I think 1.6 lbs of R 134a to recharge. Read information on the site I gave the link to and it will explain how to do this. It's not hard to evacuate and charge system it's hard to diagnosis the problem as you have already experienced.

A/C Information Forum

I have a vacuum pump, but when I changed the switch I was able to get the new one tight before all the 134a leaked out. Since the system stayed pressurized, air and moisture did not enter. Normally I would have drained the system before I loosened the old switch but I hoped they would have had a check valve about the switch.

If the system does not have enough refrigerant in it, ie the pressure is to low then the compressor will not go on. You may first need to add refrigerant with the compressor and vehicle off and then once the system is pressurized somewhat you can then add more, with the compressor on in order to get the high and low pressure correct based on the ambient temp. I don't know how much oil came out when you installed the pressure switch but you may need to add a very small amount of oil, to replace what you lost.

After checking the A/C site you suggested, one person had trouble getting a charge because the charging hose did not open the charging valve. I checked for that and with the engine off I have 90 psi static pressure on the low pressure side. That should be enough pressure to allow the clutch to engage but, since I may not have fixed my original problem with clutch control, I do not get clutch engagement. I did lose some oil so, I am planning to put in a PAG oil charge when charging begins.
Does anyone know the terminals to jump or check on the A/C module to see which signal is not being provided for clutch operation? Does anyone know where to jumper the clutch it get engagement? I have trouble finding any connections on the clutch circuit. Does the Mercedes need some sort of reset (like disconnecting the battery) if it has gone too low in pressure? If I disconnect the battery will I lose any programming like the radio?

Cyling the ignition key will allow the clutch to re-engage if it cut out due to low or high pressure. If it has a bad low pressure switch, you should be able to jumper the leads at the low pressure switch.
I am relatively certain that the clutch cycling is controlled by the AAM on a 98, not an AC module or the control unit on the dash. You might want to check the fuse list on the bottom of the fuse box cover for relays or fuses listed. Have you tried rotating the compressor by hand to see if it's seized?

Gilly

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The compressor rotates freely and does not make any odd noices during the periods it decides to work. I tried rotating the compressor using the front bolt when the car was shutoff to try to get the refrigerant around to the high pressure side where the combination high and low pressure switch is. This was a big mistake because the air ratchet was able to break the small clutch bolt. So far I am just making steps backwards.
According to a 1998 Mercedes suppliment manual I have, it shows the clutch being controlled by 1) the dash controls, 2) what the book calls an A/C module M19 (it is called a relay on the dealer microfiche and it is called an amplifier assembly on the parts bag) 3) these all go to the AAM along with sensor imputs to control the clutch. The amplifier / relay / module has a 14 pin connection and is located behind the radio. I would like to identify and test the sensors that could be acting up and preventing the clutch from getting a signal. I have replaced the switch on the high pressure line that the dealer said controlled low pressure as well. It has a three pin connection.
The list of fuses and relays that seem related to A/C are k14 comfort functions, F16 back up lights / air conditioning / diagnosis (back up lights are working and fuse looks good) and F44 air conditioning fan (this has the 40 amp upgrade and the fans run when the air works). To test k14 I swapped it with k26 radiator fan (A/C only) because they have the same part number but, it did not change my condition. Any other thoughts?

I'm having the same problem on 1999 slk 230 .... any ideas on how to jumper air conditioning clutch so I can add refrigerant? Thanks!... Ray

Start adding the freon before starting the engine, the compressor will run for a short time before shutting off and you'll be fine. Worst case scenario is you may have to shut the engine off and restart it, it will go through this cycling the AC clutch whenever you start it.
Gilly

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