Dont do that for long, youll burn up the compressor. One of the sensors on your dryer is bad and saying there is no freon in the system not allowing your compressor to engage. Freon should go in until it reaches 35 psi on low side, compressor will engage for short time then shut off pushing freon to high side lowering pressure on low side then you add more freon. This is a quick on off type process takes about 10 minutes to make sure its full. You bypass the sensors and crank that compressor all the time you are going to burn up your CCU and the compressor before long.

Ok, the air condition was working perfect until this week. Now its back to the samething, the compressor not engaging.

I grab a can of 134 with oil, dye and sealer on Monday charge the system then Bam it is working perfect again. Today the compresser stop engaging again. Look around the a/c lines looking for leaks and could not find any. Press the valve for a/c connector to charge and the Feon shot out with great force.

What is going on here? Hook up a 134 can and jumped the compressor and it still would not engage. Im puzzle at this point. I plan to get a gauge and see where the pressure stand. Any ideas to get this compressor to work. I did notice once I first start the engine the compressor engage for 2 seconds then stop.

Dirty/failing speed sensor on the compressor. Also use a reader to check the A/C system as it will store some codes. Maybe the pressure sensor on the drier is bad also, mine was on my 400E.

How to clean the sensor. Isn't the sensor located on the compressor?

Its integrated, I havent taken mine off as I have been told you have to empty the system to remove it to disassemble and clean it. Look at benzworld.org if no posts here on it. Mine works more often than not but my sampler valve (interior temp) and exterior sensor (under wiper motor) are suspect also so I will get to those first.

I got to figure out the speed sensor issue.

This morning compressor engaged and a/c cold again. Got on the highway drove high speeds a/c worked fine. After about 80 miles or so I stop at gas statition, got back on highway a/c worked then stop working after 10 miles. Played with the CCU a/c got cold again. About 20 miles more on the highway and it stop working again.

Any ideas? I plan to still get a gauge to check psi.

See post #4 from April 24. Have you checked the codes? It would be a code 6 on pin 8 the base module. If that's the problem you just need to splice a 4 to 6 inch extension on the sensor wire.

Ok, I will scan the system and let you guys know the codes I get.

Deanyel, will the wire communication error still cause the compressor not to engauge eventhought I bypass and jump the pressure switch to the compressor.

Yes, power to the compressor has to pass both those tests - either failure will cutoff operation. There's a wiring diagram on the 124 CD that will give you color coding for all the wires going into the compressor.

Wiring diagram here too - http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1994/E420/SYSTEM%20WIRING%20DIAGRAMS/4976.pdf

After several weeks I finally solve the a/c compressor not engaging problem.

Thanks for each and everyones of your input to help me solve this issue.

The problem turned out to be the ECU Base Module (N16/1). After reviewing the wiring diagram and scanning all the modules I concluded that Base Module had a glitch in it that cause the a/c compressor not to work correctly.

The W124 E420 engine bay is extremely hot as you all know and years of heat stress place on the module cause the Base Module (N16/1) to malfuction. One of the issues that I had is once the BM consume extreme heat the BM would cause the a/c compressor's magnatic clutch not to engage.

In addition, I also contribute to the problem to cause the BM to malfuction is an issue I had with the Stop Lamps (S9/1). The Stop Lamps connects has some corriosion on them and cause some wiring issues that result in knocking out my Fog lights and creating crazy codes in the ecu modules.

In closing, once I replace the BM (N16/1) with another one I had sitting around for years all the a/c compressor issue were solved