94 E420 a/c compressor not working
 

I have not driven my 94 e420 in about a year. I decided to clean it up and take it for a spin. After charging the battery, I fired it up and let it idle for about 20 minutes. Every thing work fine but the a/c compressor would not engage. The blower works fine but the air will not get cool due to compressor failure.

I checked the fuses, all are ok. Any ideas?

It is highly likely that the system is empty. When the system leaks low enough, the low pressure switch interupts the compressor clutch to prevent compressor damage due to the lack of lubricant circulation.

The first step in your troubleshooting of this problem is to connect manifold gauges and check the pressures. I would make a sizable bet that you will find the static pressure to be below 80 PSI.

Also the sensors on the dryer go bad also. Same outcome.

The RPM sensor wire to the compressor is so short that sinking motor mounts can cause a connection problem. When the system is not able to compare the two RPM signals it will not allow the system to turn on. This will actually trip a fault code.

Of course those things could be causing the problem. There are MANY things that could be causing the problem, but start the troubleshooting process methodically by checking pressures. This is fundamental a/c troubleshooting.

We can start putting all sorts of possible problems in this thread, but they only serve as distractors until the first troubleshooting step is taken.

I got a can of R134 and hook it up to the a/c connection close to the firewall driverside. The can would not empty out into the a/c system nor change tempture. I put gauge on the same connector and got no pressure. That is strange. If the system is full then I understand that it would not suck anything in from the can. If the system is empty then it maybe to low to suck in the R134 from the can also. Which wires do i use to jump the compressor? Im wondering if something is wrong with the gauge.

Is the a/c connector next to the driverside firewall the correct connector to use in refilling the system?

Yes that is the correct connector but im sure your gage is bad. Even without a charge hot air inside will show 5-7 pounds usually lol. Its probably you didnt punture the freon can really when you attached the trigger/fill device maybe.

Are you trying to charge with one of those cans with a gauge instead of manifold gauges?

No Im using a/c gauges that has a low and high side. The problem is that both fitting/adapters on the gauges high and low side are the same size and you can adjust them to fit the most connector. I punch a can of R134 with a regular refill a/c hose made to connect only to cars with R134 connectors.

Srcewed hole in can, connect to the a/c system with hose from can while the engine at idle. Turn the climate control inside the cab to high. Unscrew the valve to release freon and nothing happen just freon leaking out the hose where the valve is until I open it all the way. Compressor wont engage and can doesnt get cold nor empty.

Discount the hose from the system after closing valve to can. Hooked up one side of the gauges to the same connector, got no pressure.

Maybe you know this, but have you turned the knobs on the connectors clockwise in order to push in on the Shrader valve stems?

Yes, the a/c hose is mount properly on the R134 can.

I gotta find those cds. I have misplace my service manual cd and it has be awhile since I play around on the mb, so any help is appriecate.

No, what he is saying is that your manifold hose is not engaging the schrader valve on the AC system, so no refrigerant can get in. You need to screw the hose down all the way to make sure that it is opening the valve. If it's not working, there is something amiss with your fittings.

Thanks. I was going to say that but never got back to it.

I found a manual online with some information about a/c compressor not switching on. With engine idle and rear defrost on. I discounnect the plugs on the pressure switch (S31/1) and connect those wires together. The compressor still did not kick on. The next step suggested is to test the air-conditioner/automatic temperature control (83-504)

Does anyone have a copy of the manual section (83-504) and (83-505) PDF

Far as the a/c gauge I decide to get another one to check the pressure. Im positive the the connectors were seated on the connections properly. The gauges was most likely bad.

E420 a/c fixed
 

This was crazy but the Benz a/c clutch compressor is now engaging.

I used a jump wire again to engage the clutch, once I got it turning I got the can of R134 and hooked it up and BAM!!! the compressor begin sucking the freon into the system. The can got cool and the air inside the cabin got ice cold. :D

I tried doing this earlier but could not get the clutch to engage. I figure that the jumper wire I use to connect to the compressor did not have a solid connection to the wires, so that is why it did not work at first. Now Im keeping my eye on the system to see if it hold psi.


Thank you guys for your help.

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

A/C Compressor not engaging FIXED
 

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:D