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.

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.

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?

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

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.

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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition