A friend told me that I should try adding freon since the compressor won't engage unless there is freon in the system. So I plugged a can of 134a in, pierced the can and released the freon with the car running and the AC on. The compressor never came on, the can's weight didn't seem to change. The can did not change temperature (aren't they supposed to get cold as the gas leaves the can?).

If the system was completely empty, do I need to add more than one can to get the compressor going or does it sound like something more is wrong here?

Thanks.

Bill

You need to jump the compressor wire from the relay as soon as you open (puncture)the can. Your compressor should engage and start sucking the freon which can be confirmed by looking at the refill tube dial near the can. If compressor does not turn on after hot wiring the compressor then your compressor might be bad.

You want to determine if there is refrigerant before you attempt to charge,
Just b/c the comp does not come on can be many different things..not just R level.
And if there is NO R pressure, then you want to pull a vac first, as the system can be full of AIR.

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A Dalton

That makes sense to me, but it happened at the same time our house hold ac needed 2 lbs of refrigerant so it made sense at the time to add it to the car.

Out of curiousity, I saw where lafooy has the same car I do and was having a problem with AC (fan speed) and you mentioned putting a jumper in place to see if the fan speed would go to high. Then hsahai mentioned in a reply to my post that I could override and force the compressor to kick in. If I have freon in the system, would forcing the compressor to kick in help me determine if it is an electrical component issue? How do I do that and is it safe to do?

Thanks a bunch.

First thing you want to check is if your compressor is ok. Try jumping switch and see if compressor turn on-- just for few seconds. Compressor will kick in even if you have low freon or no freon. Once you know that compressor is ok then follow the direction as given by Arthur.

However, one big question is - Was ac working before and how long ago.? Are you sure there is no leak in the system. If there is leak in the system them ac will not hold the freon. To check the leak do it before you charge the ac with freon. Hook up vac line and see if vacum reading is persistanct for 24 hr. Arthur can put more chime on this.

Sahai

The point I was making is you look at the system pressure BEFORE you do ANYTHING..........

Jumping the sw as suggested will make the comp come on if low on R , but there are may other conditions that make a comp not come on...If there is pressure , then you could have a simple fuse problem.

The point is you do not want to be adding R to a capacity full system.
An overcharge can kill a compressor.

So, the correct procedure is a gage . If you have 70/80 PSI with the car off, you do not have an R problem of comp start..it is another problem.
But if it does start w/sw jumper , then the comp circuit is OK EXCEPT possible low R OR bad sw.
The problem with jumper sw is if the system is empty and you have a leak on the low side, you are going to draw ambient air/humidity into the system w/jumper and the comp can be operating with less oil than needed.. That is why we verify R charge FIRST....and that is why the sw is there in the first place...they do not want the comp to attemt to start when there is no/low R ..............
In your case , low R is not the main suspect b/c you state that you could not get the R to go from the can to the system w/comp OFF...that indicates the systems equalized/OFF pressure is the same as the can, meaning you have a good charge of R already...if the system were empty, the can would empty into the system until the can and system were of equal pressures..without any aid from the compressor..same as filling a flat tire.
So , if that is the case , the suspect for No Comp is fuse/clutch/etc. from another source.

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A Dalton

Did a new seach based on the very helpful information provided and saw the mention of the switch being on the dryer. I have been unable to find the dryer until now; tucked below and behind the driver side headlight. found the following:

Glass bubble shows something gurgling underneath it - it is green, which fits with the label I saw that said a dye was applied to the AC system.

While looking at the dryer I saw two connectors on it - one black with two wires and one red with two wires. The wires were unsecured and being rubbed/frayed by a pulley.

I think I may have found the problem then; I'll need to dig into it later tonight, but if the wires aren't bad, which connector should be shorted, is it the red or black connector?

Thanks again.

The sw with the wire pigtails is aux fan and the one w/spade connectors is Comp.......

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A Dalton

Here's the damaged wiring...black connectors in foreground need jumper, correct?

The damaged wires go to the aux fan sw. Don't ypu see the Pigtail on them???
The other w/spade connectors is comp. It has NO pIgtail.

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A Dalton

Problem is all the wires go into black sleeves, so the no pigtail statement throws me off. Since terminology has thrown me off the trail before, in consumer electronics if I have a bunch of wires I want to secure I can put a sleeve over them and with some types of the sleeves I can apply heat to cause it to shrink around the wiring, creating a pig tail. Same thing, right? If so, all these wires have pigtails.

Jumpered the two black connectors and the compressor did not engage.

Well, that it the terminology your chassis uses..
So.............................
Here are your two switches and they are as I posted earlier.

PIGTAIL sensor:

http://catalog.peachparts.com/ShopByVehicle.epc?q=1994-Mercedes--Benz-E4--0-Climate--Control&yearid=1994%40%401994&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6396%3AMBC%7C1530%3AED%7C10000072%40%40E420&catid=242213%40%40Climate+Control&subcatid=242240@@A%2FC+Temp%2E+Switch&mode=PA


Here is the SPADE sensor:

http://catalog.peachparts.com/ShopByVehicle.epc?q=1994-Mercedes--Benz-E4--0-Climate--Control&yearid=1994%40%401994&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6396%3AMBC%7C1530%3AED%7C10000072%40%40E420&catid=242213%40%40Climate+Control&subcatid=242239@@A%2FC+Pressure+Switch&mode=PA

And as I also told you , if you jumper the spade sensor , I doubt that you will have that turn your comp ON b/c I suspect you have an R charge.
SO, go check the FUSE is a first start in Diagnosis.

You can now see why we check the R pressure FIRST..............................................

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A Dalton

Yes sir. Checked fuse #7, it's good. Did see mention elsewhere that if it wasn't the fuse or the low pressure switch that it might be a bad speed sensor on the compressor or a faulty CCU. Is that what is left? But just to be sure, are there any fusible links on this system, or relays that can go bad?

To answer your question, was ac working before and how long ago: I don't know.

I had a 95 E320 that I purchased and sold to my 17 year old step-daughter. Arthur helped me big time diagnosing the coil problem (I missed a step in the diagnosis process). But that car was totalled out and for a replacement I found this 94 E420. The ac seemed to blow cold but it was a 52 degreee day when I test drove it. Carfax was good; documentation not so much. But it drove fine and didn't have a CEL on (does now), using built-in diagnostic led blinked 26 times.

Thought the CEL was due to the MAF(MAS?) which was dangling loosely behind driver side air intake. Putting this back in did not change the engine behavior (fine). CEL still there.

Pulled fuse box cover off, pulled the relay cover off, found blue relay with blown 15 amp fuse. Also found wire (bare) wound around the top contacts for fuses D, 13, and H.

Could aux fan have shorted out against belt pulley for AC, blowing fuse and keeping the compressor clutch from engaging?