I need some A/C help on A Diesel Mercedes PLEASE!!!

[Motorhead]

I have been working on this 1987 Mercedes diesel wagon for a guy. It came to me with the A/C not working and the compressor front seal leaking profusely.

HERE IS WHAT I DID: I pulled the compressor and sent it out to a reputable place in Florida for a reseal and rebuild. It uses a small rotary type. I replaced the CONDENSOR, RECEIVER DRIER, EXPANSION VALVE. I blew out all of the lines.
I added the recommended amount of refrigerant oil. I evacuated down the system for 30 minutes. I charged the system and fired it up.

PROBLEM: I am running too high on the high side (275psi to 300psi) and too low on the low side (10 psi at idle and down to 0 at rpm)

I am running Duracool refrigerant (replaces R12). Ideal pressures with Duracool should be 24-28psi on the low side and 175psi on the high.

To say the least... ... This car really has me frustrated!!! I have had it for 3 months doing a bunch of stuff to it. I want to see this thing go BYE-BYE!!!!!BIG TIME!!!!

ANY AND ALL HELP MUCH APPRECIATED!!

 

[Skydiver]

Is the electric fan working ? Could the radiator be clogged with debris? Try misting water on the condensor for a minute to see if the high side comes down.

Does this Mercedes run the Delco compressor? I replaced 3 on my old 81 300d.

Make sure the heater control valve is sealing off . I also replaced 3 heater control valves.



If all of the above is OK the sealed system could be clogged. Did you use alcohol to flush the system? Did you see black crud come out when you blew out the lines . "Black death" is what we call it.

 

[JoeBioDiesel]

If you're sure the charge amount is correct, and the high side is too high. . and the low side is too low... it sounds like the expansion valve might be sticking closed. Or maybe it is not getting a good indication of actual evaporator temp. I know it's new, but that doesn't mean it's good.

My 2 cents. (actual value)

Joe

 

[Motorhead]

Skydiver, The condenser is NEW. It doesn't get that hot as tha ambient temp is in the low 60's. It doesn't have the Delco/ Harrison R-4 compressor like the earlier MBZ's used. There didn't deem to be any black crud or debris in the system. Fortunately, the compressor didn't grenade.

If you're sure the charge amount is correct, and the high side is too high. . and the low side is too low... it sounds like the expansion valve might be sticking closed. Or maybe it is not getting a good indication of actual evaporator temp. I know it's new, but that doesn't mean it's good.
My 2 cents. (actual value)
Joe

Joe, I am leaning to the expansion valve also. It is a bit of a @#$%^ to get out but that is car work these days. The expansion valve does not have a temp probe to go into the evaporator. It just has a small capilary tube on the expansion valve itself. Fortunately the valve is only $20. 00 my cost. I just threw out the old one too... . I could have studied IT.

 

[Skydiver]

It sounds like an internal restriction.

Correction:When I said alcohol earlier I believe that should be acetone that is use to flush.

 

[MOates]

Might also check if the electric fan is working properly. For A/C use, it's supposed to run at low speed based on pressure.



Easy check is with the key in the run position (engine not running) and a/c mode selected on the climate control, short together the wire leads for the pressure switch. Electric fan should kick on in low speed.



If it doesn't, there's another likely cause besides the pressure switch. There's a large ceramic resistor, behind the drivers headlight and under the receiver drier, that's in the wiring to the low speed circuit of the fan. If this resistor is bad, fan won't be getting any juice for low speed ops.



That was the problem on my '87 300D Turbo sedan. Once I replaced the resistor (PITA to do without pulling the headlight), it worked fine.



Electric fan high speed is only supposed to kick in on high coolant temp. To test this, find the 3 pin coolant temp switch on the top front of the engine, right at the rad hose/thermostat connection. Short between the two leads that are side by side on the harness connector, and the fan should kick on in high gear.

 

[Motorhead]

Might also check if the electric fan is working properly. For A/C use, it's supposed to run at low speed based on pressure.

Easy check is with the key in the run position (engine not running) and a/c mode selected on the climate control, short together the wire leads for the pressure switch. Electric fan should kick on in low speed.

If it doesn't, there's another likely cause besides the pressure switch. There's a large ceramic resistor, behind the drivers headlight and under the receiver drier, that's in the wiring to the low speed circuit of the fan. If this resistor is bad, fan won't be getting any juice for low speed ops.

That was the problem on my '87 300D Turbo sedan. Once I replaced the resistor (PITA to do without pulling the headlight), it worked fine.

Electric fan high speed is only supposed to kick in on high coolant temp. To test this, find the 3 pin coolant temp switch on the top front of the engine, right at the rad hose/thermostat connection. Short between the two leads that are side by side on the harness connector, and the fan should kick on in high gear.

Hey Thanks!! This car has been a royal pain. I'll let you know what i find. I have another expansion valve ordered.

 

[MOates]

If I remember correctly, the website address is ManualsUnlimited.com - Mercedes, Ford, Nikon Service Repair Manuals . You can order on CD, or download in a zip file, the MB service manual for W124 class cars - if that's an '87 300TD wagon you're working on, it's a W124 class.



I downloaded it to my hard drive about a year ago after buying my '87 300D sedan. Cost about $25 from what I remember.



Definitely agree that these beasts can be tough to work on - MB indeed had some "unique" ideas on how to set up their cars!!!



On mine, there was a sticker under the hood where the previous owner had it converted to R134 back in '01. Strange thing was they used ester oil instead of the more common PAG oil. Perhaps it's because I've heard that on the older MB's PAG oil tends to soak thru the hoses. Also, since it was done at a shop, they didn't bother installing a R134-type low pressure connection. I had to install an add-on adapter myself to be able to check pressure and recharge the system if necessary.



If you've got any questions, I can try digging thru my copy of the service manual and see if I can find some of the answers for you.



Another thing I just remembered - if the compressor is switching on when it isn't supposed to (ie, in economy or vent mode) or vice versa, it's most likely due to the push button climate control unit in the dash. They had a problem with cold solder joints (poor connections) for the switches on the circuit board. You can pull the push button unit out of the dash and easily disassemble it far enough so that you can take a soldering iron and reflow the solder on the six solder joints on each of the pushbutton mode switches. That should cure the problem.



Same thing on my '87 300D - compressor wanted to run in every mode. Pulled the control unit, resoldered the switches, works like a champ now.