1983 Mercedes 300SD A/C
 

I have a 1983 300SD. The pressure is too high on the low pressure side and too low on the high pressure side. The compressor and dryer have been replaced and the system flushed. Would like to know if anyone can tell me what is causing this.

Another piece of information - did you replace the expansion valve?

Ken300D

Sounds like internal blockage...was the exp replaced? If so, was it installed backwards, or against the flow?

1983 Mercedes 300SD A/C
 

No, haven't replaced the expansion valve. Did flush the system forward and backwards. Everything is clear. Trying to avoid removing the dash to get to the expansion valve. Is there an easier way to get to the expansion valve? Is there anything else that could cause this? High pressure switch? Thanks for any info you can give me.

What are the pressure readings...and under what conditions ?

IIRC, you do not have to remove the dash to get the exp valve out on a w126. I hope the system was not flushed with the exp valve still connected.

The expansion valve is about six inches above the accelerator pedal. I believe in OEM condition it has insulation covering it that you would have to peel away. The expansion valve can be removed using 19mm and 22mm crowfoot wrenches - the area is too tight to get regular wrenches on them but is OK for the crowfoot.

My best recommendation on flushing is to remove the expansion valve and flush the evaporator directly with a flush gun. Inject in one tube and hook the other tube up to a short length of old garden hose into a pail. You'll never get it flushed properly and then cleared of flush if the expansion valve (or even a drilled out expansion valve) is left in the car.

Ken300D

1983 Mercedes 300SD A/C
 

Okay, compressor replaced (once with rebuilt, then with new), dryer replaced, expansion valve replaced. Took freon but won't circulate.

Husband is a professional mechanic but this has him stumped. Only thing not changed is pressure switch and lines. Lines have been flushed and are clear.

Help me please. I'm dying in this south Texas heat!!!

Contrary to popular belief, the A/C fittings on the subject vehicle are SAE, not metric.

Is the compressor clutch engaging? If it's not, jumper the low pressure cut-out switch.

Internal blockage usually results in excessive high side pressure. Inadequate differential pressure is symptomatic of poor compressor function or open expansion valve.

1983 Mercedes 300SD A/C
 

Okay, here is more info. Wondering if this could be an electrical problem in the climate control. Husband tells me there is no power going to the compressor. When ANY button is pushed on the climate control, the fuse immediately gets so hot you can't touch it (but doesn't blow the fuse). Have checked all grounds.

than you have a short somwhere

If it's fuse #11, try unplugging the blower motor. They have a history of drawing too many amps when they get old. He'll need to clean & tighten the contacts & replace the fuse. Jump the 2 blue wires at the dryer, then try to engage the comp by pressing the middle button on the CCU. If the comp comes on, he might be able to get some freon in the system before the evaporator ices up from not having a steady flow of air from the blower. Once there's a partial charge in the system the blue wires can be reconnected to the pressure switch. After the blower motor is fixed or replaced the system can be fully charged. If it still heats the fuse with the blower disconnected he'll have to start tracing for a short. Since you said it gets hot with any button, it's probably not the comp wiring, but could be the blower wiring or resistor under the master cylinder.

Yes I have heard that and it's probably true given that the system seems to be an adaptation of typically American-used parts.

But the sizes I mentioned do fit well, and when you are finished with the AC repair can be used for other things on the (mostly) metric car.

Ken300D