Unbelievable A/C pressure readings - need help - 1984 300SD - PeachParts Mercedes-Benz Forum

Mercedes Man · #1 08-28-2020, 01:36 PM

Hello.

I am installing a brand new 4-Seasons compressor. It has the infamous R-4 design. I had one of these brand new R-4 compressors that work well as Mercedes of this vintage goes for 5 years until it blew the front seal.

The compressor came with 3oz of oil, and I added 3 oz to the new
receiver/drier. I added 2 oz to the evaporator. (The compressor takes 4 oz, but it would spill out hanging on the engine, so 1 oz into the receiver drier instead.)

I pulled the vacuum for 1 hour, and performed the leak test for 30 minutes. Held 26.5 in at 2800 ft above sea level. There was no loss of vacuum.

During the charging, things were not doing well. With 100 degree ambient temperature, the center vents blew the lowest temperature at 98 degrees in the garage and 92 degrees in the street. I thought it was undercharged. I added more refrigerant.

At 100 degree ambient temperature, the high side came to 200 psi with the low side at 50 psi. Then the clutch started slipping. There was smoke. I looked at the gauge set and pressure was:

High Side: 50 psi
Low Side: 200 psi

I turned the engine off!!!!!

14 hours later, the static pressures were:

High Side: 25 psi
Low Side: 200? psi

Maybe the compressor was seized, so I check it by turning it with a wrench. It turned easily.

I am thinking there is a bad valve in the compressor or a clogged TXV.

What do you think?

Diseasel300 · #2 08-28-2020, 01:59 PM

Are you sure you have the gauges on the correct ports? The compressor cannot pump in "reverse". If the gauges are on the correct ports, verify that you didn't get the ports reversed on the compressor.

Mercedes Man · #3 08-28-2020, 02:21 PM

Dieseasel300,

Very good question. It is the first question I asked myself. I also wondered if the gauges were working correctly. I confirmed the pressures when I took the couplers off. The high side coupler came off easy. The low side couple came off hard with a loud pop.

I agree the compressor cannot pump in "reverse."

The R-4 compressor has a manifold plate with a single bolt holding down the suction and discharge lines. It makes it impossible to turn the hose around to connect them wrong.

Diseasel300 · #4 08-28-2020, 02:47 PM

Quote:

Originally Posted by Mercedes Man

The R-4 compressor has a manifold plate with a single bolt holding down the suction and discharge lines. It makes it impossible to turn the hose around to connect them wrong.

Unless the compressor is upside down. There's just not another way you're going to get high pressure in the low side and low pressure in the high side unless the suction and discharge lines (or ports) are reversed.

Mercedes Man · #5 08-28-2020, 03:08 PM

Diseasel300,

I agree with you.

This is crazy!

I am attaching a picture of the gauge set connected to the car.

engatwork · #6 08-28-2020, 06:16 PM

Did you disassemble everything and flush/blow it out to verify everything was clear. What did you charge it with?

tmessenger · #7 08-28-2020, 07:20 PM

Something has to be blocked in the compressor

Mercedes Man · #8 08-28-2020, 09:50 PM

Charging with R134a.

I decided to change the TXV. Some bubbly black oil came out of the bottom of the TXV. Is this the black death?

I blew the evaporator out. A few drops of oil came out.

I pulled the vacuum and charged the system to 70psi static and started the engine.

The readings were:

At idle:
High side: 15psi
Low side: 15psi

Engine off:
High: 5psi
Low: 65psi

Looks the compressor is not working. I am going to ask for a new compressor.

ROLLGUY · #9 08-29-2020, 08:13 PM

Quote:

Originally Posted by Mercedes Man

Charging with R134a.

I decided to change the TXV. Some bubbly black oil came out of the bottom of the TXV. Is this the black death?

I blew the evaporator out. A few drops of oil came out. .........................................................

Whenever a compressor is changed, the entire system needs to be flushed with A/C solvent and dry gas (if possible) for best results. If not, there may still be old mineral oil and contaminants in the system. The system should be PRESSURE tested, NOT with vacuum.

Mercedes Man · #10 09-06-2020, 06:06 PM

Without a problem, Autozone replaced my compressor with a new one. There is a heat wave going on here, and I drove the car over 200 miles in temperatures over 100 degrees with the a/c on. If it is cruising down the freeway at full speed, the air is just cool (70 degrees from the vent, 112 degrees outside). I even got caught in some slow traffic in 112 degree heat. The a/c simply does not work in that condition--open the windows. Only good for German heat waves.

I am convinced that the cover would have came off of the compressor if I did not put a bolt, washer, lock washer, and nut through the compressor. For only $5 in hardware from the hardware, I have prevented it. Picture attached.

This should be good until the seals blow out, and then I will be back to Autozone for another compressor.

For those who do not do their own A/C work, the Sanden retrofit is a better choice considering the short lives of the R-4 compressor.

As for the mono valve--it is gutted so that it flows hot water all the time. I spliced in a pair of cheap Ford heater valves from eBay into the heater hoses. The heater valves are controlled with a relay and vacuum switches. Now the heat turns on and off automatically when it should. Picture attached.

vwnate1 · #11 09-07-2020, 01:54 AM

I've had quite a few Delco R4 AC compressors last 250 + miles and 30 years...

The shaft seals seem weak to me, once replaced they keep working O.K. .

I replaced the original 1982 R4 on my 240D because why bother installing a new seal when I could go Sanden ? . I'm very pleased with the Sanden .

I'd love to hear more about the Ford hot water valves, what P/N & where to connect the vacuum, did you make an article about this ? .

My brothers '82 W126 blows hot air almost always, it's treeible to drive in his car .

I'd like to up grade the mono valves in my two W123' that use them too .

TIA,

JHZR2 · #12 09-23-2020, 02:46 PM

Quote:

Originally Posted by Mercedes Man

Charging with R134a.

I decided to change the TXV. Some bubbly black oil came out of the bottom of the TXV. Is this the black death?

I blew the evaporator out. A few drops of oil came out.

I pulled the vacuum and charged the system to 70psi static and started the engine.

The readings were:

At idle:
High side: 15psi
Low side: 15psi

Engine off:
High: 5psi
Low: 65psi

Looks the compressor is not working. I am going to ask for a new compressor.

Opening a can of R134 and waiting for a static pressure of 70 psi means nothing, IMO. The static pressure is indicative of the pressure in the “headspace” in a system that has liquid in it. If all you did was open a valve under vacuum, and let the tank re-flash, you just got the vapor to equilibrate, you didn’t get the full charge necessary.

You need to subsequently open the fill valve, with the engine and compressor running at 15 psi, which will introduce more refrigerant into the system (since it wants to be at roughly 70 psi in the vapor phase.

I don’t know that these results you reported are indicative of a bad compressor. Maybe they are... but you’re undercharged. The 65psi on the low side with engine off shows that. It is curious that high side says 5 psi... but I don’t know that this is related to the compressor versus elsewhere in the system...

Diseasel300 · #13 08-28-2020, 10:15 PM

The compressor is working fine, otherwise you wouldn't be pumping down one side and pressuring up the other. Nor would you be able to maintain that pressure.

Your suction and delivery pipes are connected to the wrong ports. There is no other way you can develop the pressures you're reporting.

ROLLGUY · #14 08-29-2020, 08:06 PM

If everything is stock, it should take 85% of the R12 charge for R134a. Look at the red label on the core support, and it will tell you how much R12 it should take. If you have R134a adapters, then the connectors should go only one way (low side port is the smaller one=Blue). If you put the adapter fittings on correctly (larger hose near the firewall=low side/blue, high side near condenser=red) then the pressures should be correct. Also, if you have any heat coming from your Mono-Valve, you won't get any cold from the vents. Try this: once you get it charged and back up and running, feel the low side hose at the firewall. It should be cold. If you still have heat from the vents, pinch the heater hose coning from the head near the oil filter stand. If you get cold air, either replace your Mono-Valve, or put a ball valve (3/4 PEX) in the hose coming from the head (or both). I usually put a ball valve on every one of my cars, as some heat gets into the core, even with a perfectly sealing Mono-Valve. If your R4 turns out to be bad, ditch it and go the Sanden route (just my opinion).......Rich