Hi All:
So after years of having a slow AC leak in the 87 300TD, necessitating a seasonal top-off, dreading the evaporator etc. Recently, the compressor started to make some noises, or better yet, was applying considerable load to the engine as to bog it down and oscillate the engine idle accordingly with the lurching of the AC compressor. After shutting off both the AC and then engine, I could hear what seems like a hissing sound coming from the compressor area and within seconds it stopped. Sort of like venting off.

After repeating, same results. At the time I only had the ability to test the low pressure side and it had 40psi while running (withing cooling range) and now that it's off it has 100psi on the low side static pressure, and has been holding that for 3 hrs at this point

I would have thought that with that hissing from the compressor, that the seals gave out, but it's holding a considerable amount of static pressure. What gives?

I guess there could also be some moisture in the system causing the compressor to bog, will have to get a vac pump on it after recaputuring the refrigerant, in either case, will need to open the system up and swap out the dryer.

My system has leak dye in it and darned if I can find a clear sign of leakage. I see the florescent dye around the low side fill port (obvious) and in the dryer peep glass, but the condenser, dryer, switches, lines and even the old hoses and fittings show no sign of dye. The compressor, so far from what I could access top side, also shows no major blow-outs, nor does the engine area fore and aft of the compressor, so....

Any ideas or insights welcomed.

I may end up changing out some pieces pending some further investigation. The system was converted to R134a by the PO some 200k miles ago, but it is a conversion with screw-on R134 fittings on the high and low side, leading me to believe the barrier hose is possibly original, although showing no signs of leakage. The compressor is also the early type R-12 for the early engines. Looking up replacements, they are classed by up to engine number 2xxx or after. Only the early one is classed R-12, the later says nothing about the gas. Both are Denso OE.

Is there a difference that anyone is aware of between the 2 types of compressors that prevent swapping one for the other? Was it based on the gas type between R12-and 134? Are the barrier hose manifolds different between them? More importantly, are the internals seals compatible with 134? If anyone has swapped one out, would be curious as to how you went about it. I'm looking to keep the system on R134a.

TIA

__________________
Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

Im no expert but i would not ever make any retro or changes to any other refrigerant other than the original r-12, it was designed to work that way.

The lurching and hissing probably indicates the valve plate has failed internally. It happens.

The earlier cars had 10PA15 compressors, the later ones were 10PA17. They use different manifolds, otherwise either one can be used.

These compressors can run R12 or R134a. It depends on what oil you run. PAO68 is my oil of choice, you can run any refrigerant with it and it is not hydroscopic unlike PAG.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Awesome info on the compressors and oil. Out of curiosity what kills the valve plate, not that I'm complaining, the compressor may be 33 years old with 310k miles on it, acceptable lifespan to me, but curious if something else I overlooked can lead to that?

Also would this require me to flush out the entire system? Should I worry about particulate matter throughout the system?

I'll try to get a compressor that is not pre-filled and use the oil you recommend. Do you know if New 10PA15 compressors come pre-filled with mineral oil? or require filling?

Thanks

__________________
Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

Normally I agree, this was done by the PO, not sure now after 20 years if the retro to retrofit will harm or help, but I guess if I have to go all the way it's an option.

__________________
Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

The valve plate can fail from a variety of reasons. Oil starvation, moisture contamination, non-condensibles (air/water) in the system, excessive oil, old age (normal wear), etc.

If you're already opening the system, take the time to do a flush and replace all the O-rings on everything while you have it apart. You'll flush out the old oil and any contaminants and start with a clean system with a known quantity of oil. The compressor will come charged with PAG46 oil, you can drain it and refill with the oil of your choice.

You will want to replace the filter/dryer as well. Make it the very last part you install before pulling a vacuum and charging.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Thanks Diseasel300.

On the system flush, do you recommend a chemical flush with a flush kit or a dry nitrogen flush? Or both? Any advantages one over the other?

__________________
Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

Chemical and nitrogen flushes have different purposes. The chemical flush is to remove oil and contaminants from the lines and coils. Dry nitrogen sweep is to blow out any moisture in the system.

Ideally you'd do both. In reality, I typically use a solvent flush followed by compressed air. I have a desiccant dryer on my air line, so the air is quite dry. I typically use Acetone as my solvent of choice. $13 for a gallon from your favorite home improvement store vs about the same for a pint of "AC Flush" which is essentially the same thing. Your money. Keep it away from paint and plastic, it will do neither any good but will harm nothing in the A/C system and dries extremely fast.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

As a side. Anytime you “top off” an AC system without purging the air out of the line you connect, it contaminates the system a little at a time.

__________________
Satan creates nothing: he only ruins everything. He does not invent: he tampers. And his followers are no different ~ Archbishop Carlo Maria Viganò

If the OP is going to stay with the 134A, does anyone have recommendations or experience about whether or not to change/upgrade the condenser?

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

134a and any other hfc refrigerant requires an efficient PF condenser.
the pressures are higher, and thus the temps are higher, and need an efficient way of shredding the heat.

__________________

1977 240D auto w/OM616.916 engine
2005 Matrix 5spd
2006 Prius

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Diseasel300:
Thanks for the recommendation, I'll do the acetone as I have that and a desiccant filter for my compressor, so I can skip the nitrogen for now.

I'm planning on changing the compressor, barrier hoses, expansion valve and lastly the drier. In looking over the components, The evaporator should be flush-able at the expansion valve connection as well as the hard lines using some extra tubing for the flush. Can I do the condenser in-place? with the drier and barrier hose removed? Also does the direction of the flush need to be in the same direction as refrigerant flow? For the condenser and evap specifically?

Sugar Bear and resago2000:

Still haven't decided on the final refrigerant yet. It had been converted to R134a prior, but that was marginal at best in a wagon. I have some NOS Autofrost R12 drop-in in stock that I had used in the past on other R12 cars with good results, so I'm likely going that route. The performance is nearly the same as R12 and would save me a new condenser, depending of course if that check out, leak free and the flush doesn't produce anything of concern.

Speaking of flushing and ordering parts ect. Do any of you see any concern in my drawing the system into vacuum for leak testing purposes (after evac of course). Do I run any negative risks in drawing the system now in it's current state? moving crud etc.

I would like to know if anything else is leaking before charging after all the work. My sniffer has not detected any leaks, nor can I see UV dye traces around any of the components nor thru the center vent, although that's the extent of access I have for the evaporator.

Thanks for your great advice

__________________
Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

1986 and onward vehicles already have adequate condenser and ideal compressor for R134a. Replacing the expansion valve is necessary for best performance. Using the 80% charging rule on these cars will result in a system that is grossly undercharged with predictably poor performance. Actual system charge with R134a and the correct expansion valve typically winds up 90-95% of the R12 charge (for example, my 350SD calls for 2.9 pounds of R12 [46 ounces], but takes 42 ounces of R134a [91% of the R12 charge] for correct charge. It will blow 42˚F on a 100˚ day on a stock system with only the expansion valve replaced.)

Charging a converted system is more art than science. Charge to 80% by weight, charge on an 85˚ or hotter day, all doors open, temp wheel on cold, fan on max. Stick a temp probe in the center vent. Continue adding gas in very short bursts, waiting 1-2 minutes between each addition until you reach a 30˚F drop below ambient temp at the center vent (if it's 85 outside, you're looking for 55 at the center vent). Very slowly add gas in very short bursts, waiting 2-3 minutes between each addition. Stop adding gas additions when you notice the temperature at the center vent stops falling. You'll be somewhere around 32-34˚ below ambient temp. Raise the engine RPM to 2000 RPM and hold it there for at least 2 minutes. There should be a marked reduction in center vent temps if you charged everything correctly.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Good plan of attack. Yes, the evap can be flushed in place with tubes attached to it, so can the condenser. It doesn't matter which way they're flushed, however I tend to flush "backwards" just so that if any crud is built up in the inlet side of things it doesn't get washed deeper into the tubes. No idea if it's better one way or another, but it makes me feel happy.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Leak testing is done under pressure, not vacuum. The lowest pressure the system will ever see is on the low side of the compressor, typically 35PSI or so above atmospheric pressure. The vacuum is done to dry the system, not to check for leaks. Even the best sealed automotive HVAC systems will only pump down to ~1000 microns of vacuum just due to the various shaft seals, O-ring joints, and other parts that are designed to retain pressure and not vacuum. If you want to do it by the book, run a nitrogen sweep, then pressurize the system to 200PSI standing pressure and monitor for at least an hour. If you have a tight system, the pressure will not move even 1 PSI. Pressure testing can ONLY be done with nitrogen since it does not change gas pressure with atmospheric temperature.

Don't waste your time pressure testing your system as it is now. You're replacing a bunch of parts. Only pressure test and leak check when you have it reassembled and ready to go.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]