AC homework summary - correct? - PeachParts Mercedes-Benz Forum

Cr from Texas · #1 02-25-2009, 12:53 AM

Ok, I've spent a lot of time thinking and doing my homework and need the pros to confirm and answer a couple of questions.

Vstech, Larry Bible - are you reading this?

1983 300D turbo - bought new in 1983. My AC compressor has locked up. Everything is original except for one line. Well, not quite - the expansion valve and the drier have been replaced when repairs were made and the compressor was rebuilt two years ago (mistake).
I'm still using R12 and plan to continue as I have most of a 30 lb drum that I purchased several years ago. Given the cost of new R12, I want to do this right the first time.

Here's what I know: Compressor is locked up and at least one line has an oil residue on the outside of the rubber portion. I assume, the lost oil lead to the compressor failure.

My homework on the forum has lead me to the following conclusions and questions:

1. Not much point in risking R12 loss so I'm going to go the new compressor route rather than rebuilt or used. Prices for new R4 compressors range from less than $200 to over $500. Are they all equivalent? Are there certain brands to seek out or avoid?
2. All new lines are expensive and it seems a better quality and lower cost option is to repair the lines by replacing all O rings and rubber portions. There seems to be newer and improved AC hose material of multiple wall construction. It may be necessary to braze new fittings on the original metal lines to accomodate the new style hoses and fittings. Any preferences for multiple wall types? Some use nylon and others use aramid. Looks like some of these systems end up being easy to replace the hose portion in the future.
3. All components must be flushed. It seems the best way to flush the evaporator is to drill out the old expansion valve to allow the flush to pass through and flush from under the hood. Then replace with a new expansion valve. Right?
4. Add oil to the new compressor and new drier. Looks like there are some newer synthetic oils that are rated for both R12 and 134a. Seems like the way to go?
Prefered brands?
5. Reassemble all components using the O ring sealant additive (stays soft and helps form the seal).
6. Vacuum the system to see if it holds 29 in of vacuum. Retighten if it doesn't.
7. Do a static charge with 2 oz of R22 and dry nitrogen to 70 psi. Check for leaks with electonic detector. Good, increase nitrogen pressure to more than 100 psi (how high should I go? Hoses are rated up to 500 psi but I assume this is too high. 250 psi? Check for leaks with electronic detector.
8. All good, evacuate system and fill with 2.9 lbs of R12 through the high side port as liquid (system not running).
9. Start the system and check pressures and vent temperature.

Sorry for the length of the post. If I get it down right, I'll add some pictures as I do it.

Up until last year, the AC of this car has been one of the main reasons for keeping the car as a daily driver. Even on the worst Texas days (temp over 100, high humidity), this was the best AC unit I've ever ridden with. In a matter of minutes, passengers were getting too cold. I'd like to get back to that point.

david s poole · #2 02-25-2009, 01:10 PM

all that you have written looks thorough and if followed will work well.my experience has taught me that r4 compressors come dry[no oil]and it is imperative that you add some oil and using a vise to hold the clutch rotate the compressor until it becomes easy to turn.if you don't do this you risk tearing up the new compressor in a matter of 5 mins.

Cr from Texas · #3 02-25-2009, 03:26 PM

David,
Thanks for the vote of confidence.

Can you address any of my questions?

Hose recommendations?

Max nitrogen testing pressure?

JimFreeh · #4 02-25-2009, 06:56 PM

The new hose configurations are designed to attenuate the loss of R134a. since you are continuing with R12, this is not such an issue.

For cost consideration, replacing the hoses at a hydraulic shop will give you adequate service, just make sure to index the fittings before swaging...

New OE hoses will be more expensive, but would potentially give a better fit and look under the hood. Replacing hoses will shorten the metal line just before the swage, sometimes there is not enough metal to reuse the fitting.

Use the new style green o rings vice the original black ones.

No problem using the new oil, it's compatible with both refrigerants. Adding oil to the new compressor is critical, as is overfilling the system. Do some archive searching for the amounts of oil in various components, once you flush them, they will have to have the oil replenished.

Since the old compressor locked up, you will need to make sure that you thoroughly flush both the evaporator core and the condenser. The proper chemicals and air pressure will take care of this. Since your replacing the lines and the desiccant (drier), these two components are all you need to flush.

There have been several threads about compressors, the general consensus is to buy new, not rebuilt. The MB compressor is very similar to the generic GM, but threads are metric vice SAE, and there has been some discussion about the MB location being upside down vice the typical GM installation. (See archives)

You can also test fill the system with propane to check for operation and leaks... Static pressure is around 90 lbs.

Jim

TOPGUN · #5 02-25-2009, 10:41 PM

I would use more than 2oz. of R22. It seems to be such a small percentage of the 2.9lbs. If you have a very good electronic detector, it would probably be ok. I have no technical reason for my opinion, just many years of hvac work. (too many) Good Luck

Cr from Texas · #6 02-25-2009, 11:47 PM

Thanks for the great input. My confidence level is building.
Any particular brands of compressors to avoid? Just doing a search for compressors intended to fit the 83 300D, the price range is less than $200 to over $500. Is there a difference in new quality to make it worthwhile to pay the higher price?