1983 300TD MB AC System overhaul

[dendiggity]

Hello I'm new here! Thanks for having me.

My AC system works intermittently (yes I know about the leaky monovalve) and I don't know why.

I have a new evaporator/drier and TXV valve sitting in a box. The car came to me without a belt attached to the compressor. So I put one on and recharged the system (it holds vacuum!) and got the AC working...sometimes.

Not sure how to diagnose whether the compressor is failing. It spins freely but I would guess that's not the whole story. Never heard any weird noises coming from it and the clutch does engage.

So what's rattling around in my head is that my next move is a gamble - do I evacuate the refrigerant, then put in the new evaporator and valve and recharge it again (~$150 CAD) and see if it works? Or do I just sack up and buy a new compressor too (~$300 additional CAD)?

Not too interested in buying parts unnecessarily. And I really like it when I get away with a fix that defies all the very sensible people telling me to do it the right way.

Also, when it comes to the mineral oil this old system requires (it's on R12a), I suppose there's no way of knowing how much oil is in the old system? I never added any. Probably should 'top it up'? Is it true that the compressor oil flows through the whole system, not just the compressor? If this is true, I could probably add oil at the low pressure port?

Just wondering what I may be able to get away with without a full overhaul. I will replace o rings. But I don't have a compressor to flush, nor a vacuum to dry it out. A flush seems to be a pain, but not the end of the world.

Thoughts and feedback appreciated.

[Dougflas]

I would change the compressor if OEM. I believe it is an R4 and they were not my favorite. The shaft seal after 35 years is suspect. Your hoses are also 35 years old and I would also renew them. I'm sure you do not want to hear this but do it right and be done with it. I would also go back to pure R12...it is available. Just because a system holds vacuum does not mean it will hold pressure. You can rent/borrow the needed equipment from Auto Zone. You MUST pull a vacuum.

[dendiggity]

Thanks for the response.

Any thoughts on mineral oil? Where do I find R12?

A good tip that the system doesn't necessarily hold pressure if it holds vacuum. With that in mind, I'm pretty sure the system isn't loosing refrigerant.

Any ideas that would explain intermittent cold?

No Auto Zone up here in Canada to loan us the tools...

[Cusser]

Where do I find R12? No Auto Zone up here in Canada to loan us the tools...

Is R12 still legal in Canada?

[dendiggity]

R12 is illegal in Canada. Either use R12a (only compatible refrigerant that I know of with this system) or evacuate system for PAG oil and 134a. PAG and R12 don't mix!

[bohica2xo]

An effort to cut every corner and

"get away with a fix that defies all the very sensible people telling me to do it the right way"

ends is frustration and spending twice as much money - when it comes to refrigeration.

First, you need the correct tools.
A gauge set is necessary to diagnose any system issue. Both the High side and Low side pressures need to be taken at the same time.

An actual vacuum pump is needed to remove the moisture AND the non condensable gasses from the system. You need to get down below 1000 microns of vacuum.

You can't "top up" things in MVAC - that just creates other problems. Leaks must be identified & fixed, especially if you plan to use flammable refrigerants like 12a

The first step would be to get accurate pressure readings to evaluate the condition of the compressor. It may be just fine. Could be a control issue or a clutch issue.

[dendiggity]

Lots of physics with this stuff! And thanks for the fantastic reply.

Yep, I understand that NCG's will overwork the system and compressor, and yep I understand the hazards of moisture in the system, and yep I agree that I'll need a vacuum and proper set of gauges for both high and low sides.

When it comes to tools and equipment I suppose I'll need to pressurize the system and do a leak test with nitrogen first and foremost?

As per the system diagnosis, how do I know if the compressor is good or bad, or if it's clutch related or a 'control issue' once I get pressure readings? Do you have any resources / guides when it comes to diagnosing the system with pressure readings?

Thanks again for the help.

[bohica2xo]

If the system has enough refrigerant in it to function then it can be inspected electronically for leaks.

Record the high side & low side pressures with the engine stopped. Start the engine and A/C system, then record the pressures again. Raise the engine rpm to 1500 and record the pressures again.

Post the results here.