Resurrecting a dead A/C - Where to start? - Page 4 - PeachParts Mercedes-Benz Forum

Diseasel300 · #46 04-20-2017, 11:41 AM

I don't know of a reliable way to test the expansion valve. I did a visual inspection on mine, it wasn't leaking, the powerhead was still sealed, and there was no corrosion inside or outside. It was a gamble whether or not it was ok, and it seemed that it was fine.

R12 and R134a have different performance characteristics and different flow rates. Someone correct me if I'm wrong, but R134a operates at a slightly higher head pressure and flows less mass of refrigerant than R12. If you use an R12 metering device (expansion valve or orifice) you'll be flooding the coil. Conversely, if you're using an R134a metering device with R12 refrigerant, you'll be starving the coil. Either way, you get less than ideal performance.

BillGrissom · #47 04-20-2017, 10:40 PM

I have never read that you need a different expansion valve for R-12 vs R-134A. There might be a slightly optimized design for one refrigerant, but not enough to be significant. As I understand, the expansion valve works by varying the flow to control the downstream temperature (in evaporator). On older 1960's cars it is separate components (valve + gas thermometer) so you can see how it works (at least my 1969 Dodge Dart). GM often used just a fixed orifice, i.e. no temperature control. Since it controls the flow, you shouldn't have "flooding" or "starving".

Re "do expansion valves fail?", I put a new one in the rear evaporator of my 2002 T&C van when changing the compressor and then got "no rear cooling". My wife blamed my using Duracool refrigerant (how?), though the front evaporator cooled fine. I replaced it w/ the original rear expansion valve (quickly, as refrigerant boiled off) and it then worked fine. The new expansion valve must have been stuck shut. I can't say it failed rather than was manufactured wrong. Since the expansion valve was only $8 I had thought better to change, but new isn't always better.

Diseasel300 · #48 04-21-2017, 12:14 AM

Expansion valves are optimized for a given capacity with a given mass flow rate. The orifice size does matter, the same applies in refrigeration and residential HVAC. For example, you can't use the same expansion valve for R22 or R410a, even for the same coil. If converting to 134a, just replace the expansion valve with one set up for 134a, they're not expensive.

vwnate1 · #49 04-21-2017, 09:43 AM

Thanx for the discussion .

In my r134a research, I was told that R134a works best a slightly _lower_ pressures, I took notes but don't have them handy....

Diseasel300 · #50 04-21-2017, 10:40 AM

134a runs a slightly lower pressure on the low side. High side is extremely variable depending on the ambient temperature and airflow over the condenser. 134a doesn't condense as easily as R12 does, so on a hot day (90˚ and up) the head pressure goes high and cooling efficiency suffers.

Father Of Giants · #51 04-21-2017, 03:26 PM

Quote:

Originally Posted by Diseasel300

134a runs a slightly lower pressure on the low side. High side is extremely variable depending on the ambient temperature and airflow over the condenser. 134a doesn't condense as easily as R12 does, so on a hot day (90˚ and up) the head pressure goes high and cooling efficiency suffers.

Where can I find a reliable source of R12?

Diseasel300 · #52 04-21-2017, 07:35 PM

Quote:

Originally Posted by Father Of Giants

Where can I find a reliable source of R12?

Ebay, Craigslist, dark back alleys...

I used R12 because I have a 30 pound can from 1991. Other than the SL, I have no other vehicles that use it or know anyone else with an R12 vehicle, so I figured why not use it.

Father Of Giants · #53 04-21-2017, 08:40 PM

Quote:

Originally Posted by Diseasel300

Ebay, Craigslist, dark back alleys...

I used R12 because I have a 30 pound can from 1991. Other than the SL, I have no other vehicles that use it or know anyone else with an R12 vehicle, so I figured why not use it.

How much do you have left? Could I potentially buy some?

vwnate1 · #54 04-23-2017, 06:31 PM

Am I the only one who's entire login/password info suddenly became " unknown " over this weekend ? .

Anyway, R12 is out there and it pops up in the darnedest places ~ I find old shortie (?14 Oz. ?) cans at yard/tag sales and usually buy all of it .

I also find it at Auto Jumbles although it's illegal to sell so they don't usually display it .

Frustratingly, one fellow I know has been telling me he has a giant 100 # tank of R12 his buddy dumped on him about 15 years ago, he keeps telling me he wants to sell it but when I ask " how much $ ? " all I get is silence

.

He has no use for it so I don't know what the deal is there .

Of late the price of shortie R12 cans has dropped a lot ~ it used to be close to $40 / can, now I often find it for $10 .

vwnate1 · #55 04-23-2017, 06:33 PM

THANK YOU .

This is what I'd read else where and was even given the ideal lower low side pressure to charge to .

I have it jotted down some where and hope I can find it .

-Nate

"134a runs a slightly lower pressure on the low side. High side is extremely variable depending on the ambient temperature and airflow over the condenser. 134a doesn't condense as easily as R12 does, so on a hot day (90˚ and up) the head pressure goes high and cooling efficiency suffers. "

Bimmer-Bob · #56 05-12-2017, 10:24 PM

How impossible is AC work for a noob? Mine is dead, and from what I've been told the evaporator needs to be replaced. I'm pretty sure the dash needs to be pulled for that, but I'm thinking about pulling the enitre interior over the summer, so it might be a good time.

My main concern is that besides lacking any sort of knowledge, I also don't have any AC-specific tools or anything. And even if I did replace the evaporator, what else needs to be done? I mean, obviously the system needs to be recharged, but does it need to be flushed first? I guess it needs to be leak-tested, too.

The issue is that functioning AC would add greatly to the perceived quality of the car, but fixing it professionally would be ludicrously expensive. Parts alone are within the realm of possibility - the question is, can I acquire the know-how needed to do the work myself?

Diseasel300 · #57 05-12-2017, 10:44 PM

I'd say you could. It isn't rocket science. The dash does have to come out to replace the evaporator, so if you're pulling the interior, do it at at time. While you have it out:

- Replace the expansion valve if you aren't using R-12
- Flush the lines and condenser with a flushing solvent of your choice
- Replace every single O-Ring in the system
- Drain the compressor of oil and refill with the appropriate oil (PAG for 134a, POE for R152a or any "blend")
- Install a new filter/dryer
- Install new Pressure and Temp switches on the filter/dryer
- Install new Schraeder cores

Once you have those things done, pressurize the system and test for leaks. If you pass the leak test, pump down the system into a deep vacuum (you'll need a vacuum pump for this) and continue letting it pump down for ~30 mins to make sure the system is very dry. If you have a ball valve or a manifold gauge with a shutoff knob, shut off the vacuum and keep an eye on the system. It should hold vacuum. If all is good, recharge with the refrigerant of your choice.

I'll tell you up front - the worst part of the entire job for mine was that expansion valve. Since you'll have the interior out, it should be much easier, but still be prepared for lots of swearing.

ROLLGUY · #58 05-15-2017, 10:41 AM

Quote:

Originally Posted by Diseasel300

I'd say you could. It isn't rocket science. The dash does have to come out to replace the evaporator, so if you're pulling the interior, do it at at time. While you have it out:

- Replace the expansion valve if you aren't using R-12
- Flush the lines and condenser with a flushing solvent of your choice
- Replace every single O-Ring in the system
- Drain the compressor of oil and refill with the appropriate oil (PAG for 134a, POE for R152a or any "blend")
- Install a new filter/dryer
- Install new Pressure and Temp switches on the filter/dryer
- Install new Schraeder cores

Once you have those things done, pressurize the system and test for leaks. If you pass the leak test, pump down the system into a deep vacuum (you'll need a vacuum pump for this) and continue letting it pump down for ~30 mins to make sure the system is very dry. If you have a ball valve or a manifold gauge with a shutoff knob, shut off the vacuum and keep an eye on the system. It should hold vacuum. If all is good, recharge with the refrigerant of your choice.

I'll tell you up front - the worst part of the entire job for mine was that expansion valve. Since you'll have the interior out, it should be much easier, but still be prepared for lots of swearing.

I would change the order of your list to:

- Install new Pressure and Temp switches on the filter/dryer
- Install new Schraeder cores
- Install a new filter/dryer
The drier should be the very last part installed, especially in a high humidity environment. Although it isn't the end of the world if done any other way, it is good practice......Rich

Diseasel300 · #59 05-15-2017, 04:53 PM

6 on one hand, half a dozen the other. The system is going to be open the same amount of time to do the same things. I just installed the switches in the filter/dryer while I had it in my hand. Once it goes back in the car, the system is full of air anyway. My climate is so dry that I didn't even consider the assembly order! That and the system had been in appalling condition for 10+ years anyway.

Diseasel300 · #60 05-26-2017, 11:51 AM

Well the expansion valve has given up. Guess we're going to find out how well the modern replacement works with R12. For the last few days it's been hunting, you could hear the refrigerant surging through it as it hunted. Last night it finally stuck open. I've had it come back a couple of times since then briefly, but then it opens up and gets stuck again. Manifold gauge says high side is ~125 PSI low side is ~60 PSI. When the expansion valve does cycle down, the low side goes very low (~10 PSI) and high side goes very high (~250-300PSI), then suddenly releases. Sometimes cycling back and forth several times before sticking open again. Irritating. Very irritating. Been working PERFECTLY up until now.