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  #1  
Old 09-19-2009, 07:53 PM
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134a question

Hi, asking this not because I'm currently thinking of going the 134 route on my w123, but because I do have a retrofiton my 91bmw which works great.

My understanding is that the high pressure cutoff on a 134a retrofit should be tuned to allow higher pressures than in an R12 system. Atthe same time the pressure is stressful on the components.

I do not have a phase diagram for the refrigerants... What happens if you keep the settings the same as for r12, but have 134 in there?

Thanks

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #2  
Old 09-19-2009, 08:05 PM
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R12 systems typically do not have high-pressure cutout switches. Adding one is a required part of a 134a retrofit.

In any case, the HPCO is only for exceptional conditions, such as idling with a defective mechanical or electrical fan.
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  #3  
Old 09-19-2009, 09:19 PM
LarryBible
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If the car is currently R12, why would you even CONSIDER converting it to 134?
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  #4  
Old 09-19-2009, 09:53 PM
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Because I did in 2003 when the compressor bearing started getting really noisy... Didn't really know any better, but it has worked great and holds charge...

Just curious about that stuff, haven't checked a phase diagram, but at some point it may get too cold...
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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #5  
Old 09-19-2009, 10:49 PM
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Currently...

There are NO R134a questions if you own a Mercedes that was equipped "As New" with R12a.
R134a is a fool's involvement in Masochism.
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  #6  
Old 09-19-2009, 11:00 PM
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That's R12, CI, but otherwise you are right.

Mine came with 134a and will freeze you out, but it was designed for ti.
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  #7  
Old 09-20-2009, 03:01 AM
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Quote:
Originally Posted by compress ignite View Post
There are NO R134a questions if you own a Mercedes that was equipped "As New" with R12a.
R134a is a fool's involvement in Masochism.
please read my post- it is a BMW, I did it tears ago, and it works great to date. I'm asking to gain a grasp of the effect of running 134a at "lower pressures", in the interest of longevity - can't stop optimizing.

This board has a wealth of info, thus why I ask .

Lots of folks misread my post...
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #8  
Old 09-20-2009, 06:51 AM
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I have two W123s. A Euro 300TD and a solid 240D that have good working AC systems still running well on R12. I have no plans to convert them. But, I also have another rusty 240D, that had a broken AC condenser for many years. So I decided to experiment with an R134 conversion. The compressor is definitely working harder and cooling is weaker than the other R12 systems, but it's mostly adequate for the limited use I give this car. I'm glad I didn't waste my precious R12 here because it turned out this 240D still has a slow leak somewhere - probably from the old, rebuilt compressor.
This 240D just isn't worth the cost to restore the AC.
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Old 09-20-2009, 08:01 AM
LarryBible
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I still don't get it! With R12 prices back down to something only SLIGHTLY more costly than R134, WHERE is the logic in converting?
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  #10  
Old 09-20-2009, 08:59 AM
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Originally Posted by LarryBible View Post
I still don't get it! With R12 prices back down to something only SLIGHTLY more costly than R134, WHERE is the logic in converting?
Typically, I've had to look for R12 online, or at flea-markets, or with friends that happened to have a few old cans laying around. Cheapest I've paid for R12 is around $20/can. I can find R134a most anywhere, anytime. Sometimes as low as $5/can. That's alot easier and alot more than 'SLIGHTLY' cheaper.
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Old 09-20-2009, 10:52 AM
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So if you owned an R12 car that had been equipped with R134a, which was cooling well at speed and ok at idle, would you back convert it?

-Jason
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  #12  
Old 09-20-2009, 07:04 PM
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Quote:
Originally Posted by LarryBible View Post
I still don't get it! With R12 prices back down to something only SLIGHTLY more costly than R134, WHERE is the logic in converting?
Sorry, Larry... I don't get it. You are a huge help on the board, but please note my question - in reality it has NOTHING to do with MBs, w123s, or even diesels. I asked it here because I frequent the DD, and folks like yourself are super-knowledgable.

All I am asking is a general question which has more to do with thermodynamics or the operation of the cycle, than about any vehicle...

It simply is that if there is a high pressure cutoff, installed because 134a can go to very high pressures under normal operation (compared to R12), what happens if you reduce the cutoff pressure artificially low.

I see some good points, such as less stress on the compressor and lines.

I see some negatives in that in my experience, there is a low pressure point where there is sufficient pressure for the compressor to run, but the refrigerant actually expands to TOO low a temperature, effectively freezing up the system.

I'd love to have a good grasp of the trade space. Should I re-fit my BMW to R12 since it is now cheaper? Maybe... I guess it depends on what type of lube oil is in there, right? But that is a question for another day... All Im asking is the effect of causing the system to always run at a lower set of pressures... That's all.

Thanks!
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #13  
Old 09-20-2009, 07:32 PM
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Quote:
Originally Posted by JHZR2 View Post
It simply is that if there is a high pressure cutoff, installed because 134a can go to very high pressures under normal operation (compared to R12), what happens if you reduce the cutoff pressure artificially low.
You misunderstand the reason for the HPCO switch.

R12 systems typically do NOT have any sort of HPCO switch.

All systems have a relief valve in the compressor, to prevent a hose from bursting and causing damage to other parts, or to service people.

All 134a systems, and all retrofits, are required to have a HPCO. On the Benz, this is implemented as a binary switch which replaces the pressure switch on the dryer. All replacement pressure switches have the HPCO built in, but the switch fitted at the factory did not.

The HPCO is there to shut down the compressor BEFORE the refrigerant vents from the relief valve in the compressor, so you don't vent refrigerant needlessly. The HPCO is not at all used to cycle the compressor in any case.

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