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Mercedes to 134a conversion

rwhyde on Fri June 27, 2008 5:26 PM User is offlineView users profile

Year: 1989
Make: Mercedes
Model: 300SE-126 body
Engine Size: 3.0
Refrigerant Type: 134a
Ambient Temp: 95
Pressure Low: 30
Pressure High: 250
Country of Origin: United States

I converted this car a few years ago after a compressor failure and it works just OK, when ambient is under 90 degrees or so (has tinted windows-I'm in Texas). I've converted a dozen or so other cars and they all work far better than this vehicle. I know this car didn't work particularly well when it had R-12. As a result we simply don't drive it in the summer. It is now summer and this car's number is up on my project list. What is it going to take to make it have great A/C? I'm not short on tools or capability to re-engineer this system, but would like to get as much good engineering input as possible to make the right changes the first time through to make the system perform in a highly respectable manner with R134a. I'm probably conservative on my high side reading since it has been a while, I've seen pressures up to 300psi on really hot days with the aux fan running. The problem is this also makes the engine overheat-which it never did before this conversion (cooling system is in top condition-I maintain things quite well). The vehicle always runs around 85-90C in hot weather but with the A/C going and it getting into the upper 90's the engine temperature rises and eventually overheats even at highway speeds. Cools right back down if I slow down or turn the A/C off. I assumed I must be overcharged and condenser load making it overheat, so I lowered the charge and it helped some but cooling which was marginal before 50-55 degrees went to 60 degrees. So, do I have a woefully undersized condenser that can't handle the heat transfer? Is the system capacity too small and I can't get enough refrigerant in there to flood the evaporator without driving pressures through the roof?

Matt L on Fri June 27, 2008 8:09 PM User is offline

Occasionally I do hear of people being happy with 134a conversions on R12 M-B's. These people all live up north.

If you have the original condenser, the problem is not that it is undersized, but that it is an inefficient design. It works fairly well for R12, but even then it's not the best. If you want good cooling, you should install a parallel-flow condenser. You may get acceptable performance with a PFC on 134a. You may not.

One downside to installing a PFC is that you will need to have new lines made. The other is that a PFC is very difficult to flush when you next open the system. I doubt that you're considering future flushes, but the required lines do add to the cost and hassle.

If I were in TX, I would consider using a PFC with R12.

Interestingly, my '96 with factory 134a will chill you to the bone if you put the temperature on full cool. I don't have any idea why the older M-B's were so anemic in the AC even with R12.

rwhyde on Sun June 29, 2008 12:43 PM User is offlineView users profile

I have lived in Atlanta and the Mercedes was reasonably adequate there on all but the hottest days. It is a black car with tint and if arked outside in Texas-it just will not catch up. I'll try a PFC and making the hoses is not problem-I've done that many time before for other conversions. I agree that MB seems to have gotten it right on later cars. I own a couple and they are completely adequate. I'm willing to change whatever it takes to make this old MB perform up to modern standards-I'd just like to get it right the first time.

Anyone else out there that has actually completed a conversion that can comment on performance after changes?

Thanks for the input.

tony1963 on Sun June 29, 2008 1:42 PM User is offline

I've retrofit several of these with good results. I always start with a new expansion valve, receiver/drier unit, electrical switches and a complete system flush. I most, I replace a tired old compressor, too.

My suggestion is to remove all of the parts, flush the evaporator and condenser with whatever flush you use that you are comfortable using. Then, replace all of the o-rings, parts, etc. and pull a good vacuum. Charge to specifications. The condensers on these can be flushed because they are the continuous tube design.

Always replace the electrical pressure switches. With the booster fans out front, it should cool very well.

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