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Old 03-16-2016, 12:26 PM
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'85 300D AC Service

Hi all,
I have an '85 300D that I'm trying to the AC to work well in.
When I bought the car, the AC would work reasonably well on cooler days, but not on hot days (Which defeats the entire purpose, of course). The car's been previously converted to 134A. I traced the root of the problem down to a bad expansion valve (leaking). I've talked to the two local european specialist mechanics, and they have both suggested that I fix the leaks, then vacuum and fill the system. I already had one of them recover the remaining charge.

My plan is to replace the expansion valve & receiver dryer and refill the system. I realize that I can't guarantee this will work forever, but I am lacking the budget to do a total rebuild. I have already cleaned and straightened the condenser coils, ensured the aux fan works, and plan on cleaning the evaporator before I refill the system. (My dash is out, so I'm also rebuilding all the vacuum pods, replacing all the vacuum fittings, and generally making sure the vacuum/electrical side is in tip-top condition). I've also rebuilt a set of HVAC controls, found a non-toasted CCU, added a 2a breaker to the aux water pump for future prevention (which I have disconnected currently, with a new one ready to install), and will be rebuilding a monovalve (hopefully I have a Bosch core in my collection that I'm going to rebuild)

My question is, what should I add as far as oil to the system? The receiver/dryer was leaking oil onto the kick panel padding, so I would assume I need to add some oil.

Are there any tips to getting the charge pressure right on a 134 converted system? I've found some charts on the correct temp vs pressures to achieve.

I realize there's been a lot of discussion on this, but much of that was "convert back to r12" from when it was still reasonably available. I have no plans on doing so. There also seems to be a lot of conflicting information, so I'm just trying to figure out the basic procedure in straight-forward terms.

I'm well aware that I should be doing a complete service, draining and replacing everything, cleaning it all, adding a better condenser, etc, but in reality, I just want the system to work. If the compressor fails in a few years, I'll have the system evacuated and rebuild it completely - but for now I'd just like to be able to drive my car on warm-to-hot days. (Have a late model car for 100+ degree days)

Thanks for any advice.
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Old 03-16-2016, 10:30 PM
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Start here - A/C recommendations for the MB vehicles

134A is garbage in these cars plus it helps kill the POS R4. You will never have good performance with it unless you toss in a parallel flow condenser.

Use 152A if you want R12 performance on the cheap. $50 buys a 12 pack of 12oz cans.

A/C Charging with Easy Cheap Safe Alternative Refrigerant - Page 5 - RX7Club.com
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