PeachParts Mercedes-Benz Forum - View Single Post - Sudden loss of AC performance, Monovalve?

Diseasel300 · #11 05-21-2020, 01:57 PM

I suspect you may be grossly overcharged. Is the suction line back to the compressor cold or frosty? Which compressor does the M120 use? 10PA15 or 10PA17? Both were used on the W126 era cars with the 17 showing up in the early 90s. The difference is the displacement (15cc vs 17cc). Keep in mind that none of the temperatures or pressures from the FSM will apply on a converted system and even less so on one running a parallel flow condenser. The parallel flow condensers cannot stack liquid in the tubes like a series-flow coil can, and they are very sensitive to charge.

Based on the head pressure and suction pressure you have at 70F, I'd wager that you're grossly overcharged and simply flooding back to the compressor. The PF condenser will hold considerably less gas than the series coil did, so your starting point for charge should be ~50% of the original weight of R12.

My preferred way to charge an unknown system is to wait for a warm day (85˚ or so) and charge by vent temperature. Set up your fans however you want and run the system on the highest fan speed, max cold, and a temperature probe in the center vent with all doors and windows open (leave the top up so you don't have sun bearing down in the cabin area). Very slowly add gas, waiting 1-2 minutes after each addition until you hit 30˚ below the ambient temperature. If it's 85 outside, you're looking for 55 at the center vent. Even more slowly add gas, with even longer waits after each addition until the vent temp doesn't change. You'll be 30-32˚ below the ambient temperature. At that point, the system should be properly charged. It doesn't sound scientific, but it's an old-school way to charge a system with an unknown charge requirement.