Mercedes benz R129 SL fault code 73

[jimdollar1]

Hello from Scotland. The aircon on my 1993 Mercedes Benz SL 500 is giving me a fault code 73 "A/C Compressor signal open circuit". The aircon does not blow cold. I know I can go through a Mercedes Benz test routine to see whats wrong but wondered if anyone has come across this before? Many thanks, Jim.

[Cusser]

First guess would be that low refrigerant level is getting the pressure switch to do its protection job, and thus preventing the circuit to the compressor clutch from "closing"/completing. So the center part of the AC compressor doesn't engage or turn.

Have a shop look at this, they can actually pull out and measure the refrigerant level, and find/fix any leaks. If it's never had its AC serviced before, might just be low due to attrition over the years. I had this on my 2004 Frontier, no real leakage, just attrition out the compressor seal, as designed.

[jimdollar1]

Thanks Cusser.I had it into kwikfit recently who evacuated the system then said they couldn't regas it as it had a leak.I bought an STP EzChill and it was going well and was in the green zone when suddenly the pressure went to zero however a few minutes later it went back suddenly but would not stay there for long.

[Cusser]

Thanks Cusser.I had it into kwikfit recently who evacuated the system then said they couldn't regas it as it had a leak.I bought an STP EzChill and it was going well and was in the green zone when suddenly the pressure went to zero however a few minutes later it went back suddenly but would not stay there for long.

1. Don't EVER add anything that contains sealer or conditioner to the system. Refrigerant is fine, so is appropriate type of refrigeration oil if low, and UV dye is OK.

2. Don't use Kwikfit or brake place or quicky lube places for AC work, use an independent ASE mechanic or a dedicated Auto AC shop.