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Mercedes air conditioner

System pressure is OK at 27psi. Compressor does not engage when AC turned on. Was told it may be AC relay. Can you tell me where this is located on this auto? 1986 Mercedes SDL

Are you measuring the 27 PSI with the compressor not running? If so, the ‘static’ pressure sounds much too low, indicating that the system is low on charge. I’m not sure what it should be for your system. Does it use R12 or R134A? If you don’t know the difference, it’s definitely time to take it to an expert.

The system is R12. Reran pressure check. 47PSI with compressor engaged and 47PSI with compressor off. No noise from compressor, so assume it is OK. No bubbles in sight glass. A/C has become slowly less effencient over past several months. System was professional serviced about 3 yr ago. Does not appear to have leaks as pressure is maintained. Looking for electrical problem, ie. relay (which was recommended on this site) to avoid major expense. Checking A/C relay was recommended. I don’t know it’s location.

So the compressor engages, and no pressure change? Doesn’t sound like a relay problem if that’s the case. Gradual loss of cooling sounds like a leak, clear sight glass might mean it’s really low. Maybe some air in the system, too?

OK, I don’t know how much you know about A/C systems, so I’m going to get a bit elementary. Are you sure the compressor is actually engaged, or are you going by the switch position? If it is truly engaged, the outside face of the electric clutch will be rotating with the rest of the compressor’s innards. My guess is it is low on R12, and not actually engaging. Good luck finding someone qualified to fix it who actually has some R12 to sell at a reasonable price. It will be far cheaper to change it over to R134a.