This week the A/C failed on my '96 E300D. Leading up to this event was a hissing noise that came from the air vents in the center of the dash. This noise is only present when the A/C was on and it had been present for several weeks. But the A/C kept working. Then about 2 weeks ago the driver side vents started blowing warmer air (ambient outside air) while the passenger side vents blew regular cold air. Now both sides are warm (ambient).

I had my mechanic look at it but they don't do A/C. He said the refrigerant systems seems OK because the system is still pressurized. I suspect a vacuum leak in the HVAC unit. Is it possible that a leaky line prevents the cold air from coming out even though the actual A/C is still running fine?

You have insufficient refrigerant flow, which causes a hissing in the evaporator and warm left-side vents.

The insufficient flow is caused by either low refrigerant level or a restriction. To tell which, evacuate the system and charge by weight. If it functions correctly, you were low. If the left side is still warm, you have a restriction.

Sounds like you have a good sized refrigerant leak. The hissing sounds are an initial indication of low refrigerant. On the 210 chassis, warm left cold right is a further sign of low refrigerant. You likely still have some refrigerant in the system, but not enough to provide any cooling. In a mild climate like Michigan most folks won't notice degraded performance until about half the refrigerant charge is gone.

Find a mechanic that does a/c and have 'em take a look. If the location of the leak is not obvious, have it charged up and dye added. When it stops working again the dye will have marked the location of the leak.

- JimY

Or he has a restriction. In fact, if his dryer is original, it is likely the first design which did tend to plug. There's a TSB on this.

Blindly filling the system is not a good idea because of the possibility of a restriction. The pressure transducer is at the dryer outlet, so the high-side pressure can spike but the climate-control unit won't know it.

Mine read 350 psi at the high-side service port, while the climate-control unit reported 3 bars.

Thanks for the replies everyone. Sounds like I better take it to someone who knows about MB A/C systems. I'll report on the findings once it's fixed.

Mike.

That's a good idea. Ensure that they charge the system by weight. Some shops charge by pressure, which works great for US cars with CCOT systems, but not as well for European cars with TXV systems.