Hey All,

Over the summer I'd been driving my '90 350SDL until it came into some transmission problems (documented here). It's drive able, but on a long trip for Thanksgiving from ATL To VA, I noticed no heat. I get warm air radiating from the defrost vent if the car is stopped, and if I increase fan speed the air turns cool. ( also to be mentioned, I believe I'm going to need to replace the vacuum pods behind the dash as I only get air from the defrost.) A/C does work and blows cold, compressor engages and everything. The mono-valve was replaced just before that long trip, and if I unplug it the symptoms persist. Fuse #14 is melted in its place on the fuse block, but appears to be intact and the horn even works.

When that all happened, I started driving my '85 300D during the winter. The heats works well and despite 330K on the clock, the car drives very well. Yesterday it got a little warm here in the A, so I went to turn the cabin temp down for the first time since November. Boom, full heat all the time. I have searched and it looks like the monovalve could be the culprit. It hasn't been replaced in my 40k miles of ownership and symptoms seem to fit. Last summer I had the AC recharged and it was working well, but now the compressor doesn't seem to engage. Could it possibly have leaked all of its freon? Belts are new.

I'm planning another long trip from ATL to Houston in a month, and would like at least one car with fully functional HVAC! Any pointers would be appreciated. The W123 can be an easy fix I think, the 350SDL has me stumped.