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.

If the monovalve is functional, continuous 12V blocks the flow of coolant through the heater core. There's also the auxiliary pump between the monovalve and the engine water pump. I don't know what happens to cabin heat when the aux fan fails. How badly do you need heat between Atlanta and Houston in March?

There should be a low pressure switch in the AC system most likely at the AC drier behind the left headlight. It's the one with spade connectors on it, not the one with pigtail connectors. Electrical continuity across the switch connectors means sufficient charge to engage the compressor. Or momentarily bridge the wires to see if the compressor engages as you'd have to do charge it.

Sixto
83 300SD
98 E320 wagon

I have a simple multi-meter but am completely ignorant when it comes to using it. How can I check the voltage at the valve? I suppose it could also be possible that the new valve was faulty? I've read on here that it's happened before.

Begin by pulling the valve and examining the diaphragm. It's bad if torn. You can unplug the connection to the monovalve and note whether you have heat.

I was told when I asked that you won't be able to directly measure voltage and remember a pic or vid where someone demonstrated a working one by connecting a noid light.

__________________
85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

I left the valve unplugged a few weeks ago to test that theory out. Symptoms persisted. I will try to pull the valve tomorrow to make sure it's not improperly seated or torn. If everything looks good, what would my next move be?

The monovalve should be wide open without power so step one on the SDL is to unplug the monovalve to confirm that you get heat.

My earlier statement wasn't clear. The monovalve gets a continuous 12V when the ACC is in any mode (except off?). ACC pulses the ground signal to control heat.

Sixto
83 300SD
98 E320 wagon

Right, done that and no heat. At least, no usable heat. I can feel warmth from the defrost vent but once I start driving or increase fan speed from low, it cools.

Something here might help - 300SD Heating Problem, HELP (I'm Cold)

Is your engine staying at proper temperature of 80-85*C when warmed?

Sixto
83 300SD
98 E320 wagon

Engine temps are always rock solid right in that range. On both cars, actually.

After doing some more reading, it looks like it could possibly be the AUX water pump, monovalve, or the pump relay that are causing the no-heat issues in the 350. Hoping it's not the pump, that looks like a PITA to replace.

Quick update, the monovalve diaphragm in the 300D was completely shot. It's so deteriorated that I I almost didn't realize the two sections were supposed to be one!

I'll get that swapped pretty quickly. Maybe I missed it being mentioned in here, but will a damaged monovalve prevent the AC compressor from engaging?

The monovalve will have no effect on your compressor engaging.

The Auxillary pump is a simple job to replace. The pump is mounted in a rubber holder and will slide out of the holder. No need to remove the metal bracket holding the rubber holder. Before inserting the new pump into the rubber holder spray the inside of the rubber holder with a little WD40 or a little soapy water. This will make the new pump slide into the rubber holder easier. The rest of the job is to plug in the wires and remove replace the 2 hoses.

This is in reference to the 350? Everything I'd read lead to me to believe it was a hassle. Do you know of any write ups that would pertain to this car?