I've got HEAT! Boy, do I have heat. And, yes, I searched first.
 

The saga thus far:

Oil was changed to 10W 40 from 15W 40 in early November in preparation for the winter. Anti freeze was also partly changed (didn't drain the block, just the radiator) then.

Noticed engine temp was a little bit higher. Consistent, though.

Planned on replacing the thermostat but haven't gotten around to it yet.

Recently, the heater stopped working. I had noticed that the vents in the middle of the dash weren't functioning and I figured that this was related to the fact that I could look into the vents and see some sort of white plastic that had seemed to peel away from the top of the "vent foyer" (for lack of a more accurate term) behind the vent-venetian-blinds and fall down. I was putting off dealing with this until it was warm enough to make it more likely that I could take off the dash without cracking it. (I might crack it anyhow, but don't want to fiddle with it in this cold snap.)

BUT, now the fan blew nothing but cold air at any setting.

So, looked for mono valve and unplugged electrical connector.

Voila, HEAT!

But, nothing but. And, I was idling while this was going on and saw the engine temp drop right down to 80 after unplugging that.

If I rotate the temp. adjusting wheel until it is all the way to the coldest setting, the heat goes off. But not otherwise. No modulation of heat.

Checked fuses. All OK.

Any ideas?

Sounds like a bad monovalve. Take it out, remove the 4 scws at the top and pull out the solenoid/diaphragm assy and see if the rubber is broken. If so, a $20 part will fix it to new.

might be the monovolave combined with a bad in dash temp sensor and/or a disintegrated tube behind the glove box.

OK. I was thinking about replacing the monovalve but know less about the heating system than I do about the rest of the car. (If that is possible.)

So, is that the main purpose of the monovalve? (Heat regulation, that is.) Does it do anything else?

Thanks a lot. :)

The monovalve controls the water flow through the heater core to regulate the heat.

If you leave it unplugged, you will have full heat all the time. In the coldest position, the temp mixing flap (if there is one on the W123) will be to full cold, and the AC will run, so the air will be colder.

If it doesn't work properly connected, check the valve itself -- the rubber bellows goes bad and leaks, and if its' really bad it will corrode the armature of the solenoid and it will intermittantly stick open or closed randomly. Replacment cures it -- if the armature is corroded, you need to replace the solenoid coil as well.

If the heat won't regulate properly, there are other things to check -- the foam hose to the interior temp sensor, one of the temp sensors bad, bad pushbutton control unit, etc. that will have to be chased down.

Peter

Replacing the monovalve insert will repair about 90% of the heat/no heat related problems with these cars. Some also say it might be the aux. pump, but the monovalve usually fixes it. A simple repair.

Alan Hamm

Changing the monovalve fixed the interior heat problem on both of my 300D's. It took about 15-20 minutes to replace each of them. A very easy job.
Good Luck!

Monovalve
 

Does the 126 chassis have a Monovalve also....I have the same problem with a 1984-300SD....thanks Jim

Yes, 126s have mono valves... I think most models do... Or did.

Does the monovalve insert physically move up and down within the coil? I'm talking about the shiny cylinder that you can see poking above the assembly?

Or is it just the internal section of the monovalve that moves?

Ken300D

That shiny cylinder is held in up position by a spring. This is a position when the valve is open (full flow and heat). When electric current is applied the cylinder will be pushed down against the spring, closing the valve (no flow, no heat).

1. When the spring is corroded the valve will be closed at all times at idle or low RPM. At higher RPM the valve will open by the pressure of water. In other words you will have cold, heat only when keeping RPM high.

2. When the coil is bad the valve will be open at all times. This means full heat at all times.

3. When the rubber is broken water will enter the cylinder area. The seal doesn't appear to be absolutely tight, I think there can be some limited water in the cylinder area without affecting its operation. In that case you will have the valve operating sometimes and sometimes not.

The climate system is designed in such a way that should the controlling unit be faulty you still should be able to turn the heat completely on by rotating the wheel to the hottest position or completely off. No modulating of temp takes place. In your case, when you are able to turn the heat completely off (or completely on) but nothing in between I am afraid the valve is OK and the fault is with the controlling unit itself or the thermistor that tells the unit what temp your environment inside is.

A very strange aspect of monovalve failure or deterioration, is that the heat might work sometimes. In my 300D, I could be driving along with heat working fine. Slight adjustment, heat gone completely, never to return until a new fresh start, when the car is cold, maybe.

After replacing the monovalve insert, that is the first thing I will always check when I have a heat problem on a 124 or 126.

Alan Hamm

Just wanted to say thanks
 

I have printed out this thread and am hoping for warm weather (relatively) this weekend and decent conditions so I can make the change.

I'll be taking out the glove box, too, to see if the hose behind it is rotted out and if that makes a difference to the flow to the center vents in the dash.

I think I'll have to leave investigating the "peeling" plastic I can see through the center vents until the summer. Longer days, more light, warmer temps, fewer gumption traps. :)

Thanks everyone.

Just remember that you will not get heat from the center vents. That is by design.

I hope that this does not sound like too much of a dumb question, but does anyone have a good description of what each of the buttons on the W126 climate control do? I know what the two outside ones do, but the three in the middle I'm not sure of. I'm pretty sure that I have to replace my mono-valve so that is probably why I don't seem to feel any difference when any of them are pushed individually. I have discovered that if I hold two buttons at the same time I get a combined effect that has offered some results in changed air-flow in the cabin. Are you supposed to do this?

One last question, are you supposed to be able to get heat to the rear passengers through the vent at the end of the console?

A MB mech told me 'never', is this true?