Not a new topic at all but I could use some brainstorming.

Just returned from a trip to Charleston SC, a total of 1250 miles. Drove the 1983 300CD. Car ran great as expected but ... froze our arses off on the Virginia to SC border leg.

For some time I had been having the common heater issues at highway speeds. Since I was using a cheap aftermarket monovalve solenoid, I fell for the urban legend about them "blowing closed" at high speed and just before leaving bought a high buck OE solenoid for about $130.00. After installing it there was absolutely no change in operation.

The system regulates perfectly at around-town velocities so I do not suspect the control pushbutton assembly or hose blockages. I tried pulling the connector completely off of the monovalve solenoid expecting it to default to full heat. It did not. There was no change, that is heat at low speed and no heat at high speed. The thermostat appears to be regulating the coolant temperature o.k., in the 82 to 84 degree range as expected this time of year..

I know there are a number of you out there with similar issues. Any discoveries to share?

#1 Question :

What temperature did the dashboard coolant temp read at highway speeds ? .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

82 to 84 deg C. At one point in the trip, I even tried the old trucker trick of partially blocking the radiator with cardboard. No change.

For what its worth department, I repaired and replaced everything to get heat, with limited success, on an 84 300d. What finally fixed it permanently, was replacing the controller behind the glove compartment. As it turned out, that was the easiest repair I did on the car. I plan to take the old one apart one of these days to see if the board can be repaired.

Thanks. This one never occurred to me. I'll take a look after I try couple more things.

It occurs to me that, if your heat doesn't work with the monovalve unplugged, the problem cannot possibly lie with the temp controller.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Good point. Thanks.

Today removed the entire monovalve assembly took it totally apart and inspected it. I found a big, hard clot of calcification blocking the output side (lower outlet). I was excited about this since I thought I had found the problem.

The clog was so old and hard that I had to drill it out. Reassembled and installed the monovalve and guess what; NO DIFFERENCE. Still low speed heat regulation and cold air above 60 mph.

Still stumped. Can the aux pump block the flow if it is not in the circuit? Remember, it regulates fine at those speeds where the pump is supposed to work. When the pump is not working at higher speeds, can it block the flow?

If no answers from the brain trust, I'll test the pump to see if it is working correctly, then bypass it to see if that will allow flow.

You should have already tested the aux. pump and tried it again after the clean out with the monovalve unplugged .

Keep us posted and don't despair, you or someone will figure it out in due time .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

That part you drilled was a ball check valve , pops up with spring,test by lightly pushing
down and see or feel if pops back up from top side, FSM says to replace if not working.

Mono valve can become hydraulically locked if water gets on both sides of the rubber
diaphragm,even if new part. Test,remove mono valve repair part,pull back diaphragm
shake hard to remove water, if no water not the promlem.reinstall,if now works mono valve bad.

Test water heater hoses ,is temp.hot on both sides,in and out.

What leads you to believe that the aux pump operation is speed dependent? Urban legend?

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

FSM description indicates that aux pump provides coolant flow at slow speeds.

I think that would be more of an explanation of purpose than it is a description of operation.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Yes. They mean slow engine speeds, not vehicle speeds. I just got a 1984 300TDt and it's got heater system issues in spades. Haven't gotten to probe what works and what doesn't. It's got a manual switch hooked up and a ball valve under hood but I didn't feel a lot of anything.

Anyway back to your issue. Can you sit in neutral at highway engine speed and get heat? Or better yet get hauling down a big hill on the highway and drop it in neutral and see if heat returns while coasting at speed(DONT HIT NEUTRAL BY MISTAKE!!!), rev match or wait till your slowed down alot to put it back in gear. Maybe a heater hose or component is internally failing and blocking flow under higher pressure when the water pump is running at highway rpm, the way a brake line can fail and become a check valve not allowing the brakes to release.

Sent from my SM-J737A using Tapatalk

__________________
WARNING!!! VINTAGE MERCEDES MAY MULTIPLY UNCONTROLABLY!!! I have tons of Sl/Slc parts w108 w111 w126 and more. E-mail me with needs
BirchsgarageMB@gmail.com

Nate, I think the aux pump is probably o.k. since I have beaucoup heat at slow speed but it is worth a try.

I never give up.