No heat on highway, but lots at idle
 

Alright guys, I'm having a bit of an issue with the Benz that I'm noticing now that the weather is getting colder. At idle I get loads of cabin heat, however when I'm on the highway, I get barely anything. :confused:

Coolant level is up, and the thermostat is only a few months old. Anyone have an idea on what could be the issue?

Car in question is the '85 300SD W126.

Torn monovalve diaphragm.


PeachParts FastLane Mercedes Parts Store

Thanks for the info. Is there a "how to" out there showing how to replace it? I'm mostly familiar with VW diesels, and don't have a manual for the Benz. (yet)

take out the 4 screws in the top of the valve, remove core, replace with new core.
on your car, it's below the windshield inside the fake firewall about the center of the car.

I understand a torn diaphragm will result in heat all the time.

Sounds opposite from a malfunctioning recirculating pump tho.

Any leaks in the system letting outside air in? Such as a pod flap that stays open.

That would reflect a deficiency in understanding. No heat at high RPM is the classic symptom of a torn diaphragm. There are plenty of reports on this forum that support that concept.

So what would cause heat all the time? That is another issue I have. Thanks

I thought a bad t-stat would cause coolant to constantly pass through the radiator. At idle, no air moving over the radiator, so he would get heat. At road speed, it would be over-cooling the engine... right?

Depends on how it failed, but the symptoms as described are classic symptoms of a torn monovalve diaphragm.

heat all the time would be a stuck open diaphragm in the monovalve or it could be a defective CCU pushbutton unit. that symptom is classic for a siezed aux pump that has burned up the CCU board.

A poor electrical connection at the monovalve would be a common cause.

Sorry, can't be the first issue, ask tango.

My thermostat is almost new. I bought the car last April with a bad thermostat that was stuck open. Replaced it in about March.

Thanks for the info guys. I'll order up that valve here later this week along with a new water pump since mine is leaking a tiny bit from the shaft seal. :(

Answer
 

Answer:
* Blown CCU fuse.
* Bad mono valve.
* Bad ground to the mono valve.
* Failed push button unit.



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Agree
 

Textbook symptom of this issue.