What is this part?
 

1985 300d

Can anyone tell me what his part is I can’t seem to locate it in EPC. The part is leaking

I'm going to take a guess. I think it has to do with emissions or something. Some sort of switch over valve?

Looks like the monovalve to me. Heat stuck on full blast?

Not Emissions
 

It’s the monovalve. This valve allows the flow of water in and out of the heater core. You can buy a kit to rebuild it. Not a very expensive item, but then again, expense is relative 🙄. That $178,000 Mercedes S550 Maybach (I think that’s what it was) I saw at the dealership last week is not expensive to some people...it’s ridiculously expensive to me 😁. I is po!

It might be more accurate to say that the valve regulates the flow of coolant through the heater core. Depending on temperature selection, the valve may or may not "allow" flow.

Thanks everyone!

The monovalve housing has a weep hole, about where the coolant leak starts. Thus, I doubt the housing is cracked, but also insure the hose there is tight. The good news is that would mean just the innards need replacing. The MTC kit cost ~$30, but some say the electromagnet is too weak to always work reliably. If so, that kit should stop the leak, but might give "full heat" symptom. There is always M-B brand for $$$. Another alternative is to install the complete monovalve from a 1986+ car. One guy fit it in the place where the aux pump sits (that part isn't needed).

BTW, your connector is clocked different than in my two 300D's. Mine is on the passenger side (i.e. turn yours 90 deg cw, looking down). Google Images to see who is right, but should work either way. If yours is wrong, it suggests the innards have been replaced before, which would be expected after 33 years.

I've had a monovalve apart many times to study its operation and never noticed a weep hole. You sure about that? The MTC kit is more like $12 to $15 but don't waste your money on it from what I've read. A genuine Mercedes kit is available for around $40 that does not have a weak spring/ diaphragm like the MTC.

Yes, you are correct. I just purchased the genuine Mercedes Monovalve Heater Repair Kit. I did some research and the consensus is.... stay away from aftermarket parts on the Monovalve.

This is excellent information!