Short version, friend of mine has a 85 300TD that is her only car (original owner). She has no heat, car doesn't warm up. I said thermostat. She takes it too her mechanic of 30 years and he replaces something and the car overheats 20 minutes into her way home. She claims the temp gauge pegged . Excellent. Mechanic won't answer his phone.

I show up and pick her up, but first I am going figure out what went wrong. My competency with car repair is very high so don't be afraid to use some detail. For whatever reason I haven't found any good info about what this is and what it does.

yes, that is the monovalve, and it's the heater coolant flow valve.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

So it's two different parts or it is one part that has two names? Where would you suggest buying one?

Yes, mono valve, the red arrows point to it, the blue arrows point to the housing.

Seems there is an issue regarding part #293 or #125. Or perhaps the housing itself has developed a crack.

If that doesn't open, you won't get heat.

Engine overheating is probably a different issue, until the mechanic talks, you don't know what he replaced, receipt?

Good luck.

Attached Thumbnails

 

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83 SD

84 CD

And he probably didn't "burp" the system after draining the coolant.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Yes, and if the vent line from the top radiator tank to the reservoir is blocked off it will be near impossible to burp it.

To the OP, notice what Mike caught. Make sure the hose I described is in place and that the air is bled after your repair.

you were confused as to what it does, and I was explaining to you it's function.

finding it is another problem...

here on this parts sales site... the solenoid is listed as NLA...
Pelican Parts - European Automotive Parts and Accessories - Porsche • BMW • Mercedes • Volkswagen • Audi • Saab • Volvo • MINI

here's a link for the entire part... $$$

until you find out what is leaking, and why the car is overheating, I'd install a Pex ball valve, and operate it manually...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Yes, the system needs to be burped. Take the top hose off the radiator and fill with 50/50 coolant/distilled water mix. I start the engine with the heater set to "high". Of course this is after a very thorough flush of the coolant. I've used both the prestone back flush kit (only install using a spare heater hose when you are backflushing, then reinstall the original heater hose because Prestone backflush kits are known to leak during everyday driving) and the famous Mercedes Citric flush (some people have had problems with the citric flush but it seems to go fine for me).

After all that, the car may need a new heater control valve (monovalve) repair kit which is a lot less money than an entire new heater control valve. It is my recommendation to stay away from the MTC brand and go with genuine Mercedes though a bit more $$$$$$, your stress level will remain lower.

http://www.pelicanparts.com/cgi-bin/ksearch/PEL_search_2014.cgi?command=show_part_page&please_wait=N&SUPERCAT_FLAG=Y&make=MBZ&model=3306§ion=ventht&page=1&bookmark=2&part_number=000-835-06-44-MBZ

W123 ones are quite expensive, though I find these cars in the junkyard often enough now. If my housing failed (OP's appears cracked), I would just run 5/8" heater hose (I use silicone, lasts forever & more flexible) where the monovalve sat, then install a later one from a W124 (cheaper, I have one from ebay) where the aux water pump (electric) sits (front passenger side). The aux pump isn't needed (perhaps in Germany or Alaska) and can even cause a dash fire. One guy here did that. You would need to extend the monovalve wires. For now, do step1 to get thru the winter. That might even keep the babe on your leash. If the relationship progresses, work on a used monovalve. One guy even replaced it w/ a manual on/off valve for winter vs summer mode.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Just to update, the mechanic is a retired MB dealer tech with 40 years of experience. He works on a short list of cars for people that he knows. I'm fairly certain he put a used monovalve on the first time, one he had laying around. It was leaking and caused the overheat. He apologized for not test driving the car, fixed it, and dropped it off working fine.

The heat worked properly for about 2 weeks, the coolant temp would get to 80. Then the engine stopped warming up properly and was not producing heat. I drove the car myself, even with the block heater running prior to start up, after 30 minutes of driving the temperature needle was barely above 40. I made a grille block and the needle would get up to 60 and hold while driving, but there was still just cold air coming out of the vents.

The car has been back to the mechanic friend twice with no resolution, it's well over an hour drive each way, quite a trip to make with no heat for a 71 year old woman. Now he is sick and can't look at it again until further notice. My friend is concerned the mechanic might be suffering from the early stages of dementia, though I wonder that about her when she tries to relay what he told about the car and seems to have no idea. The mechanic did not return any of my calls so I'm in the dark.

I was going to try and not get in the middle of this debacle but it is getting a bit ridiculous.

I'm thinking the recent overheating took a toll on the thermostat and it is stuck open. So this is not a monovalve issue now, but the mechanic is still chasing that prior problem. Could be both though.

double post

For whatever it's worth, I experienced the above symptoms when my thermostat failed in the open position.

Were you ever able to ascertain if the mechanic actually replaced the thermostat? Or were his repairs limited to the monovalve?

A thermostat is not very expensive; I'd probably replace that and put new coolant in. Lots of info on this forum on both the proper coolant to use and how to properly "burp" the system after replacing the thermostat and refilling the coolant if you use the "search" feature.

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Christopher
'06 Mercedes E350 station wagon (silver/black)
'85 Mercedes 300D (black pearl/palomino)
'85 Mercedes 300SD (smoke silver/burgundy)
'79 Cadillac Sedan DeVille

'05 Toyota Camry (because always running is nice)

'85 Mercedes 300D sold back to orig. owner 8-1-06
'84 Volvo 264GL Diesel, owned 2000-2013

I do not know, unfortunately. Seems like there is an appointment this week.

Well, I saw the car yesterday after a short trip and noticed antifreeze dripping on the ground. Popped the hood and the monovalve was gone, the two hoses were joined by and clamped to short piece of metal tube. It was leaking at the connection, which seemed to be on tight enough but to an improper intermediate pipe. I don't know what exactly was done but she was charged $375 for this work. Perhaps it was to cover ordering another part. The saga continues. It puts out heat regardless of the dial setting.

The car needs a working thermostat and cooling system. The thermostat can be tested prior to replacing. Quality parts will work. Knockoff imports perhaps or not.

The heat system needs a working mono valve to control heat. There are also other components such as the push button unit and vacuum pods.

Deal with the mono valve and cooling system first. Send me a PM if you want a FSM to the 126 cars with similar systems.

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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.