While waiting for suspension parts and info, I found a German website that has some information about the heater control and other topics if you search around.

This link was an interesting read about the heater automatic control:

Heizungsanlage, Funktionsweise und Fehlersuche

I was able to translate by right clicking and selecting translate.

These are links that might shorten your search time for more info on the 124 and others:

W124-Archiv

Mercedes-Benz W 124 Forum :: W124-Technik Forum :: Suche - duoventil

Mercedes-Forenliste

95 E300D

I've ordered what is probably an 'off brand' flow valve from Amazon that should provide a new coil, housing, plunger, and seals for my monovalve's water flow.

It is returnable and if quality is suspect I will do so. Price is 59.90 and has the 1147412049 part #.

I give it about a 50% chance of working but the low price prompted me to try it. Hoping it is better than a used one. If installed will keep my old one as spare.

That sucks because I plumbed a used W124 mono-valve in my 1985 300D (W123), thinking they are more available and cheaper. I got it cheap on ebay some years ago. I installed it where the "aux water pump" sits (tossed that useless part), extending the wires, w/ hoses where the OE one sat. But, mine looks a bit different than the links.

Another approach is what I did in my 1984 300D. In the spot where the OE mono-valve sits, I installed a vacuum-spring heater valve (new Ford or such, $8 ebay), controlled by a little 12 V vacuum switch (1980's Dodge). Others do similar using later M-B parts, indeed a vendor kits that. With this setup, you can see the push-button controller output. It constantly turns the switch on and off, which kind of surprised me since I thought it operated the mono-valve as a continuous "servo control", i.e. proportional between 0 and 12 V, not on-off. But, the output might be via a relay on the pushbutton module which would mandate on-off control.

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

Received 'off brand ???' from Amazon. Measured dimensions and coil ohms.

Appears to be 'well made' knock-off. Measurements are almost identical to original. Ohms are about 19.5 for knock-off. Old coil measured 12 but might have internal shorts, I don't know what new original specs are.

There is a printed part # on new part. RD38310003 manufactured in Sept 2020. Searching that part # brought up two other sources. About $35 plus shipping and $45 with shipping and 1 year warranty.

I haven't looked at the innards of the new one yet, but will after a short trip to Kentucky.

I think the flow paths are as follows for my 95 E300D, let me know if you know/think I'm wrong.

1. water flows from drivers side rear of engine to the monovalve next to battery - either through the heater or through a hose bypassing the heater.
2. heater valve solenoid on the monovalve is energized if 'heater flow' is not requested by the climate control computer. This seems to result in the solenoid being powered on most of the time. I do not believe the aux flow motor is turned on during this time.
3. If 'heater flow' is requested the flow solenoid drops out and the aux flow motor turns on. This cycle repeats with a faster tempo for more heat.
4. If heater valve solenoid fails it typically fails so there is constant flow through the heater. Designed that way on purpose for emergency heat.
5. Heater solenoid valve has two seal surfaces that control path. Top one nearest to solenoid is open to allow flow to bypass heater and the bottom one is closed to block any flow through heater. This is the solenoid energized path. When not energized the top seal is closed and the bottom opens allowing flow through heater. There are two return hoses from heater.
6. The windshield water heating path returns to the small hose connection at the bottom of the monovalve front chamber.
7. From the design, if the lower seal plunger got stuck in the closed position. The upper seal could be closed when more heat was requested.
This could result in no flow. Probably doesn't happen enough to be concerned with ( except to clean the lower plunger of corrosion if apart ).
8. Aux water pump is primarily to have good heat control during low rpm when there is less water pump flow. If the aux motor with vanes was removed and a sealing plate placed in it's place wouldn't the 'auto' heat control still work satisfactorily, especially in warmer climates?

Questions: Do the pump vanes impede water flow if motor not turning? From looking at the vanes and chamber it seems that flow would be impeded.
If so, wouldn't this slow down the flow returning to the water pump and maybe result in higher rear engine temperatures?

Is there another unimpeded path/hose of the rear engine water flow back to the water pump to allow better cooling of the rear engine area?

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Don't know that answer to your question, but the monovalve was dead in a '95 E300 I bought a couple years ago, and I replaced it with the cheaper valve for a gasser 124, which doesn't have the heater core bypass. Blocked off the bypass. No discernible negative impact after two years of driving.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC