Do you get hot air through the center vent in EC mode and max cold temperature settings, and the fastest speed from the blower motor? If so, it's the MV.

by in-large the problem with heat tends to be with the monovalve. I just replaced the sleeve about 2yr ago and I think once before that....

I was having the same symptoms before, that is of inappropiate heat.

Replaced with the monovalve repair kit and everything is working fine again...I think they just wear out fairly fast. Couldn't really tell anything was wrong with the old one visibly, but the repair kit is fairly cheap so I just decided to replace it.

I have the aux water pump disconnected anyway, but it didn't really solve the problem for me...The monovalve was the culprit in this case.

If you haven't done the monovalve repair kit, its certainly worth a try and its easy to replace and much cheaper than the PBU etc.

Make sure you are getting a 12v signal (connect multimeter to connector on monovalve) and make sure you are getting 12 v when the A/C is on and cold air is supposed to be blowing...if you aren't you've probably got a bad PBU. If you are getting 12v, probably bad monovalve.

Even if you aren't getting a 12v signal and suspect the PBU is bad and don't want to spend the money, you could run a wire over from the fusebox over to the monovalve and splice it in...this is what I did before I replaced the PBU a long time ago (when I couldn't afford the PBU) and it actually worked (except couldn't automatically get heat anymore). In the winter, disconnect the connector and you always get heat.

After I bought a replacement PBU, I removed the temporary "splice" and everything is back to normal.

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1) 84 300SD 285K+ Miles (Ver Calif)
2) 84 300SD 175K+ Miles (Ver Federal)

I do get heat out of all vents, but it is intermittent and temporary. It eventually goes back to a semi normal temperature. Sometimes, though, it gets stuck on full fan and full hot, no matter what temp setting.

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David
1986 300E Anthracite + ECodes + MB Mileage Award

If you get an intermittent and/or constant behavior, could be due some rust inside the Monovalve (solenoid exactly). Sandpaper (500 or thinner) the inside of the solenoid and the piston, could help to restore its normal work. Please do this with a cool engine.

It is a good idea to get the repair kit for the MV first. Due passage of time , o-rings and rubber parts inside could be fixed to the metal part, and get damaged/cracked when disassembling it, producing coolant spilling out.

The little spring could be broken too. I don't know if the repair kit includes one. If not, you can get one taken the damaged sample. I don't have yet the lengh of this spring but, completely compressed, it must be fit exactly inside the piston.

Being this solenoid a one way device, when it reaches a 12V signal, it blocks water flow (going downwards in this case). When the 12V signal stops, the spring pushes the piston upwards to allow hot water come inside the heatercore, sucked in the aux pump direction again. This may take place several times per minute to reach the asked temp inside the cabin.

Hope this will help you.

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Thank You. This helps immensely, but, I thought the W124 MonoValve was not repairable (no kit avaailable).

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David
1986 300E Anthracite + ECodes + MB Mileage Award

Don't worry. Here you can find more pictures that explain all what you need to know.

I have had to completely fix the solenoid because its coilcore was broken, filtering coolant.

In picture "1" is the solenoid completely disassembled, with its two new o-rings, new coil, new spring and new heavy duty plastic coilcore.
O-rings and steinless steel spring were obtained at homestores.
Plastic coilcore was made with a lathe (taking the broken sample), and coiled in a electrical motor repair shop.
To calculate the amount of wire in the coil, it is necessary to know the wire diameter and its weight. So, if this should be the problem, take all the damaged/unrolled wire with you. Maybe you'll have to unsolder the coilwire extremes from the plug (pic "4B")

In pic "3" you can see the broken coilcore near the new one.
You can use (well, the lathe operator) any hard plastic capable to handle the hot coolant temp to make it. And there are a lot of plastic types.

In pict "2A" is the "hard to find" rubber sealant. If after reassembling your unit you get coolant through it, make your own soft o-ring putting silicone sealant over it, allowing to get dry before reassembling the MV.

Why all this work? (it's not too much). In Chile, where i'm from, a new MV costs about US$ 300, and i did all this for less than US$ 60.

What else?

Hope this will be useful for you guys.

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Hello All,

It was the Monovalve. Although the old one tests within range (12.6 Ohms), and seems to work (can feel the click), and looks OK upon internal inspection, it must have been sticking in the open position. The new one is in and this problem has ceased. Thanks for all the consternation on this one!

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David
1986 300E Anthracite + ECodes + MB Mileage Award