Our 91 350SDL never had heat all of last winter and the panel would control whether you wanted air flow (luke warm) to floor or defrost. After the #5 fuse blew last week and the AC stopped working I got full 170 deg. heat out of the defrost and some flow thru the dash. I installed a new fuse and we are right back to no heat again. I would simply remove the fuse again for this winter but it also is the power for the trans electrics and makes it shift weird so we need to find out what is the problem ???

When my AC Clutch blew in my 300sdl it took out #5 also - with the same result you have. I had to put a towel over the dash vents until parts arrived.
I suspect (guess) the monovalve goes into the open position when there is no current. A master of the 126 ACC hopefully will chime in.

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TX is correct.

The monovalve defaults to an open position. It requires power to fully close.

Couple of things to check:

1) Make sure that the monovalve diaphragm is not compromised. Probably OK because you do have heat at certain times, but, you need to pull it and rule it out.

2) Turn the temperature dial all the way up to max. until you hear the click. Then run the vehicle and see if you get any heat.

Report back.

Absolutely correct.
The #5 Fuse feeds the A/C Compressor Clutch, The Auxilliary Water Pump and the ACC. If the fuse blows it will cause the ACC to switch to a 'default' mode which is Full Heat from the Defrost.
In my SDL the Aux. Water Pump was drawing Hi Amps and the Clutch was shorting out - not sure if these were related, but disconnecting the Pump and replacing the Clutch fixed the problem.

Try disconnecting each device in turn (replacing blown fuses as required) until you figure out what is at fault. With any luck the ACC will start working normally after the problem is fixed, although I have heard that in some cases it can be damaged by 'circuit' problems.

Good Luck

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1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown

Where is the aux. water pump located ? Brian, I have tried every possible method to get heat last winter with the controls and I tried to max the control knob early this morning with no change in air temp, but in the AC mode it cranks out acceptable cold air (I live in Arizona) and the control knob works when set at differrent cold temps. I also do not know where the mono valve is located, or if it is the problem. It seems to me now that after the fuse blew and I was able to see that it can produce VERY HOT air thru the defrost that there is no hot water going thru the core that the fan blows thru when the circuit has voltage. I do not understand how the system works, if it is vacuum controlled, or is this monovalve electrically controlled, or is it even the item that controls hot waterflow ??? Thanks for all of the help.