I accidentally shorted the wires to the monovalve when the key was on and now there is no voltage going to it meaning coolant flowing through it and HEAT. I checked all the fuses and none of them are blown. Does the ACC drive it? Anyone got a shematic of the monovalve and how it's powered?

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

One of the monovalve terminals should have fused power at all times when the key is in "Run" or "Start" -- check with a voltmeter between the terminals (one at a time) and ground. I no longer have a 123 so can't say which fuse.

The other terminal is a switched ground and yes, it goes through the ACC. Shorting the monovalve terminals has probably blown something in the ACC. If this is in your '83, that design had no internal protection like the design in the later W124 cars. You may have smoked the ACC.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

http://userweb.windwireless.net/~jimc/mamerepairs.html#acctest

Some info at the above link. Info on schematic sent.

Check fuse 14, but there's no guarantee. Per other posts and the schematic, the monovalve receives +12 from the the fuse box and the ACC modulates ground to close it. Smoking the ACC shouldn't affect +12 at the valve relative to ground. Yes, it would mess up a reading across the pins, but shouldn't affect a measurement to the chassis.

I've traced it down to the mono valve is not getting a ground. Mostly likely I burned the the temp dial switch, or the trace going to the switch. For now I grounded it with an alligator clip to shut off the heat and will have to pull the ACC and see what's up inside. The connector shell on the monovalve fell apart (no longer there). With exposed terminals, it was easy to short out. Anyone have a spare monovalve connector shell?

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

With the CCU "Off," do you have continuity to ground at the monovalve negative terminal?

Not sure I would run the car with constant 12 Volts across the monovalve. For testing purposes it's OK but I've never put an oscilloscope across the monovalve to see the waveform and measure the voltage when the CCU has the monovalve completely turned off. I wouldn't want you to burn out the monovalve's coil on top of your other problem.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

The monovalve isn't regulated by variable voltage. It normally has 12-14 volts applied via a switched ground at all times when the key is on and heat is not called for.

When the "OFF" mode is selected, the CCU provides a purely mechanical connection to ground. The electronic module for temp control is not involved and there is no "waveform."

No.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Thanks for the concern but no worries. I checked the wiring diagram and it is completely safe to ground the monovalve and doing so will not blow the CCU or drain the battery. The CCU temp dial switch (in the off position) provides a ground to the monovalve, nothing else can do it. There is no waveform, it is either on or off. Relay and diode logic cannot provide waveforms.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Apparently I was misinformed in my early days -- I thought the CCU provided a pulse-width modulated square wave to the monovalve, with the CCU adjusting the duty cycle to change the amount of hot coolant the valve could pass. Thinking back to 2006, I had to repair my CCU (some PC traces had been fried) and I do remember that the CCU in the W123 is a very simple thing with just a few relays and resistors. Perhaps I am confusing it with the W124 CCU, which is much more electronic.

Today I tried to make some measurements but it's so hot I cannot get the monovalve to open except in "Defrost" or all the way to the high end click stop in "EC." In either case there is no voltage at the monovalve and it is wide open.

In cooling modes, the monovalve is grounded by the CCU so that it stays completely closed, as you say. I'll repeat my tests tomorrow when it's cool but for the moment I stand corrected -- thanks to you and Tango.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

A lot of people were/are mis-informed, myself included, from threads in this forum stating it's PWM. That is until I opened up the CCU in my 83 300D turbo, and saw only relays, diodes, resistors, rotary and push button switches. I looked very hard for integrated circuits, transistors, capacitors but none was to be found. These components are necessary to generate PWM outputs. I wonder who started this PWM BS?

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The CCU is primarily a switchboard. The brains are in the electronic module for temperature control, a separate component.

It may indeed be pseudo-PWM at a very slow rate, e.g., 10 seconds "on," 10 seconds "off." Relays and temperature sensors could do something like that. It is clear from the way the system is built that the CCU has to be able to adjust the flow of hot coolant. There is no other way to create "warm" air, unlike the CCU in model 201 (190D/E), which uses a "blend air" system and does NOT have a monovalve.

The 124 CCU, which is full of electronics, may have true PWM -- I'll check later this morning -- and since Mercedes says the two systems are "functionally similar," someone might have assumed something inappropriate. It's probably best to not use the term "PWM" with regard to the 123 CCU and simply say that the 123 CCU can control heating by turning the monovalve on and off.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

The CCU (defined as the push button unit) only controls the monovalve directly in the OFF and DEFROST modes, or when the temp switch is set to the detent MIN position in the other three modes. Otherwise, the monovalve is controlled by the electronic temp control module.

Yes, I forgot about that electronic module up under the dash on the passenger side. However, if we need to get picky about our definitions, we should define "CCU" as the whole shootin' match while the thing with the dials and push-buttons is more properly abbreviated "PBU."

And therein lies another source of confusion: in the 123 the CCU's electronic module is separate from the PBU while in the 124 the electronics was reduced in size and stuffed into the PBU. Those of us who have or had (me) both models have to be especially careful what we say so that we don't cause confusion to the new guys and gals.

Thanks for the reminder.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970