A Different Climate Control Issue
 

MY 300SD will either freeze you or roast you. There is no modulation of heat or A/C. The blower speed increases or decreases normally but the temperature in A/C mode is 36 degrees which is fine in the summer but on a 75 degree day a little cool, and when the blower slowes down it feels even cooler. Likewise the heat blows hot when called upon but never is tempered.

My manual states there is a 5 second pulse signal (ground side) to the mono valve. Obviously this is not happening. Does that signal generate from the Electronic Switchgear or the Control Head? Anyone else seen this issue? I did an extensive search and have not seen this addressed in the past. Hope I am not duplicating. TIA.

Ahhhh . . . I had something like that when I first had my 123 car. I discovered that the pushbutton assembly had previously been fried (stuck aux water pump, probably) and someone had done a poor attempt at repairing the pc board traces. I rebuilt the board and the problem went away and has not returned in ~2 years/25000 miles.

THe controller indeed uses pulse-width modulation (PWM), a big word for simply turning the monovalve on and off (closed or open) rapidly. The pulse width is controlled by the electronic circuitry as a function of temperature, thermostat, and perhaps other sensors – for a 1980s car it's fairly intelligent. However, other than full-ON or full-OFF (the click stops at the ends of the thermostat wheel) and Hi-LO fan speed there is no manual control, an error on the part of the designers, like those of the F4 Phantom, who didn't give the fighter a cannon because guns were obsolete in the new era of stand-off missile warfare.

Another possibility is that the monovalve is somehow sticky such that it will move to full open or full closed but nothing in between. Now, I don't like that theory because it isn't really consistent with how the monovalve works. I mention it just to be Monday morning anal.

Jeremy

Teke out the four screws from the top of the monovalve. Carefully remove the wires connected to it. Remove the top. Check the valve rubber for tears. Then check the pistoning action of the valve insert inside the valve housing (you will see what I mean when you have it in your hand) You might have to go after the metal parts with some steel wool if they have been exposed to water. Carefully check the wiring going into the connector from the firewall. It gets brittle and can short out. In my case on the 300SD the wiring broke at the solder joints on the connector. A quick resolder and it worked fine.

The A/C is never modulated. It runs "full bore" whan selected and, when working properly, the system controls the temp by adding heat as necessary to maintain the selected temperature. Your symptoms suggest the possibility of a faulty temperature control unit.

Start with the cheapest/simplest first. Could be the monovalve and that is where I would start, but to me it sounds like a bad CCU (push button unit).

Fixed!
 

Last night I hooked a labscope to the monovalve and found it was only full on or full off, no modulated pulse so I removed the control head and found a burned strap on the circuit board. Repaired it and all is working well. I checked the monovalve to make sure it was not sticking and causing an overload situatiion. Current draw was 2.3Amps. This AM it is still working propely!

I've removed and installed the rubber seal a number of time but have never been inside the valve to check it out. Do the innards just pull out for the aforementioned steel wool treatment? Mine seems to be sticky even after replacing the rubber part.