So i've done a lot of reading and I'm still having a hard time telling whether my problem is due to the CCU or monovalve. I just took the Benz out for about 15 minutes and had full heat the entire time. I go from full heat on the dial to full cool and I don't notice any different in temp. ALso, if I go across the row from defrost to those other buttons to the right I notice NO difference. However, there is a difference b/w high and low fan. The difference in fan speed is the only differnence I notice out of all the buttons. Temp stays at full heat it seems.
I know my vacuum is shot in the cabin so that could explain why I'm not noticing a diffence b/w which pods are blowing air. Currently I get defrost and the pods against the doors blow. I bought a used CCU and that's what I'm using now.
Is this my monovalve failed and stuck open or is it my CCU?

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1983 300D TurboDiesel

Have you checked the ACC fuse?

You can check the monovalve wires. One should have constant +12V, the other should be open for full heat, ground for zero heat and a pulsing signal for controlled heat. The signal vs temp wheel setting should tell you whether the problem is at the monovalve or upstream.

Sixto
87 300D

It takes just moments to remove the mono valve, you can inspect it for a torn diaphragm.

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83 SD

84 CD

Full, uncommanded heat isn't usually associated with a torn diaphragm.

It could be (and often is) just a poor electrical connection at the monovalve. You can check the control circuit as follows (key on, CCU off):
One terminal at the monovalve should have system voltage present. The other terminal should have continuity to ground. If either is not present, there is a control issue.

And when you turn it to a heater/defrost setting you should see the voltage at the terminals cycle on/off about every five seconds. Be careful you don't short the two terminals together or you could fry your CCU.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

That is incorrect. The monovalve is controlled via a switched ground; voltage should be present at one terminal anytime the ignition switch is on.

Full heat all the time and in all settings means that the monovalve is in its default full open position. The cause is either a bad monovalve or a failure of the circuit that controls it. As Sixto says, check the fuse first. Don't just look at it, remove it, hold it in your hand, make sure it isn't cracked across in the middle and make sure the ends are not corroded.

If the fuse is good, check for voltage across the two terminals of the monovalve with the CCU set for full cooling. If you measure 12 Volts, the CCU is probably OK and the monovalve is probably bad. If you measure 0 volts, the problem is probably in the CCU.

You can also test the monovalve by removing the plug and (for a couple of seconds only) put 12 volts across the terminals (either way). You should hear a click as the solenoid moves and you may see a little sparking as you touch and remove the wires. If the monovalve doesn't react, remove it and take it apart. Failure modes are (a) torn diaphragm, (b) jammed diaphragm, (c) rust in the solenoid coil due to an internal coolant leak, (d) open coil wire and probably others that I can't think of now.

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"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

See post #4, someone disagrees.

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83 SD

84 CD

There are two messages here -

1) A torn monovalve diaphragm is a monovalve failure.

2) Full heat all the time is not a typical symptom of a torn monovalve diaphragm.

You can have one situation or both.

Sixto
87 300D

Perhaps I should have been clearer and stated 'across the terminals' vice 'at the terminals'. When the ground connection is removed, the voltage across the terminals goes to zero. When the CCU re-applies the ground, the voltage goes to 12V. The method of cycling the connection (whether the hot side or the ground side is cycled) has no affect on the voltage seen across the terminals.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)