All,

I tried many things but I have had no success. The temperature from the vents is boiling hot. Whether the AC is on or not, the vent is always hot. I checked the sensor tubing behind the glove box and it is good. I replaced the monovalve diaphram, I resoldered the ECC unit. AC is on, charged and the low side is cold. What am I missing? I noticed that when I turn the temp knob there is no "click" when I move it between hot and cold. Use to do that. I would appreciate any ideas.

85 300d w123 202k miles

The item in the Yellow Circle is the switch that allows the Hot Coolent to flow or not to flow to the Heater.
I suppose you could find where the Coolent flows from the Engine into the Heater and where it flows out and simply block them off until you find out what is wrong.

I am not sure what happens if you simply disconnect the electrical Connector on the Switch. It May be that will shut it off and cut off the Coolant Flow.

Of course that is a separate issue from the Vents being open.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel

The mono valve is controlled by variable voltage.

12V is mono valve 100% CLOSED.

Zero volts to the mono valve = 100% OPEN.

Climate Control:
http://www.peachparts.com/shopforum/showthread.php?t=142408




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1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

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The monovalve is controlled by constant voltage and a switched ground.

Does your horn work? Check your fuses.

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1984 300D, 228k, Light Ivory, Java MB-Tex

the control is on the ground side of the circuit.

* If you have 12V (+) positive to the mono valve.
* You can close the valve with a jumper wire.
* From the battery (-) ground terminal to the mono valve solenoid (-) ground terminal.

This is a common diagnostic test and field expedient road repair/band-aid.

My best guess would be the ECC unit has failed.

If you have +12 at the monovalve but your system is blowing heat, that probably means you don't have ground at the monovalve. There could be different reasons.

For example, a smoked ground trace on the temp wheel, as described here: http://www.peachparts.com/shopforum/showthread.php?t=300751&highlight=monovalve

Notice the temporary fix was a clip to ground, with the permanent fix in post 20 (ignore the PWM discussion as irrelevant to your problem). A temp clip to ground MAY assist in troubleshooting, but be careful you don't short the wrong pin to ground. ID the +12V pin FIRST!

Before digging in to the CCU, I'd recommend an inspection and continuity check of the monovalve connector. Ensure the metal sleeves fit snugly on the pins for the monovalve - you may need to g-e-n-t-l-y squeeze them to mate that connection tightly. Turn the CCU off and check continuity to ground. Turn it full cold and check continuity to ground. If the monovalve is not getting a steady ground, it's going to open and let heated coolant into the heater core.

All good advice, I will try what you recommend and see what happens. Thanks

I'm so glad I got a new Behr last summer... I'll be treating that thing like gold now. Biggest item of maintenance I can see from my old evap coils is that they were severely clogged. Any restriction on airflow would cause stress on a blower and therefore limit it's lifespan.

I'm looking forward to getting that AC box cleaned back up ready to go back into the '85 TD.

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'85 300TD "Puff The Magic Wagon" - Rolling Resto
'19 Mazda CX-9 Signature - Wife's sled
'21 Morgan 3-Wheeler P101 Edition
'95 E300d - SOLD
'84 300TD "Brown Betty" - Miss this one
'81 240D "China Baby" - Farm grocery getter

Where should I look if I'm not even getting the 12v to the mono valve? My CCU has also stopped clicking when moving the wheel from hot to cold.


PS: I know all my fuses are good however I think this issue first started when my blower fuse holder melted. The blower motor was serviced and I've since moved the circuit to a new fuse holder but none of this had any effect on the valve.

Look to see if the key is in the RUN position.

Then advise what vehicle you are asking about.

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Oops forgot to add that its a 1981 300TD and yes, I made sure the key was in the run position. Everything works well, including the aux pump, however I'm still not getting power to the valve.

If it is blowing hotter air than what is inside the car, definitely hot coolant is flowing thru the heater core. Do as suggested in post #2 (as a test and/or to get thru this summer). Block off the hose going to the heater core. Careful, since it splits into 2 on the firewall, so don't block just half. Easiest is the single hose coming out of the rear head on driver's side that loops past the oil cooler. Buy a big $1 clamp at the hardware store. As mentioned, unplugging the mono-valve will give you "full heat".

Re knowing your AC is working, if the return tube (in front of valve cover) is cold, it is working. Your heater may simply be over-powering the AC.

Unfortunately I have no AC, I'm suppose to be working on that now, not this. The valve is definitely wide open since the heat is hot enough to burn your eyes out on any setting, and fully cook a chicken at the same time. The mono valve doesn't have voltage on the positive terminal so jumping the ground terminal to the battery's ground did nothing for me. Where does the 12v originate? (And don't say "from the battery" to all you smarty pants out there. )

The wiring diagram shows 12V+ coming from Fuse #14 straight to the monovalve. The fuse chart reflects this. Click the link for a large scanned image of the wiring diagram.

http://i.imgur.com/boj8wly.jpg

See the fuse chart:

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1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

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