1987 300tdt Hot in Summer and Cold in Winter (CCU issues)

[1987-300TD]

Latest project is to fix my climate controls. A/C sometimes works during the summer but more importantly right now, heat does not work during the winter. :-|

I am unsure where to start. I have read several threads regarding the CCU (Climate Control Unit). What I have done so far:

* Set the dial to Max Red (85) before it clicks
* Set the fan to Max
** I get cold air pumping out the center vents; very light cold air pumping out the driver's side vent.

Where do I start to troubleshoot this beast?

Over the summer I had tested the CCU vacuum ports (the ones that control the dash pods). All ports held 23 bars except port 4 which held 0.

The defrosters seem to work on high whenever I have needed them.

Thank You in Advance!!!

[jay_bob]

Your best bet is to locate the online service manual for your car.

Either startekinfo.com, click on MB Workshop Resources, page through to 124 Service Info

Or w124-zone.com, you will have to register but it's free.

Chapter 83 is the complete troubleshooting guide for the system. They want you to use a special breakout box to test the sensors, but if you are handy with a meter, have a copy of the schematic, and can read between the lines of the procedure, you can see which wires to test by hand.

If you do need a climate control module try the junk yard, but know they made a change in 89 to the aux fan control function, so you'll need a module from an 86-88. Either a 124 (300D/300TD or the gas powered E class models) or 126 (300SD or any of the gas powered S class).

If you come up short at the boneyard there is a company called Programa that will rebuild the units for a couple hundred plus your core.

[1987-300TD]

Thanks for the insight Jay_Bob!

I tried replacing the CCU in my car with one I picked up a while ago from the local Pick-A-Part. I had the same results. In particular, no heat.

I did a little more research. Could it be the mono-valve instead of the CCU? I have not tested it yet. Have read about rebuild kits. As I do not have any service records from the PO, I do not know if it has ever been replaced.

Thank You!

[jay_bob]

It is entirely possible that the mono valve has failed closed (i.e. no flow). Note that this valve is electrically closed and spring open, so it defaults to the open position (allows flow).

Try this test, it's pretty easy to do:
Remove the mono valve and substitute a suitable piece of pipe temporarily and see if you get heat.

Also the bad vacuum actuator is not helping things. I assume you meant the other actuators held 23 inches of mercury (not bars). 23 bar would be over 300 psi, which you could not generate with a mityvac and would burst the actuator diaphragms rather dramatically.