Diagnosing a heating/cooling issue on a 85 300D Cali version. So I've got the CCU/PBU slightly out, perhaps a couple inches, of the dash (I took it out of the plastic mounting bracket) and am holding it in my hands. If I move the unit I can hear various clicking noises inside the dash where it would mount. I have it outside the dash because I have a couple of CCU/PBU that I am testing. Some make more clicking that others when they are slightly moved. Sometimes the clicking causes a reduction in the speed or force of air from the vents. (I have the fan on high) Has anyone ever noticed this or better yet, can someone explain this?

See if the plugs are properly fastened on the sides. See if the temp control wheel module is not loose. The same goes with blower module on right side.

If you've removed the CCU completely off, see if you hear any rattling when you shake it. Could be a sign it needs to be resoldered.

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My '82 300 SD had a peculiar clicking from the glovebox side of the ccu. Recently the ccu failed, so I pulled it and replaced it from a parts car. Now I have lots of fan and vent controls, but no temp modulation (always hot). Also, the clicking from the glove box side is still present. Any ideas?

If the "clicking" sounds like something rattling around, it's possible that something has broken or come loose. You should be able to pull the bottom cover for an inspection (I haven't worked on a 123 PBU for some years).

There are several small relays that could also be making the clicking noise. They respond to commands from the electronics and the switches to control various functions. If their solder joints are coming loose (sometimes called a "cold solder joint") or if the two cables are not making good contact with the plugs of the PBU they will energize and de-energize and that could cause the clicking noise. You can unplug the two connectors and check the pins. They should be shiny clean; if they are tarnished (they are silver plated) you should clean them. Be careful to not scrub off the silver plating. I use metal polish and Q-tips. If you use steel wool, use 0000 grade and be very gentle.

The lack of temperature control (always hot) may be caused by a disconnect in the wires between the PBU and the temperature sensor in the center of the dash. If you pull the glove box liner you can get to this sensor and make sure the two wires are properly connected. I had that problem.

Another possible cause is failure of the diaphragm in the monovalve (controls the flow of hot coolant to the heater core) or the wiring to it or the coil. The monovalve is "fail-safe" in that a power failure opens the valve; this is so you'll always have hot air to clear a fogged windshield.

You can test the monovalve by unplugging the connector and briefly connecting 12 volts across the two pins (polarity doesn't matter, it's just an electromagnet). The monovalve should "clack" when you apply power to it. It is possible for the seal to fail and allow the coil to fill with coolant, which then corrodes the mechanism into a solid lump of metal and can fail either open or closed.

You used to be able to buy a repair kit for the W123 monovalve but I've heard they are no longer available.

Jeremy

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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

Thanks, I'll check and clean the pins (carefully). I replaced the mono valve plunger / seal earlier this summer. Hope it lasted longer than 5 months!

See the link below for climate control electrical testing and repair.

Middle-aged Mercedes Electrical Repairs

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Junkman, I had no idea the depth and breadth of the information available! I promise that I have searched before my querry, and would not have asked the questions I did had I seen this treatise before. My only question is .... when can I send you components to rework? (cruise control, CCU / PBU)

Apparently, Jim no longer offers this service.....Does anyone know of another avenue for the repair of a CC amp and the PBU?

Old thread I am bringing back. Does anyone know who is currently repairing the Bosch Temperature Regulator?

Track down user James Dean. He does these climate controls, ECU's for multiple makes and models.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.