PeachParts Mercedes-Benz Forum - View Single Post - Climate Control Unit CCU/PBU and "clicking"

Jeremy5848 · #4 12-01-2011, 08:31 PM

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