Okay, so I did a search and found out that chances are good that one of the relays on the ccu is probably going bad or I just need to do some cleaning back there, but what has me concerned is that even with the ccu turned off I'm still getting a couple of clicks when I turn the key to the number 2 and 3 position. Is there a relay buried back there that is also part of the ignition system?


As for trouble shooting the CCU; the aux pump has been disconnected for a year so I know it's not fried. My blower motor started going out once in a while a couple of months ago, so I replaced it with one from the junkyard which may also be on it's way out. Today when I got the cracking noise, I couldn't get the blower to come on with the defrost setting, but the AC compressor kicked in. On the other settings I was getting moderate air flow from the vents with no AC. I shut the HVAC system down for a few minutes, then turned on the AC and whola had good cold AC for an hour. Could worn brushes in the blower cause problems elsewhere? Thanks

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'81 300SD

IIRC an '81 has two modules - the PBU and a separate box that, for one thing, serves as the blower control. I've never heard of that other box going bad, and very rarely do resistor banks go bad. Rather than put more thought into this, I'd send the PBU to have the solder joints checked and/or reflowed. Or maybe you possess the necessary skills and visual acuity.

I don't think blower brushes can affect ACC since there's no feedback other than through the cabin temp sensor. I suppose the blower can draw too much current depriving the PBU of what it needs. At some point the fuse will blow ending all doubt.

How's the aspirator motor?

Sixto
87 300D

Is the picture of the box the one you're talking about? This is a spare that the PO gave me with the car.

Is the aspirator motor the fan that gives you just a little bit of air flow when the PBU is in the off position?

Are there no relays to clean on the earlier W126 HVAC systems? Maybe I was reading threads that pertained to later models.

I'm thinking of giving these guys a call and see what they charge to rebuild my PBU. General Development Laboratories - Home Page

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'81 300SD

I don't see a picture.

The aspirator motor draws air over the cabin temp sensor. I suppose you could feel a draft from the vents since it dumps into the blower motor housing. But with the AC off air should only go to the windshield defrost registers.

You can try this since the 126 isn't very different - W124 Climate Control Vacuum Valve Repair DIY

The early 126 PBU is very simple. I'm sure you know someone who knows his or her way around a circuit board. Try that first.

Sixto
87 300D

New development: twice today when I shut the car down it shut off slower than usual and the clicking occurred at the same time as the delayed shut down. What's really odd is that the PBU was in the off position. Does this mean that there is a vacuum valve on the back of the PCU that might also be going bad? I ran the mityvac to the red/green line going to the HVAC system and it slowly drops 1 Hg per 30 seconds from 20 Hg's which is about what it's always been since I bought the car two years ago.

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'81 300SD

The intermittent problems are, as Sixto stated, likely due to solder joints in the Pushbutton Controller. The circuit boards are arranged in an "H" pattern and the joints tend to crack where the boards meet. It is probably what is causing the excess relay cycling, as well.

The red/green line is what supplies vacuum to all of the switchover valves, located behind the side panel on the passenger side of the center console. From there, the vacuum is routed to the various flaps to direct airflow. The pushbutton controller operates these according to which mode is selected. 1Hg/30 seconds drop isn't bad and probably isn't a problem source.

If you do a search, in this forum, for "reflow solder" "climate control" and look for threads with photos, you will probably see the circuit board joints that need re-soldering.

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Sam

84 300SD 350K+ miles ( Blue Belle )

I removed the PBU today and cleaned the pins on both sides with Deoxit. Good news is that there's been no clicking sound today and I was getting center vent action for almost the entire time of AC and economy operation which is a huge improvement over what I was getting before. The bad news is that the whole system died on me a few miles before getting home. After playing with the Bi-level and defrost buttons, all was good. Knowing that the vacuum pods on my center vents are good kind of made my day.

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'81 300SD

Classic sypmtoms of poor solder joints. Follow the excellent advice provided in post #6.

Yes, I plan on resoldering the board. I have read that if you do a poor job, you could make things and worse. Also, GDL will not rebuild a unit that's been tampered with. Maybe I'll practice on one from the junkyard depending on price.

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'81 300SD