I did a search and didn't find anything about this. My 126 CCU went bad (I thought). I replaced it with a rebuilt one. Still has this problem:

Works fine in Heat mode and EC mode
In AC mode works fine for a while, good cold air, fan varies accordingly. Then, after a while I begin to get clicking noises. Like the sound that the old cash registers made when printing receipts. It does it sparadicly, then it becomes constant. The funny thing is that if I turn the unit off, it still does it!!!! What's with that?

I seems to be related to something loose. Hitting bumps seems to make it worse.

During this clicking fit the fan and flaps change sparadicly. The A/C is still cold, the fan still blows just changes speeds.

This is one goofy gremlin!

I have the same noise on my 87 300TD.

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The funny thing is that if I turn the unit off, it still does it!!!!
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What if you keep the O button depressed? If I do on mine, it stops. It seems we have the same problem.

No, pushing the "O" button for off, even holding it continues for a while, then slowly quits. Must be related to heat or something. Almost acts like there's something loose when it happens. I've wiggled wires, but they're all grouped together into to connectors and there's no way to tell which wire it would be. Besides, how do they go bad when they're so tightly bound? Maybe the acutal connector itself? Would electrical contact goop help? Surely it's not in the CCU itself, again?

Try checking your vacuum switchover valve. Listen to where the clicking is coming from. It will be from behind the dash. The valve will have both electrical and vacuum connectors attached to it.

The vacuum switchover assembly is on the passenger side of the console. This noise is coming directly from the CCU itself.

It is definately coming from a loose something. When it starts you can tap on the dash and make it click, bumps in the road, etc.

But it doesn't do it at first. Seems like when it gets to that intermediate stage where it begins switching to maintain temp is when it happens.

What's got me so confused, if it was electrical then why does it continue even with the CCU off? Where's the power coming from?

Could it be in the Temp Control Unit below the glove compartment?

Had this identical problem with my now sold 190. After a few trips to the mechanic, replacing the ACC Potentiometer (sp?) fixed the problem forever.

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Chris

'04 ML500 - 53k, Inspiration Edition, Desert Silver
'11 Audi A4 Avant - Brilliant Black
'87 300SDL sold
'99 C280 Sport sold
'85 190E 2.3 sold

Where exactly is this ACC Potentiometer? Can someone post a pic? I thought this problem was common on '85+ Benzes with the Climate Control feature. I always thought it was the Climate Control Module or box that controls the fans and the temp.
BTW, mine does the same thing..300SDL. I live with it so far. In Florida, it's been pretty hot. So I keep the air blowing..

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99 C230K Sport
87 300SDL
81 300SD

I think, correct me if I'm wrong, the pot is the temp sensor in the roof, by the light. I took a pair of needle nose plyers and wiggled mine good and so far it has not repeated the problem! If it does I'll replace it. You have to remove the light to get to it, not hard though.

Just had a similar problem with the CCU on my '85 500SL. Only it was the CCU itself. One way to find out if it's the box is to get someone to put their hand on it while it's acting up. (Mine mounts on the firewall behind the glove box on the passenger side.) You can definitely feel the relay(s) sticking in the box.

Pulled the box and opened it up. Cleaned some contacts and relays that were actually dirty, but it didn't fix the problem. One incidental (?) side effect of this was that when stopped and it acted up the RPM would surge from 475 up to about 800. Don't know what was causing that.

The really neat part of this is my CCU #000 822 12 03 was priced at $258.00 through the MB dealership when I first started looking for the part. Once I figured out that I'd have to replace the box I started inquiring of places like FastLane to see what I could get it for through them. Lo and behold!!! MB had raised the price on the CCU to $1,574.00. No kidding!!!!!!!!!!!! FastLane did offer it to me for about $1,200.

That was still a big chunk of change so I did a bit of inquiring and found another unit with a different part number that I bought for $125.00. It's missing one pin on the side that has four pins. Put it on yesterday and I've got heat, air, and a fan. Don't know what I'm missing and probably won't find out without a schematic.

FYI, I also went ahead and replaced the Overload Protection Relay since I'd had to replace the idle control valve, idle control unit, and now this. She's running like a top now.

Cheers!

Don

The cabin temperature sensor in the W210 cars (at least) is a tiny fan in the same assembly as the reading light. It is similar to the cooling fan in a computer (as far as the motor is designed) and it drove me nuts on our car when I first got it. It sounded like it was coming from the dash until I leaned forward and traced it to the headliner area. It runs for about five seconds even after you shut off the car. If this is your noisy thing you can try cleaning it by vacuuming it or removing the light/sensor assembly and blowing canned air through it, mine got a lot quieter after some dust removal. HTH,
-Tom

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RIP "Betsy" (Oo=*=oO)
'96 E 3 2 0 (W 2 1 0) M 1 0 4 (Totalled)
Spaces in sig so as not to screw up the SEARCH; every time someone searches for that MB they don't want my sig!

2004 Audi A8L
'98 VW Passat 1.8T 5M
'87 Alfa Romeo Milano 2.5L 5M
'67 Impala convertible, 327cid

Hey guys,
My 82 300D has the same clicking (Just started last week). It is definately from within the CCU and I also think it must be electrical, as it produces significant static for my radio when it does it. If anyone has the simple (read: cheap) solution, please let us know!!
Paul

After eight months has anyone figured this problem out yet???

Thanks,

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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000

sorry, i forgot! after a few days, it just disappeared. Must have been another one of those 24 hour MB bugs

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Paul
2004 E500 4matic; 72,000mi

I have the same issue with my 1988 300E. I also have a hacked up AC wiring harness though. But the car is at the independent shop now and I should find out what gets replaced to make this incredibly annoying relay clicking go away. I have read posts here that point to a blower regulator relay being a problem. On the E that part sells for 250-300 bucks. I'm betting on the CCU itself though.