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Discussion Starter #1
My ACC has never worked since I've gotten the car, but now that the warmer temperatures have come around again, I'm thinking about just getting it serviced. I have a 1980 300D with the vertical push-button control unit and the only thing that works on it is the defrost button. The other buttons do absolutely nothing when pushed. Anybody have any ideas of what the problem may be? Could the control unit itself be bad?
 

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1981 Mercedes 300D
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Hiya there - do you happen to have any other vacuum related problems on your car? I rec...

Hiya there - do you happen to have any other vacuum related problems on your car? I recently diagnosed and repaired the vacuum system on my uncle's '78 300D. His climate controls did not work (except in defrost) when there were vacuum leaks in the vacuum system (ie: door locks, cruise control, fuel shut-off, etc). You might want to check your vacuum system for leaks first - I've heard the early model climate control is very tempermental with even the slightest vacuum leak.

Anyway hope this helps get you started..
 

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Discussion Starter #3
Hmmm.......

Thanks for replying! I've had vacuum problems before, but I don't know of any now. The vacuum system from the firewall back (door locks, fuel flap and trunk) is disconnected and plugged from under the hood, as I have a bunch of leaks in some of the door lock mechanisms, and I don't think the trunk or fuel flap actuators work. So that eliminates all of that. I had a hole in the vacuum pump diaphragm and I repaired that, and now I have a 25 inHg reading on the vacuum line that goes from the vacuum pump to the brake booster, when normal for that line is 21-23. And the car shuts down immediately every time.

But I guess there still could be a leak somewhere. I wonder if the leak is in the climate control system itself......
 

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You might want to check the vacuum lines under the battery tray going to the climate contr...

You might want to check the vacuum lines under the battery tray going to the climate control system. Quite often those things get eaten away by battery acid! Not a fun job if this happens.

Also, there is a sensor on the engine that is for the climate control - if this fails, the climate control will think the engine hasn't warmed up and won't allow the climate control to kick in. They put this in here so that people wouldn't get cold air blown on them if the weather is cold...kinda dumb. Its located on the same side of the engine as the glow plugs. There will be two sending units - one going to the gauge, and the other going to the climate control system. If you disconnect the line for the climate control system and ground it, and your climate control still does not turn the fans on in any mode other than defrost (it might take a second or two)- the problem is elsewhere. The older control units rely on vacuum and that temperature sending unit to turn the fans on (except in defrost). Good luck, and keep us posted!
 

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Discussion Starter #5
All relevant!

Wow, everything you've said is relevant. I've never looked under the battery tray, but must say, it is quite corroded! I broke the tip of the one sensor that goes to the temperature gauge and I don't get a reading on the temp. gauge anymore. However, the climate control didn't work before the sensor was broken either.

I wonder if I get a new sensor if the climate control will work, as I did "fix" all of the vacuum problems I had. When you say to ground it, do you mean the sensor in the engine or its attached wire? Thanks again, now I have more stuff to look at!
 

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Hello again!There should be two sensors on the engine block - one for the temp gauge, ...

Hello again!

Sorry if I wasn't clear earlier - you should disconnect the wire going to the sending unit, then connect the wire to a suitable ground like the engine block or negative terminal of the battery. If you ground the wire that goes to the sensor for the temp gauge, your temp gauge will peg out (and possibly be damaged if you keep the ignition switch on the 'on' position for more than just a sec). The wire you want to ground is the one for the climate control. If I remember correctly, the wire is connected to the sensor that is closest to the front of the engine. Keep a close eye on the temp gauge - if it pegs immediately after turning the ignition switch to the 'on' position...you've got the wrong wire grounded. The wire for the climate control doesn't have anything to do w/ the temp gauge in the dash. Oh...please be careful and try not to ground the glow plugs or 'wires' going to the glow plugs...lots of current going through these...a fire could result of the glow plug wires were grounded and the ignition turned on...


Also - have you checked the fuses? Sometimes these fuses get corroded and the terminals they sit on need to be cleaned. There should be one 'hidden' fuse over by the relay panel on the driver's side of the engine compartment (this is how my uncle's '78 was anyway...). The relay compartment is close to the fuse box, its located on top of the wheel well adjacent to the fender. Its normally covered by a black plastic cover...remove this to reveal the relays (normally two relays here), and a round housing for a glass-type fuse. Check this fuse, its part of the climate control system - its easily forgotten about as well!

I just hope that the info I've given you is relevant to your 1980. If my memory serves me right, the stuff I've told you to look for so far has been stuff that was relevant to my uncle's '78. Hopefully I haven't gotten stuff in there that applies to the other climate control system offered from '81-up. Anyway I'm glad I could be of some help to you - let me know if you need any more info!

Even better, if you have a chat program like MSN, AIM, or ICQ - maybe we can chat there sometime! It will be a little easier to try to figure things out that way! :) Anyway good luck with everything...again let me know if you have any other questions.
 

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Discussion Starter #7
All of the above check out ok

I checked the vacuum lines under the battery and they're ok. All the fuses are good and so is the amplifier fuse by the relays. Just wondering, anyone know what kind of fuse that is? Mine is just a glass cylindrical fuse that says something like "T2/250V" or "12/250V" and that's it, and it also has something like sand or silica gel in it that you can see through the glass. I've never seen one like that, and I've asked around before and nobody else knew what kind it was either. I also tried grounding that sensor's wire and the ACC still didn't come on.

I also checked that dark green vacuum line with a yellow stripe (I think it's the ACC) that goes into the firewall near the brake booster and it didn't hold a vacuum. So, I'm thinking there might be a vacuum leak somewhere in the numerous vacuum lines that make up the climate control system. Am I correct in thinking that? (I don't know if it should hold vacuum or not when the car isn't running and the ACC is switched off) I might just have to bust out the instant messenger!
 

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I'd like to ask a parallel question: I have an '84 300SD and when I push the max AC button, most often the defroster comes on, but occasionally the AC will work and blow cold air. Pushing the button always engages the compressor. Think this is a vaccuum problem? That would be my first guess. Also, sometimes when I am lucky enough to have the AC come on, it will just switch over to defrost in a few minutes. The switch itself was replaced a short time ago and I haven't removed the panel yet to see the connections on the back for myself. I have my fingers crossed for an easy fix...I need AC in Los Angeles!

Thanks,
Gene
 
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