Just another Idea!!!!!!! I buy and restore and re sell A few of these every year .It,s Interesting How Many Problems on these cars follow Yr. after Yr. I had a stubborn 88 300 sel with the same problem and after checking all systems ..It turned out to be a Slightly loose belt. The climate control systems on these cars are a bit over engineered, The slightestslippage of the belt system and the sensors shut down the system ..Sometime you can get it back up by just resetting it by turning the key off and back on again .It tells the computer to start a new...Mb are notorious for Tensioners. Like water pumps I learned to use only MB parts and that goes for Tensioners too. Some after mkt. parts work well but In this area Go the extra bucks and stick with Orig. Parts...(I hope you are buying Wholesale otherwise It hurts.. The thread on the Fuses is also very true ..We Replace fuses and clean the fuse holders every time we replace belts and hoses Approx every 2 years. And always use the MB fuses (special alloy) They work the best and Are in the most part "Trouble free".Why cant MB simplify their A/c systems.....Something like The Japs have done to their cars (simple and trouble free)... and very COLD TOO ..Pete

Well, still having Part of my mind is better than none.... LOL
My manual was printed in 1988.... and it lists ac 1 and ac2 systems... but both look like vacuum primary....
What is his system called ?
Maybe it is due to not being a 123 ?

No computers on the 84 to my knowledge so I think that rules out slipping belts. I have a somewhat similar problem on my 85 but it does it on the heat cycle and not AC and it doesn't do it all the time. I am betting it is the switch/controller. The only other option seems to be the monovalve but I don't think it can cause these kinds of problems.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

my 83 was doing the same thing but only when I wanted heat. Fan would run for a few minutes and then it would die. There was heat in the vents, just could not make the fan blow. Every once in a while it would come on and work fine. I pulled the control unit and on the bottom is one screw holding a cover that when removed you see the bottom of a board with copper traces. Mine had a refurb sticker and the copper had been resoldered so I just reflowed everything and it cured the problem. I think heat and vibrations crack the copper which gets fixed when resoldered which had over time cracked again. Still working after a year so cost to fix was as close to zero as you can get

Mine didn't have any damaged copper but after warming up the old solder I got it working. I seen one hairline crack where one side circuit board meets the top one but the real problem was just in the fan blower switch. For the cost of 15w of electricity for 5 mins I don't think I'll complain. It is so much more pleasant to have heat on demand rather than trying to convince that SOB to turn on.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

I pulled the monovalve this morning and inspected the rubber parts. They look fairly new, I didn't notice any tears, holes or rips. I think my next move will be to pull the control unit and give a visual, see if any thing looks funky.

What's wierd is that when I turn the heat on it does the same thing, runs for about 5 minutes and quits. I tried turning off the car and restarting and it starts but quits again, and when he heat is on the aux pump for the heater hose kicks on. The people who had it before me replaced the ignition switch, any body think that might cause a short?

It really seems like an electrical problem because when it quits the seat belt light comes on for a few seconds then quits. I can hear that the compressor shuts off as well.

I will you an update on how the control module looks.

Unless I missed it, no one told you to check the sensor hose behind the glove box. Pull the glove box and look for a foam hose running horizontally connecting a plastic pipe to the sensor at the top center of the dash.

I fully expect that it will be totally missing. They disintegrate. Go to home depot or somewhere and get a length of foam pipe insulation, cut to fit and slip it in place.

Good luck,

I think the foam pipe is only on the earlier 126.120 with the sensor vent in the dash. By 1984 (the original poster's year), The sensor vent was moved up to the dome light area and the foam tube was no more? Serial # 12601 (and some misc.) was the approx. cutoff, I think. Been wrong before; I might be this time, but my 1984 300SD lacks the dash sensor vent foam tube and is superceded by the dome light area plastic tube.


Edit: He has a 123 not a 126, so my post has no value for him.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

I will check behind the glove box. My car does have the sensor in the middle of the dash.

If the air is set at minimum with the a/c on, would this sensor still shut down the system completely?

Today it ran a whole 15 minutes before shutting off. I ran up and down the highway today a total of 100 miles and it would kick on occasionaly but not often.

I've been reading a lot and found an article, someone recommeded, by Steve Brotherton at www.mercedsshop.com/diy_stevebfl_0.htm
and he was discussing a compressor control relay, that when in a diesel you need more power it disengages the compressor. He says it will also disengage the compressor in the event there is problem with the voltage.

Does anybody know on an 84DT if there is a Base Module, Klima or MAS?

Could the low/high pressure switch on the drier cause this?

Like I said when the air is on and blowing it is cold, and when the heat is on it's hot, and when it quits the seat belt light comes on for a few seconds.

Thanks for all the advice. This is the best web site I have ever been on. The comments, stories and advice is excellent.

I don't think that any of the 123's had the sensor by the rear view mirror. That started with the 124's.

The symptoms match the hose being gone.

Yes, this is the best site I've ever seen.

Have a great day,

O.K. Just checked for that foam hose, no hose, not even any thing in there that even looked like it was a hose, just some dust.
Thanks for the info. I'll put a picture up tommorrow. The car is really nice, I want to have everything working perfect.

Any idea of links for tachs that are stuck? Mine won't move above 2,000 and the clock doesn't work.

Thanks

Remove the drivers side kick panel and reach up behind the cluster and disconnect and reinstall the round connector on the back of the cluster several times. You may have a dirty pin on that connector that supplies power to the clock and tach.

Don't use rubber or plastic hose for your sensor connector. Use foam hose. This is because the foam hose will conduct no heat and will work properly for your climate control.

I am really disappointed in my fellow mshoppers. The foam hose is the most common problem with the climate control and no one else homed in on this. I leave for a few months and everything just falls apart here. JUST KIDDING! I don't think anyone that responded has an automatic climate control car, so they wouldn't know about this problem. They were just trying to help.

Have a great day,

Larry, I didn't say foam hose because he said everything shuts off - fan and all. Doesn't that sound like an electrical problem?

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

Not necessarily an electrical problem. If the sensor is indicating the need for no a/c, then the a/c will shut down altogether, same thing for a false indication regarding need for heat.

Have a great day,