81 300sd dash control buttons question

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how can i check if i am getting power out of my dash control buttons when calling for a/c? does it go right to the pressure switch? i have no power there. -gary

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    Gary, a general comment. No dash button will operate a power device directly... they operate intermediate relays and that is where you will likely find your problem
cheers

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Guenter Scholz wrote:

And only a simple relay if you're real lucky. In the case of MB, they like to mix all kinds of vacuum and electrical widgets in systems, particularly the climate control, which is very complex. It's about as far away from a straight wire to the compressor as you could get. A simple vacuum leak in the air conditioning controls can disable things. As I recall, in my 80 300SD, the blower won't even come on unless vacuum pressure is present.
If you're trying to track this down yourself, I'd get a repair manual for the car that includes circuit diagrams of the electrical and vacuum systems. Then you can track it back to what works and what doesn't. If you plan on keeping the car for awhile, it's money well spent.

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I tried repairing a pushbutton module once. Spent days on it, ohmed it out, replaced burend traces, all checked out. Woulnd't work. Got a good used one. Still works today.
I've repaired a lot of eletronic gizmos from camcorders to alternators but this thing is a bloody nightmare.
--
Need Mercedes parts? http://parts.mbz.org
Richard Sexton | Mercedes stuff: http://mbz.org
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Richard Sexton <See http://rs79.vrx.net for address> wrote:

completely apart put in new relays on the ciruitboard and checked every component and resoldered all traces (well it was over $300 CDN) but no cigar >:-/ finally bought one.
cheers, guenter
ps my predcictions for tomorrow D (1) A (1) germany wins on shootout by 1
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Vacuum pressure? That a definite oxymoron, sort of like jumbo shrimp!

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why not, use a '-' ,,,, re jumbo shrimp, had some for supper tonight actually.... geeez, they were the size of a small lobster
cheers, guenter
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Not really, the pressure measurement is just a negative value.
I believe that most of the time what the vacuum does is remove the air from one side of the piston and the atmospheric pressure actually does the work.
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Roger Shoaf

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Do you have access to a wiring digram of your vehicle? If so I'd trace the wiring from the AC compressor back toward the control buttons. Look to see if you have a relay in the system which could be the problem. On my '81 300D the fan will not work if the fuse is blown which also (apparently) provides power to the AC compressor as well. One other thing to check is to make sure the two plugs with all that wiring are firmly pressed onto the male pins on both sides of the control module (the one with all the buttons and switches in your center consule). Don't know if you have a parts yard in your area but I've bought that control unit with all the buttons, along with the circuit board and rotating temperature switch, and also with the fan switch, for about $25 with a guarantee. Of course I have to take them out myself but it's well worth the trouble. Only problem is I usually have to get to the yard as soon as the MBs are brought in to get the best parts. Good luck!
wrote:

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First, check the freon pressure... should be over 100 PSI when not running and engine off. Without this, your compressor will never come on.
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Tiger wrote:

i don't have power at the pressure switch with eng running and fan on high and dash temp wheel to max blue. system, seems to work fair [45-50 degrees] when wire to press switch is hooked dierect to battery even though i still have some air bubbles in sight glass. working backwards from pressure switch what is next thing i should check other than the dash control assembly which i have switched 3 different ones. -Gary
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I don't have the specific wiring diagram for your car but I'll check this weekend and see what happens on the 300D from the pressure switch back. I'm naturally assuming you've checked all of the fuses. Not sure if the AC compressor/pressure switch are on a separate circuit on your vehicle but that's usually the first thing to check.

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yes, checked fuse, 12v in and out of #11 with eng runnibg , a/c on max, fan on max. Ernesto wrote:

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gulp.... just bought a factory mbz heat/a/c manual on ebay.... paid through the shorts... $129.00!! got in a bit of a pissing contest with a few other guys [musta been in my same predicament] :) ... how bad did i get gouged?? http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item )0001077382 damn i need this a/c to work and just know i can do it myself with this groups help and a couple schematics... got 3 mbz books, haynes, chilton etc... never have i found an answer to a single one of my questions in one of em.... gonna take a week to get it, will keep asking questions in the meantime ok? appreciate any and all your help... -gary zdilly/gary wrote:

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Bought a complete set of four factory MB manuals 3-4 years ago, paid roughly $300 for the set. In as much as the printed versions are practically unavailable today that might not be too bad of a deal, especially if it helps you solve the problem. I know there are CDs available but I understand the photos are of very poor quality. It takes a long time and a lot of scanning to get a proper set of manuals put onto a CD. You'd think they could do a better job than that.

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i bought a w126 cd a while back on ebay.. couldn't make it run.. my son said i dont have a big enough graphics card. i have a dell 1150 inspiron laptop. anyone else make these work? -gary Ernesto wrote:

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Yes, the old cd manual gives us hard time with XP... I figured it out.
First, install that old Adobe Acrobat 3.01 that came with the CD (there is a folder in the CD content if you can't find it) Adobe Acrobat Reader 7.0X... and reboot if required... Then go ahead and boot up that CD...
That should work.
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thanks, will try it today. while i wait for my new a/c manual i bought on ebay, i figured i would buy and install a new dryer and expansion valve. and purge the system. i also have a near new compressor i had on a sd my son wrecked. think i'll put that on. where's best place online to buy dryer and expansion valve? -gary Tiger wrote:

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Oops... I realized I lost alot of instruction...
1. Install the Adobe Acrobat 3.01 that is in CD... 2. Reboot... 3. Install the new Adobe Acrobat Reader 7.0x... 4. Reboot if necessary 5. Load up that manual.
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i did the adobe 3.01, 7.0 is about a 7 hr download on dial-up so will go to hot spot to get it today. thanks for the info. Tiger wrote:

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