81 300sd dash control buttons question

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

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.

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.

Vacuum pressure? That a definite oxymoron, sort of like jumbo shrimp!

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!

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

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

!! I know exactly what you mean. Had my fuel relay module 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

why not, use a '-' ,,,, re jumbo shrimp, had some for supper tonight actually.... geeez, they were the size of a small lobster

cheers, guenter

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.

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.

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.

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.

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.