Anybody 'splain the ACC servo functions? - PeachParts Mercedes-Benz Forum

unkl300d · #1 12-13-2006, 11:21 PM

OK I searched the archives and found some hints about bypassing the servo on a non-turbo 300D. Also some reference to the digital acc control and the other bypass kit.

However, I read several variations of climate control functions working or not working or uncontrollabely working as a result of a servo bypass etc. (not the digital bypass).

Question. How exactly does the ACC Servo function in relation to the dash ACC control unit and the CC amplifier?

For example, my 1979 300D servo seems to be internally frozen such that it causes an electrical drain on the battery unless the CC amplifier is removed.
Of course the car got no heat with this bad servo and good amp.
My dash CC unit buttons all operate to power on/off the blower and appropriate vents work for defrost and air. Just no heat.
My Air conditioning unit switches on with the dash switch even though the CC amp is now removed.

I am trying to figure this out.

It will help me assess which route to take.
Simple bypass or digital unit. Not interested in another servo unless the replacement is cheap i.e. expect it to last 9 months!

What I am leary of is doing one or the other 'fix' and later getting some other associated climate control problem which will defeat the 'fix'. esp. if the 'fix' is expensive.

SO, what are the mechanics of this torpid system?

Anybody know??

Best.

winmutt · #2 12-14-2006, 11:53 AM

Quote:

Originally Posted by unkl300d

Question. How exactly does the ACC Servo function in relation to the dash ACC control unit and the CC amplifier?

You mean ACC amplifier right? Sounds like your aux water pump is causing the drain perhaps? It gets its power from the ACC amp and seems to be most common cause for blowing the amplifier. Of course there could be a leak in the servo housing thats shorting the servo motor but I think you would have blown fuses and sparks if that were the case.

Essentially there is a motor in a box, this motor has an arm on it that makes two things happen. At certain points during the radius it opens and closes both electrical and vacuum circuits. The ACC amplifier takes the weaker signals from the temp sensors and up converts them to a signal that can switch 12v analog appliances made in the 80s.

This is of course one of the few AMERICAN designed components of this car *cough* *cough* . Look around here there is a a link to a Chrysler shop manual that explains it much more thoroughly than the its MB counterpart.

tscorda · #3 12-14-2006, 04:53 PM

If you have a fax I will send you info on Servo Valve. Private message me.

Tom (former 1977 300D owner)

Moorewr · #4 12-14-2006, 07:24 PM

Sounds like my wife's 77 300 D, except no battery drain. I opted for the digital replacement from diesel giant, because we bought the car new and plan to keep it forever. Fairly simple to install and works fine.
Good luck
Ron Moore

rhodes2010 · #5 12-14-2006, 07:28 PM

Here is the definitive explanation of the ACC Auto Temp II.

http://www.imperialclub.com/Repair/Lit/Master/281/cover.htm

Good reading, enjoy.

unkl300d · #6 12-14-2006, 10:49 PM

Great replies! Thanks.

I am getting there in learning how this thing works.

One other question.

Regarding the aux water pump on this non-turbo car. I'd like to test it with a 12V DC power source. The wires on the pump are brown and whitish colored
I believe.

What is the color code? Neg= ? POS= ?

I used a hose to listen (like stethescope)to the aux pump and it seemd to be whirring while car was on (when amp was connected).

I'd like to do more tests.
Some other archive outlines how to disassemble the aux pump to clean and bring back to life.

Thanks

Denis