Cruise control accelerates but won't set - '83 300SD - PeachParts Mercedes-Benz Forum

ThePhoenix · #1 07-26-2009, 01:17 AM

First of all yes, I have read just about every post in all 216 threads after searching for "cruise control"...

So here's my story. Like most MBZs of the vintage of mine, the CC has given up the ghost. In an effort to revive it I visited my local pick-a-part and grabbed a couple of amps and actuators. I installed the actuator with the least current draw and noisemaking and one of the NTM (new to me) amps and took her out on the highway. I was excited to hit the accelerate switch and feel the gas pedal fall from under my foot, only to be let down as soon as I let go of the switch and watch the MPH drop like a stone...

I know I saw a thread somewhere that explains the basics of how the system works, but it's late on a Saturday night and I'll be darned if I can find it. I seem to remember it saying that the speed holding function relies in part on a MPH reading taken from the speedometer. Before I go pulling more amps from the scrapyard or sending one of my existing ones along with an actuator to GDL, I'd like to find out if the speedo reading is the problem. How can I test this?

Many thanks!

micalk · #2 07-26-2009, 10:32 AM

The cruise control, I believe, works off the same signal as the speedo. I don't think the speedo generates a separate signal for the amp, though the signal to the CCA is routed through there. There may be an op-amp or some kind of isolation amp to keep the speedo internals and CCA input separate. But the signal is a series of pulses. If you have an oscilloscope you can look at the signal, or you _may_ be able to verify its presence using an AC voltmeter. But if your speedo works, your CCA should be getting the signal.

Only amps for an SD will work with an SD, though an SD amp may work in a 300D. They don't go both ways. Part numbers are your friends.

btw, Peter at GDL is a wealth of info, and very helpful. I had a CCA rebuilt there for my SD and I'm very satisfied (three months and counting!). He was great before and after the sale.

ThePhoenix · #3 07-26-2009, 12:06 PM

Quote:

Originally Posted by micalk

The cruise control, I believe, works off the same signal as the speedo. I don't think the speedo generates a separate signal for the amp, though the signal to the CCA is routed through there. There may be an op-amp or some kind of isolation amp to keep the speedo internals and CCA input separate. But the signal is a series of pulses. If you have an oscilloscope you can look at the signal, or you _may_ be able to verify its presence using an AC voltmeter. But if your speedo works, your CCA should be getting the signal.

Only amps for an SD will work with an SD, though an SD amp may work in a 300D. They don't go both ways. Part numbers are your friends.

btw, Peter at GDL is a wealth of info, and very helpful. I had a CCA rebuilt there for my SD and I'm very satisfied (three months and counting!). He was great before and after the sale.

Thanks for the info. You are the first person I can recall who has said that the amps are model specific for the SD. I will check the p/n's...

micalk · #4 07-26-2009, 12:37 PM

Quote:

Originally Posted by ThePhoenix

You are the first person I can recall who has said that the amps are model specific for the SD.

check out the last post in this thread for a bit more info..

http://www.peachparts.com/shopforum/showthread.php?t=151696

badgator · #5 07-26-2009, 03:51 PM

Old box would acc and not hold or hold and drop.

I went to u-pull-it, pulled 2, paid for 1, $7 - large relay, didn't work.

Returned later 10 min, EXchanged for 2nd, still works a year later.

30 day no hassle warranty . upullandpay.com

doxland · #6 07-28-2009, 06:22 PM

I sent our amplifier to this place and it came back perfect. Dealers were hopeless.

http://gdl-online.com/begin.html

Cr from Texas · #7 07-29-2009, 02:09 AM

I can't find the thread - If you do, send it to me.
IIRC, there is a post that describes the speed setting being stored in one or more of the capacitors. There was a post that gave radio shack part numbers for the capacitors. If I find the post, replacing capacitors is an easy job - well at least is was to fix the clock.

AlanTbird · #8 07-29-2009, 09:21 AM

Mine was doing the same exact thing for YEARS and about a month ago as I was playing with the lever as I do to entertain myself...it came to life at 80 mph (WE CAN DO 80 mph HERE) and held nicely for about 20 miles then quit.

since then...sometimes it works fine...sometimes stops for a few miles then suddenly comes to life madly accelerating.

I feel that something is loose or dirty some were and one day when I have nothing better to do...my plan is to go over the system and clean all the connections...starting with the ones under the hood that have been exposed to 23 years of diesel soot.

SD Blue · #9 07-29-2009, 11:24 AM

Quote:

Originally Posted by Cr from Texas

I can't find the thread - If you do, send it to me.
IIRC, there is a post that describes the speed setting being stored in one or more of the capacitors. There was a post that gave radio shack part numbers for the capacitors. If I find the post, replacing capacitors is an easy job - well at least is was to fix the clock.

Here is a great link describing the amp circuitry: http://forums.pelicanparts.com/showthread.php?t=394659

For folks reading this post, keep in mind that, from their website, the policy of GDL is to not accept the amp as a repairable core if they can detect self-attempted repair. They don't mind if you open the case but re-soldering they won't accept. A strange policy but maybe they have seen a few re-soldering butcher jobs and the customers felt they could perform miracles. Having worked in circuitry repair, I can relate.