Cruise Control question - PeachParts Mercedes-Benz Forum

probear · #1 07-31-2007, 09:22 AM

When I originally got my SDL, a little under 2 years ago and 39k miles ago, the speed control worked intermittently then quit several weeks later. With a little procrastination and normally not using it anyway, time slipped away until yesterday.

Just for jollies, as I do weekly, I try the speed control, and it worked. Not one variance for over 45 miles. Later, I tried it on the way home. Good thing I payed attention. It seemed to lock on accelerate

.A tap on the brakes, it disengaged normally. Tried again, same thing. On the third try, it acted normal again, and reset normally.

Sooooo, what part of the system does it sound like it intermittently bad?

bodyart27 · #2 07-31-2007, 10:39 AM

There is a brain (controller) under the dash by your left knee, a sensor in the gauge cluster, the stalk, and the actuator under the hood that moves / holds the throttle linkage.

Typically the brain or the actuator craps out, not the sensor in the guage cluster or the stalk. I first sent my brain in to programa to be repaired, but in the end I found out it was the actuator. What I might suggest is sending BOTH into GDL to be bench tested. Then you'll know which is crapping out. The brain can be resoldered/repaired, but I believe the actuator has to be replaced.

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

Good luck!

Jeremy5848 · #3 07-31-2007, 11:54 AM

Depending on what is wrong, the actuator (which is mechanical, uses either vacuum or an electric motor, lives in the engine bay) can be rebuilt. But I agree with the others, the electronic box under the dash is probably at fault. Rebuilt units are available on an exchange basis from FastLane and other sources for about $200.

Rich1Merc · #4 07-31-2007, 02:45 PM

On my 1984 123 300TurboDiesel, cruise will pulse (fall off slightly then regain speed in 2 second pulses) when the turn signal is actuated.
It will also slow down with the use of power windows or sunroof.
I am suspecting that there is a poor connection somewhere which could be a high resistance ground somewhere.
Any thoughts would be very much appreciated.
On my 1976 114/115 300D the cruise control stalk will accellerate the car, but it will not hold any speed. When stalk is released there is no cruise. Pushing it upward will accellerate the car as long as it is held up.
Again, thanks for your help.
Rich

TurboWells · #5 07-31-2007, 03:33 PM

Quote:

Originally Posted by probear

Just for jollies, as I do weekly, I try the speed control, and it worked. Not one variance for over 45 miles. Later, I tried it on the way home. Good thing I payed attention. It seemed to lock on accelerate

.A tap on the brakes, it disengaged normally. Tried again, same thing. On the third try, it acted normal again, and reset normally.

Sooooo, what part of the system does it sound like it intermittently bad?

I had this exact same issue with my 87 300SDL. About 50% of the time the cruise would engage and perform flawlessly. The other 50% of the time activating the cruise would cause the car to go into endless acceleration.

I toyed with all the connections for a few months to no avail. I eventually purchased a used amplifier off eBay for about $50 and haven't had the issue return.

In retrospect, I believe the problem was a cold solder joint on the amplifier and I probably could have corrected the issue with a soldering iron. I was hesitant to use the soldering iron when I wasn't sure the issue was with the amplifier.

-TurboWells

jshadows · #6 07-31-2007, 04:17 PM

The problem with soldering that I've seen is there's no turning back. The rebuilt units require a core that has not been tampered with last I checked...I've tampered with mine to no avail, so need to go through the full troubleshooting to make sure it's definitely the amp. If it is I'll keep re-melting joints and hope for the best or find a tamper free unit off a parts car.