Intermittent Cruise Control 300SDL - PeachParts Mercedes-Benz Forum

shertex · #1 05-11-2012, 09:27 AM

Just acquired a 1986 300SDL (from the one, the only Brian Carlton). He indicated that he had had GDL rebuild the cruise amp (this was back in 2007) but that it never worked for him and he chose not to pursue the issue.

So I thought I would see what was going on. I removed the lower dash panel and saw that the amp was disconnected. So I connected it, seating and re-seating it several times to make sure I had good connectivity. And I did the same with the cable from the actuator.

And, lo and behold, the cruise worked perfectly! No idea why it didn't work for Brian....maybe bad contacts.

Having no idea the last time the throttle linkage had been lubed, I thought I better deal with that. Kind of hard to get to on the SDL.....under the intake manifold. But I did the best I could, certainly slopping some ATF on the actuator at points other than the pivot points.

Take the car out this morning and cruise doesn't work AT ALL. So frustrating....I think, did my lubing the linkage mess something up? So, again, I seated and re-seated the connections.....still nothing. Then, about halfway through my 30 minute trip, I tried again.....and again the cruise worked perfectly! I put it through its paces through the remainder of the trip. Worked absolutely perfectly.

I think it's a fair assumption that the amp rebuilt by GDL is solid (although there's of course the possibility that it's not).

Any thoughts as to what's going on?

JamesDean · #2 05-11-2012, 09:43 AM

David,

While you were under there did you verify that the amp was indeed a GDL unit? It might have been switched out at some point seeing as it was unplugged.

I would double check the voltage signals at the cruise amp connector. Make sure you're getting appropriate voltages on the stalk pins and brake disengage pin.

Alternatively:
Perhaps you have a bad wire somewhere? Did it look like anyone modified the cruise control harness down under that panel?

I had a gentleman stop by my place a few weeks back as he was having some difficulty with his repaired cruise unit. It turned out that it was something with wiring related.

When I installed and test drove it, I had the amplifier hanging down by the brake pedal, the unit worked flawlessly for well over 20 miles I'd say. After he secured it up with the panel the unit worked intermittently. Perhaps you are experiencing something similar?

I would take a multi-meter and test continuity between the cruise amplifier connector and the actuator plug.

Also, my offer to test out the amplifier as I mentioned in email still stands should you wish to send it. No charge except for shipping.

shertex · #3 05-11-2012, 09:46 AM

Thanks, Kris. So am I correct to assume that there's NO CHANCE that my ATF slop could have caused the cruise to stop working for a period?

JamesDean · #4 05-11-2012, 09:50 AM

You're correct. The transmission will have no effect on the cruise system as far as I am aware, unless the speedometer needle also stopped, which would mean the speed signal stopped.

The cruise control speed signal is shared with the speedometer gauge, its generated in the tailshaft of the transmission.

The cruise unit keys on this signal to move the actuator arm forward/backward to increase/decrease throttle as this signal changes. It does not care if the transmission slips, flares or is sloppy. Its only concern is maintaining that set-point by moving the arm of the actuator.

shertex · #5 05-11-2012, 09:52 AM

Sorry for my lack of clarity. My question is not regarding the transmission, but the ATF I used to lube the throttle linkage. I got it on the actuator and wondered if this could create a problem.

Quote:

Originally Posted by JamesDean

You're correct. The transmission will have no effect on the cruise system as far as I am aware, unless the speedometer needle also stopped, which would mean the speed signal stopped.

The cruise control speed signal is shared with the speedometer gauge, its generated in the tailshaft of the transmission.

The cruise unit keys on this signal to move the actuator arm forward/backward to increase/decrease throttle as this signal changes. It does not care if the transmission slips, flares or is sloppy. Its only concern is maintaining that set-point by moving the arm of the actuator.

JamesDean · #6 05-11-2012, 10:00 AM

Quote:

Originally Posted by shertex

Sorry for my lack of clarity. My question is not regarding the transmission, but the ATF I used to lube the throttle linkage. I got it on the actuator and wondered if this could create a problem.

Oh! No I doubt that caused anything. I, too, use ATF to lubricate the linkages. As long as everything is connected back up as it was you should be fine mechanically speaking.

shertex · #7 05-11-2012, 10:06 AM

Quote:

Originally Posted by JamesDean

Oh! No I doubt that caused anything. I, too, use ATF to lubricate the linkages. As long as everything is connected back up as it was you should be fine mechanically speaking.

I didn't even disconnect anything....it's really tight down there. Just tried to work some ATF into all the pivot points.

shertex · #8 05-11-2012, 01:05 PM

Later today, I tried it again and experienced a third condition. This time, the cruise would catch briefly then disengage. That, of course, is more symptomatic of a bad amp. Even though it's a GDL rebuild, I guess I need to be open to the possibility that there's a problem with the amp. So Kris, I may end up having you take a look at it after all....I'll play with it for a while, though.

Brian Carlton · #9 05-11-2012, 01:10 PM

Well, you've certainly managed to get more out of it than I could.............

I'd tend to put my money on GDL..........and consider that something is intermittent in the actuator.

shertex · #10 05-11-2012, 01:14 PM

Quote:

Originally Posted by JamesDean

David,

While you were under there did you verify that the amp was indeed a GDL unit? It might have been switched out at some point seeing as it was unplugged.
.

Yes....it had the typical GDL seal (although the seal was broken....I guess Brian wanted to take a peek).

Brian Carlton · #11 05-11-2012, 01:20 PM

Quote:

Originally Posted by shertex

Yes....it had the typical GDL seal (although the seal was broken....I guess Brian wanted to take a peek).

I never went inside it..........????

shertex · #12 05-11-2012, 01:21 PM

Quote:

Originally Posted by Brian Carlton

I never went inside it..........????

The seal is definitely broken. But it's a clean slice...so maybe something inadvertently cut it.

yuke · #13 05-11-2012, 04:56 PM

Shertex James Dean got my cruise working after many months of itermittent functioning he is excellant on cruise problems. I think in the end my problem was a lose connection somewhere behind the instrument cluster as when on dean's advice I pulled the instrument cluster out and fiddled with all the wires and connections I could touch low and behold my cruise has worked flawlessly since and I am very happy and keeping my fingers crossed that it keeps working. Dean told me that somewhere behind the cluster the cruise signal juntions with the speedo signal and I think that is where my problem was. Thanks again Dean and good luck with yours Shertex.