weird cruise control failure

[delucat2]

I’ve searched extensively without finding this particular mode of failure, so I’ll post.

I just acquired a very nice 88 560SL. PO couldn’t tell me much about the car’s history. My first – and only – experience with the CC was on the test drive. When I pushed the lever to accelerate (held up and released for a second) at about 60 mph on level ground, someone ordered full speed ahead, opened the throttle fully, forcing a downshift to 2nd, rpms and speed soared – throttle held open - for about 2 seconds. Someone figured out that there was a malfunction of some sort, and the throttle cut out completely, then went back to full throttle for a second or two, back to no throttle. The CC turned itself off as I tapped the brakes.

Does anyone have any experience with this mode of failure? Kind of frightening! No indication that the CC wants to hold current speed, only that an application of full throttle is warranted. Speedo is perfectly smooth at all speeds. I just did the taboo brake bulb replacement Wal –Mart cheapo bulbs - but I don’t personally have any belief that this effects the system in any way.

Any ideas? I have already pulled the troubleshooting info from GDL’s site, and will conduct this weekend. I’m a fairly adept electrical troubleshooter – 15 years on boats with electrical connectors in engine rooms that are full of very corrosive swarm moist salt air. I must say that 90% of the time the components people think are bad are really not. It is most likely poor contact in connectors, poor conductors (frayed/cracked wires and poor ground), and dirty bus bars or battery posts, so I think that I’ll be looking to do a complete plug/terminal/harness inspection before I remove any component.

Regards,
tony

[Gilly]

Sounds like the switch and servo motor are working, and the speed sensor must be working to. Although it kind of flies in the face of the logic you like to use, it sounds like the cruise control amp (control module)

Gilly

[Fred P.]

There is a breather hose going into the inside through the firewall. It is black, and about half again as big as a vacume line. It goes in by the vacume lines on the left side. It is plugged and the servo can't breathe

[NEAL560SL]

I had this identical same thing with my 86 560SL. I followed previous posts and removed the CC amp and resoldered every solder point on the circuit board and it has worked fantastic all summer.