DBF

1992 D-250 Auto.

I'm running down the road, turn cruise control on, and hit Set. The RPM jumps up and the truck increases speed rapidly with black smoke, lots of noise, everybody running for cover and wondering what that nut is doing.

Touching the brake pedal turns off the cruise control and all goes back to normal.

The diagnostic tree in section 8H of the FSM doesn't have a path for this problem.

Obviously, the adjustment clip is installed on the bell-crank end and the clevis clip is installed at the servo, otherwise the servo wouldn't pull the cable and get full throttle.

Vacuum is also good, or it wouldn't work. Plus, I measured it at 27" idling.

The servo plug electrical contacts have been cleaned and the electrical continuity checks on pp. 8H-7, continuing on 8H-8, are what the book says is correct.

There are no unexplained Fault Codes - just 12 & 55. No lights on the warning panel.

For maybe-related things: The speedometer is smooth, so no sensor problem there. The transmission shifts into O/D normally (pot installed vice TPS).

I'm stumped. I guess I'll clean the contacts on the PCM plug/socket next, but I sure could use anyone who's had this problem and solved it to speak up.

Regards, DBF

Gumby

I had the same thing happen when I installed my pot. I spent some time adjusting the pot & the runaway cruise went away.

DNRCustoms

if u are running a Pot controlled overdrive, switch the outside wires and re-adjust it. Get out on the open road. turn pot either full clockwise or counterclockwise. mantain your desired o.d shift point. Then slowly rotate the **** untill it kicks into O.d . from there small adjustments will get you right where you want to be.

DBF

Yup, that was it.

I'd put the pot in a nearly inaccessible place so I wouldn't hit it and screw up the setting. Then it worked fine for several years.

I must have moved it somehow.

Thanks to all responders....

DBF