Cruise control problem... won't stop accelerating

This problem use to happen only ocassionally. Now it happens all the time. Now it happens every time I engage the cruise. I turn the cruise control on, and press the set button at a constant speed. Cruise control takes over and begins to accelerate. While it is doing that the coast button does nothing. I thought the Accel button may be sticking, but the cruise control won't engage with the button depressed.

According to the FSM, the PCM controls the Cruise servo, which is the same one the speedo gets the signal from. And the speedo works fine. The PCM gets the vehicle speed from the speed sensor on the rear of the transfercase. The servo has a constant positive going to it unless the brake pedal is pressed. The servo has three solenoids. Two controlled by the PCM, one by the power input from the brake pedal. The two controlled by teh PCm are for the vacuum and vent on the servo. When you engage the cruise the vent solenoid is grounded, closing the vent to keep the servo at one position. When the PCM wants needs to decelerate the vent solenoid is un-grounded and the valve opens, allowing the vent to open (vent to atmosphere I believe), which is the same as when teh cruise is off. When the PCM needs to accelerate the vent solenoid is grounded as is the vacuum solenoid, allowing vacuum in and closing it off to the atmosphere.

So with that in mind.. at rest with the cc off, the vent valve is open and the vacuum valve is closed. Nothing is grounded. When the cc is engaged and has no need to decelerate the vent is closed and the vent solenoid is grounded. When teh PCM wants to slow down to the selected speed the vent valve is cycled open and closed until the set speed is reached. On acceleration the vent solenoid is grounded (vent is closed) and the vacuum solenoid is grounded (vacuum opened).

There is also one other solenoid called the purge solenoid, which gest the + from the brake switch (as long as the brake pedal is not pushed it gets power) and the other end is grounded. So, it is closed until you hit the brake.

So... from that, it seems like the PCM is bad.. for the following reasons:

1) if its was the servo, I would notice other symtoms, like the cruise not working at all or my XJ wanting to accelerate even with the cruise off.

2) It can't be the wiring because after the XJ gets over the set speed the PCM will want to slow it down, removing all grounds from the solenoids (like when the CC isn't engaged) until it gets to the set speed. So, even if the wires going to the vacuum and vent solenoid were some how fused together the PCM still should remove ground from both of those when the XJ has exceded the set speed.

Any ideas?

-Chris

I´d call Jeep and ask them if there is a TSB on the cruise control. Chances are your problem isn´t unique. Safety problems are rather imperative to correct now. At least get something on paper, that you asked. Sounds like a good situation for the old FAX machine, to have a paper trail and record of correspondance. I never accept the first "no" I hear from the dealer or the manufacturer. Every beef I´ve ever won, from the dealer or manufacturer is becuase of the thickness, completeness and wieght of my paperwork.
Get names and keep a diary of correspondance, to go along with your pile of correspondance. Anytime I talked to somebody new, I´d mail/fax/e-mail them the whole package, they give up faster, when they see your serious.
If it is an existing safety TSB, Jeep may fix it fast and for free.
Last bit of advice, if it isn´t written down, it never happened, best to start a record right away.

I'm going to stop by the dealer tomorrow. No need for a paper trail or any of that. I am on first name basis with the parts and service guys. They have done a few favors for me in the past including borrowing tools and manuals. Good guys.

Anyway, I tired to get some idea of what is going on, and it appears it isn't the PCM. Hooked up a test light to the various wires on the cruise servo, and from what I saw it appeared that the PCM is doing its job correctly. The PCm only grounded teh vacuum solenoid when it was actually going slower than when I engaged it.

-Chris

Its fixed. The servo was bad. Dealer had it for only $35. And I thought it was going to cost over $100. And they had it tehre too!