Cougsfan

Every once in a while when cruising along at 60 mph or so and hit a fairly mild bump, like when the road changes from asphalt to concrete on an overpass, my truck can do funny things. First the cruise control might kick off (pushing resume will speed her back up to set point again). Secondly, if the cruise control isn't on, sometimes the truck will downshift and stay downshifted until I back off the throttle. It is far more apt to do this on a hot day or after a lot of miles. Some days I can't make it do it, other days it does it all the time.
I suspect the TPS. I have read threads on drilling holes in and cleaning the the TPS, replacing the TPS with a potentiometer, and of installing a capacitor between a couple of the wires.
Am I headed the right way? Which of those seems like the best route?

CarlJensen

Typical TPS or brake light switch problem. Go over some bumps and hold the brake pedal up when it starts going goofy on you. If it stops replace brake light switch. If not it's a very good chance it's TPS. Don't bother cleaning, just replace.

Cougsfan

I have replaced my brake light switch once already to get the abs/brake light to go out, and to just make the cruise control work at all. I see how it could make the cruse control kick off but am not sure I see how it can cause the transmission to downshift. I have read numerous posts about people replacing the rather expensive TPS and it not solving the problem.

slevhansen

Try to check the voltage at the TPS, I did that a while ago, it was 0,15v, so I made the mounting holes a little bigger, got it to 0,5v, and that made HUGE difference.
It is an easy fix..... I did it

Slev

delsilver

I adusted the voltage on Mine, it was .6 and i took it down to .2 and made a huge difference. I had to baby the throttle to get it to shift into overdrive.
There is a nut on the bottom of the TPS that is turned to adjust the voltage rather than drilling out the mounting holes. Very small increments make a big difference. I sharpened a paperclip and pushed it into the the center wire of the plug from the backside so i could read the voltage chage while turning the nut. There is a very good write up on this but not sure right now where to find it.
I did this before changing to the potentiometer because if you do that you no longer have the kickdown function in overdrive, have to manually diengage with the OD button.

infidel

It does, the tranny kicks out of OD when you apply the brakes.

Similar symptom problem I ran into was when a fellow replaced his stock sized batteries with ones that were shorter.
He cranked down on the hold down bolts without looking at where the extra length went.
It punctured a wire loom under the driver's side battery but only shorted when he hit a bump and the wires hopped.
The short caused the PCM to shut down for less than a second, cruise and OD kicked out.
The tach also dropped but was hard to see because it did it so fast.