Hi,

I've been having an intermittent issue when driving on the highway, with cruise on or off, of RPM fluctuation by 100-200 RPM's on both flat ground and up inclines. Does not happen all the time and I cannot pattern it. I believe it is caused by the TC unlocking and locking repeatedly. The truck revs over 2000RPM at freeway speeds so I don't believe load is an issue causing a need for downshifting; there is enough power to climb without the truck needing to dump lock-up. I've replaced the TPS last year due to another issue so that can be ruled out. Any ideas what else I should be looking for / tests to perform? I will be doing a tranny oil change soon so if I need to dig deeper to look / replace stuff then I would like to do it all at once.

 

Possibly brake switch..... Usually its the TPS though.

 

Brake switch eh? Something to do with cruise control?

 

Nah. PCM disengages TCC when you step on the brake. If the switch is not quite right, PCM thinks you are stepping on the brake.

 

Researched it a bit more and looks like the TPS or brake switches are the culprits most of the time for most brands. Went riding up north yesterday, truck was fluttering whole way up but seemed OK on the way back except for after I hit traffic (and was braking a lot). Makes me think the switch might have something to do with it. I might just throw a switch at it since they are so cheap and get more technical if it doesn't fix it. I had troubles with brake lights staying on in the past, but that was the brake switch housing flopping around in the mounting bracket. We'll see! Thanks HeyYou.

 

Be sure to let us know what ya find.

 

I swapped the switch in and went for a test drive, truck was behaving itself with cruise on. I took apart the old switch for kicks and one of the contacts was disintegrated and a lot of carbon deposits inside. The new switch made me scratch my head for a second, I didn't realize the plunger is adjustable in length so I sat there wondering why the new one was a lot longer than the old. Just pull back the brake pedal, install it, and let the pedal do the adjusting of the switch for you.

One thing I don't like is that if I pull up on the brake pedal (say, if I were to bump the underside good with my foot on accident) the plunger adjusts to the next position shallower and the brake lights end up staying on all the time. Not sure if I'd end up doing this in practice as I pulled up intentionally with my hand the first time this happened, but nonetheless I don't really like it. I might try and move the switch bracket a bit to fix that (looks adjustable).

Anyway, I'll check in later once I've test driven it some more and see if this problem is gone for good.