I have a 2004 Ram quad cab w/ Hemi motor and auto tranny - 2wd.

The vehicle goes into what I think is "limp mode". The front brakes lock up, and the truck won't shift or rev high. The speed sensors on the truck (according to my mechanic) are basically fried and melted after this happens. I've taken it to a few different shops and no one can fix it. Just replacing the speed sensors the truck will run for about a 2 wks and do it all over again.

Please forgive me as I am a professional nerd, and know nothing about mechanic work.

 

Limp mode isn't going to lock any brakes. Sounds like the front calipers are locking up, especially with the "... fried and melted ..." Time to find another shop that can work on brakes.

 

Why wont the motor rev high, and why after letting the truck sit over night, it starts working again for a few days?

Thank you very much for trying to help me out here. I'm lost.

I also read that limp mode will force you to pull over. Maybe I misunderstand it - what exactly does it do?

 

Your not resting your foot on the brake pedal while driving? Could be the brake light switch.....Under the dash mounted on the pedal...If the computer thinks you have the brakes on and are accelerating-driving it will pull power (close the throttle...it is run by the computer..drive by wire) Maybe a bad power brake booster pulling the brakes on? But then the brake lights (outside) should be on. Does the cruse control work? If not there is most likely a problem with the brake switch....when you push the brake it will turn it off....well not the cruse light on the cluster but it won't engage to hold speed. If they have a scan tool they should be able to read whether the computer is getting a released or applied signal from the switch. Just something to check..

 

No this is not happening. Are there any other suggestions I might try? Can someone please explain what limp mode actually does then?

 

Rev high when? Limp mode pulls the power back when the tranny is not happy.

I've done enough brake stands to know the computer doesn't pull the power back just 'cause the brakes are on and you're accelerating.

I'm suspecting a dragging brake pad building enough heat to cause the wiring to melt. I guess it's possible something is trying to draw so much current through the wiring that it's getting melted (but I doubt it - that's just a wheel-speed sensor).

 

Sounds to me like a sticking caliper and/or an internally collapsed brake hose that is not letting all of the fluid pressure release. This will cause tremendous heat and if the sensor circuit shorts to ground, it could be reeking havoc on the pcm.

Limp mode is a default mode that when the PCM isn't happy, it defaults to that will run it in open loop and limit power until the problem is corrected.