I have a 2004 F-350 super duty power stroke 6.0L diesel with a 6-speed manual transmission. The truck has 59,000 miles on it. Cruise control works fine on flat roads and downgrades but shuts off on all upgrades. I'm thinking brake pedal sensor. What are your thoughts?

 

Being intermittent it is going to be tough to find. This might be a stupid question with the engine you have, but the engine is keeping up on steep hills? If the actual speed gets too far away from the set speed the cruise will automatically drop out. Nothing you can do about it except downshift and when you do that you will lose the cruise anyway. Which brings up another point;

You should have two switches on your truck since it's a manual; One on the brake pedal like all cars and trucks, but you have another one on your clutch pedal. If you push the clutch in while in cruise it should shut off. See if you can look up where the brake pedal switch is located, sometimes they had a redundant pressure switch on the master cylinder. This is the famous switch that was burning people's garages down, see if your year has a recall for it. They were famous for brake fluid leaking into the pressure switch and causing the switch to heat up and catch fire.

On any of these interlocks, the only way I know to pinpoint the problem is to jump that particular switch out and see if the problem goes away. Of course you want to be on your game when doing this, know it may not cut off the way you normally deactivate it. But if you do one at a time, you might be able to pinpoint which one it is if the problem suddenly goes away.

 

I watched the tachometer and listened closely to the engine a couple times after the shutoff. The RPMs remained the same and the engine did not sound like it was struggling. But now that i think of it, I was told that the technician/mechanic that did the "bulletproofing" of the 6.0L said he felt the clutch slipping in overdrive. The owner of the shop test drove it and said he could feel the slip as well. Could the cruise be shutting off when it feels the slip? Perhaps the sensor is interpreting the slip as loss of power in the engine.

 

If set mph , declines to i believe 5 mph under , cruise will shut down , but only require you press resume .
I can't remember , but I think the servo for the cruise , is operated by vacuum, held in the black reservoir, passenger side .
On a diesel , that is filled sontonspeak , by the vacuum pump , also in that same area .

 

There is no decrease in MPH or RPM when shutoff occurs.