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Discussion Starter #1
I have a 2002 gmc 2500 dmax/automatic allison...

when I tow my horse trailer up a fairly steep hill or mountain it does not shift to a lower gear until it is bogged down/straining at 45-55 mph. If I run it with cruise control that baby kicks down to a lower gear exactly when it needs to and climbs aggressively as it should.

Any tips or diagnosis would be helpful? The trailer is only 4-5000 lbs empty.

My mechanic thought I may need to manually drop it in 3 from D... but when?

Thanks
 

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What tire size? A lot of things enter into this. Engine power ( fuel filter ); engine capability to make power. Need more info on the truck and what it was put through.
 

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the tires are stock, I would need to get the exact size of the tires.
If stock not necessary. You ask a lot and don't give much info. We don't have crystal balls. Fill out a signature and give full history without leaving anything out. Otherwise there is no help.
 

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Discussion Starter #6
Mike, the truck is as if it came from the factory, no modifications, chip use etc. Maintenance performed. No chrystal ball needed. It is a 2002 model dmax with at transmission.
 

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How many miles on the truck?

What's your maintenance status?
>>Miles on trans fluid/filter...intervals?
>>Miles/gallons on fuel filter...intervals?
>>Injector rates/history?
 

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Steadily apply firm but even pressure to the skinny pedal on the right with your rightmost lower appendage until it will not go any further (floor it) if it still wont downshift you got a problem, These things dont downshift much if you dont give it more throttle unless they really bog down. If you hold a steady throttle going up a hill its gonna slow down until it needs to shift, not when you want it to. Giver a little hell next time and see what happens, Thats all your cruise control does anyway, applies more power when it needs it.
 

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Discussion Starter #10
Houston, we have a problem. My leg (the right one) cramps from pushing the skinny pedal down to the floor. Hell has been given and the problem exists and thus the reason for the post...Thanks
 

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Truck needs to see the doctor then, Its possible that you have a failing TPS sensor on your pedal. Does it seem to have normal power empty at WOT? cruise control bypasses the tps so if its bad your cruise control wont know it and the truck will act normal, No cruise and it goes back to the tps and engine monitoring. Bad TPS signal and the truck wont know exactly how much throttle your giving it
 

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The TPS sensor is on the pedal. I think the 02's were still drive by wire, it is a variable resistor that tells the engine where the gas pedal is. They call it an "accelerator pedal position sensor" They only run around $40-120 if yours is shot
 

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I wouldnt say Im experienced I just am able to think through systems and look where others might not. Im not saying thats what the problem is but its one thats easy to rule out with either a scan tool or a multimeter if necessary and pretty cheap to fix if its bad.
Its one of the only things taken out of the loop when you push the cruise control button
 

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Discussion Starter #17
OK, had new APPS installed (accelerater pedal position sensor) and wow what a difference...solved this problem and it now has all the power needed. 98 dollars plus 35 to install, no codes ever came up. Like a brand new truck now!
 
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