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John, GMC Technician
Category: GM
Satisfied Customers: 9176
Experience:  ASE MASTER TECHNICIAN/40 years experience
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2004 GMC c2500: stays..throttle..degrees..voltage drops to 1.5 volts

Customer Question

Can you answer questions about a 2004 GMC C6500 with and 8.1 L engine.

When you press on the throtle it goes up to 3000 and then immesiately falls to 2000 and stays there. The throttle positioning sensor drops to 22 degrees and the voltage drops to 1.5 volts. We have replaced the throttle positioning sensore and the APP peddle and this made no difference. The APP goes to 100% and stays but the voltage drops to 2.5 volts. If the truck is not running and the key is on the throttle positioning sensor goes to 100% angle and the voltave goes to 4 volts however, when the truck is started it goes to the above mentioned settings. Is this the sign of a default or limp mode. It is also a manual transmition.
Submitted: 7 years ago.
Category: GM
Expert:  John replied 7 years ago.
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john :

Hello & Welcome to JA,

Expert:  John replied 7 years ago.
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Customer: replied 7 years ago.

The only response we have recieved to date is Hello and Welcome to JA.


What additional information do you need?

Expert:  John replied 7 years ago.

What code are you getting ?


Does this vehicle have a PTO ?

Customer: replied 7 years ago.

We have it hooked up to a Genesis and are getting engine readings but no trouble codes.


Yes there is a PTO it operates the dump bed.


Expert:  John replied 7 years ago.

Ok,check the data parameter for the PTO. If it shows that the PTO is active ,the computer will trim the engine rpm to a max lower rpm.


Here is a GM bulliten on PTO switch

PIP3187A: Engine Has Lower Power Check PTO Switch - keywords 6.6 6.6L 8.1 8.1L allison automatic diesel fuel L18 lack LB7 LBZ LLY LMM MAF no pressure RPM sluggishness sponginess speed - (Jun 9, 2009)

Subject:Engine Has Low Power, Check PTO Switch

Models:2001-2007 Chevrolet Silverado Classic
2001-2007 GMC Sierra Classic
2003-2009 Chevrolet Kodiak
2003-2009 GMC Topkick
2007-2009 Chevrolet Silverado
2007-2009 GMC Sierra
Equipped with the 6.6 diesel or 8.1 gasoline engines


This PI was superseded to update model years. Please discard PIP3187.


The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

A customer may have a concern of low power. The dealer may notice that the engine will not go over 2000-2300 RPM.

A low power concern can be induced if the Power Take Off (PTO) switch is on.

If a low power concern has been duplicated, complete the current SI diagnostics for any trouble code or symptom found. During diagnosis be sure to check the PTO switch. When the PTO is enabled, it may limit the engine speed between 2,000 and 2,300 RPM (RPM that is reached may be dependent on actual PTO settings).

If the PTO switch is on, lower than expected readings on the FRP Regulator Fuel Flow Command, Turbocharger Boost Pressure, and Mass Air Flow readings may be noticed.

It may also cause changes in transmission operation such as long or delayed upshifts.

Evaluate low power after PTO has been disabled.

John and 4 other GM Specialists are ready to help you
Customer: replied 7 years ago.

Thanks for your time it turned out to be that the traction control button was on.

Expert:  John replied 7 years ago.
Good to hear you found it,hope you have a good evening.