I agree with checking the wiring under the hood. My throttle issue was making the truck go into limp mode. Limp mode causes the engine and tranny to act just like you describe. My scams would get codes for throttle voltages. Turns out ONE wire I had repaired under the hood didn’t have a proper enough repair. It was one of the throttle 5v circuits. There are 2 for redundancy. While inspecting the main harness I found lots of shiners(wires so squished the insulation had split exposing the copper, hence “shiner”) under the engine cover. I removed the throttle harness all the way back to the alternator and replace it(soldered in a spare I had). Just some .02$. Good luck.
2004 Tahoe Throttle issue
- Thread starter Twithers
- Start date
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I did order a new throttle body, so I'll go over all the wiring when I replace it. The thing is that I don't have any codes related to the problem, or apparent limp mode. I even went into the other modules with my bidirectional scanner, but I didn't find anything. With the scanner connected, I can watch the real time throttle percentage as I drive and even getting onto the highway, cycling through the gears with the pedal to the floor, it never goes higher than 55% (57% with my other scanner). I think it's either a wiring issue, or I have to replace the gas pedal assembly, to replace the pedal position sensor. Those run around $200, so I figured I'd try the cheaper throttle body and check the wiring. I was hoping that someone else had come across the same problem and found a wire causing it. That way they could help me narrow down where to look. There are alot of wires involved and alot of them are deep under the dash.
rockola1971
Full Access Member
There is no such thing as a "passing gear". What happens is when the throttle is actuated past a certain percentage then the transmission is commanded to downshift one lower gear than what is already at. On the highway you typically are in 4th gear (OD) and the tranny is commanded to downshift into 3rd, RPM's go up and tada, you have torque available for use (to speed up).
There is a very valuable clue in the OP's first post. "The scanner shows the throttle input only goes to 57% and no higher than that". That would explain why you dont get the downshift for passing. The reason you dont get above 57% is simple. The Accelerator Pedal Position(APP) "Sensor" which is just a potentiometer has a worn/dead spot in one of its elements which is only giving you a little over half (57%) travel reading output which is typically in the area of throttle position most used for highway/interstate driving so that spot on the potentiometer receives the most wear when a vehicle spends alot of time on the highways. The APP "sensor" and the throttle position sensor (TPS) on the throttle body have nothing functionally to do with one another but they are each part of the throttle system which also includes the TAC (Throttle Actuator Control) Module and the motor that drives the throttle blade on the throttle body. The APP inputs it signal in relation to accelerator position to the TAC Module. The TPS gives feedback as to the position of the throttle blade in the throttle body to the TAC Module. The Throttle Blade positioning motor is commanded by the TAC Module to got a 0-100% angle based on input of the APP to the TAC Module.
Now if the TPS signal and the commanded Throttle Blade Position do not agree then you get "Reduced Engine Power" on IC and tada Limp Mode. The 2 common causes of this is a dead/worn out spot on the TPS and/or break in the wire (not necessarily the insulation) of the wiring harness going to the TPS. That is why replacements typically come with a new plug and 12" of pigtail with butt splices (which I do not recommend ever using on an automobile)....SOLDER THAT WIRE SPLICE!
Bad APP sensor should not ever cause a "reduced engine power" or limp mode. The TAC Module doesnt know that you should have more than 57% travel of your APP. As far as it knows, you are just cruising at 57% throttle and it is just fine with that.
If you really have gremlins then it is possible that the voltage is low enough to the APP so its output voltage can never go above what the TAC Module interprets as 57% throttle. But If I were not able to check voltages I would just shotgun this one with a new APP or Pedal Assembly if you cant get the APP separate.
If you are really interested in seeing more about this system and a vehicle specific example of real world voltages then have a read here : https://www.lsenginediy.com/gm-gen-iii-ls-pcmecm-electronic-throttle-equipment-guide/
There is a very valuable clue in the OP's first post. "The scanner shows the throttle input only goes to 57% and no higher than that". That would explain why you dont get the downshift for passing. The reason you dont get above 57% is simple. The Accelerator Pedal Position(APP) "Sensor" which is just a potentiometer has a worn/dead spot in one of its elements which is only giving you a little over half (57%) travel reading output which is typically in the area of throttle position most used for highway/interstate driving so that spot on the potentiometer receives the most wear when a vehicle spends alot of time on the highways. The APP "sensor" and the throttle position sensor (TPS) on the throttle body have nothing functionally to do with one another but they are each part of the throttle system which also includes the TAC (Throttle Actuator Control) Module and the motor that drives the throttle blade on the throttle body. The APP inputs it signal in relation to accelerator position to the TAC Module. The TPS gives feedback as to the position of the throttle blade in the throttle body to the TAC Module. The Throttle Blade positioning motor is commanded by the TAC Module to got a 0-100% angle based on input of the APP to the TAC Module.
Now if the TPS signal and the commanded Throttle Blade Position do not agree then you get "Reduced Engine Power" on IC and tada Limp Mode. The 2 common causes of this is a dead/worn out spot on the TPS and/or break in the wire (not necessarily the insulation) of the wiring harness going to the TPS. That is why replacements typically come with a new plug and 12" of pigtail with butt splices (which I do not recommend ever using on an automobile)....SOLDER THAT WIRE SPLICE!
Bad APP sensor should not ever cause a "reduced engine power" or limp mode. The TAC Module doesnt know that you should have more than 57% travel of your APP. As far as it knows, you are just cruising at 57% throttle and it is just fine with that.
If you really have gremlins then it is possible that the voltage is low enough to the APP so its output voltage can never go above what the TAC Module interprets as 57% throttle. But If I were not able to check voltages I would just shotgun this one with a new APP or Pedal Assembly if you cant get the APP separate.
If you are really interested in seeing more about this system and a vehicle specific example of real world voltages then have a read here : https://www.lsenginediy.com/gm-gen-iii-ls-pcmecm-electronic-throttle-equipment-guide/
Has anyone separated the pedal position sensor from pedal and replaced it? I've been able to find potentiometers that should be compatible replacements, but it would be best to get more info off the original pedal sensor to make sure it's right.
It makes sense that there would be a dead/worn spot inside the potentiometer at the point where the pedal sits most often. I don't understand why the signal stops at a certain percentage and no higher. Since the sensor hasn't failed completely, if there is a worn spot, when watching the pedal percentage as you push the pedal to the floor, you would see it cycle from 0 up to the dead spot and then jump up to the point past the wear spot and continue climbing. The full stop at the 57% doesn't make sense to me.
I have a background in component-level electronics, including some micro/miniature repair. Most potentiometers that begin to fail over time, are losing connection internally because corrosion builds up on the circular series of contacts inside, causing a break in the circuit. It's probably overkill, but it makes me wonder if it's not too difficult to pull the sensor from the pedal, could the potentiometer be opened and the contacts cleaned to fix the problem.
It makes sense that there would be a dead/worn spot inside the potentiometer at the point where the pedal sits most often. I don't understand why the signal stops at a certain percentage and no higher. Since the sensor hasn't failed completely, if there is a worn spot, when watching the pedal percentage as you push the pedal to the floor, you would see it cycle from 0 up to the dead spot and then jump up to the point past the wear spot and continue climbing. The full stop at the 57% doesn't make sense to me.
I have a background in component-level electronics, including some micro/miniature repair. Most potentiometers that begin to fail over time, are losing connection internally because corrosion builds up on the circular series of contacts inside, causing a break in the circuit. It's probably overkill, but it makes me wonder if it's not too difficult to pull the sensor from the pedal, could the potentiometer be opened and the contacts cleaned to fix the problem.
Mudsport96
Full Access Member
BEFORE YOU GO ANY FURTHER!!!!!!
Read my reply in your thread about the wrong ecm.
03 to 05 vehicles have different programming than 06 and 07.
The computer is lost because it cant read the pedal position over 57%
Read my reply in your thread about the wrong ecm.
03 to 05 vehicles have different programming than 06 and 07.
The computer is lost because it cant read the pedal position over 57%
rockola1971
Full Access Member
Well that changes things. Wondering if OP's Tahoe with engine swap ever worked correctly past the 57%? I had assumed we were troubleshooting a failure after things worked correctly. Shame on me.
Mudsport96
Full Access Member
Here is a snippet from gearheadefi.com
There are many threads on the differences in the GM Gen 3 Truck DBW systems. The short answer is that 2003 to 2005 models used one system of parts, while the 2006 and 2007 (classic) used a different system of parts.
If you wish to program a 2006 or 2007 PCM (1MB P59) with a 2004 calibration, and it is DBW, then you will also need to change the TAC Module, APP (pedal), and the short wiring harness that connects them.
Here is a thread from the EFILive Forum:
https://forum.efilive.com/showthread...-my-2006-truck
There are many threads on the differences in the GM Gen 3 Truck DBW systems. The short answer is that 2003 to 2005 models used one system of parts, while the 2006 and 2007 (classic) used a different system of parts.
If you wish to program a 2006 or 2007 PCM (1MB P59) with a 2004 calibration, and it is DBW, then you will also need to change the TAC Module, APP (pedal), and the short wiring harness that connects them.
Here is a thread from the EFILive Forum:
https://forum.efilive.com/showthread...-my-2006-truck
Gotcha. I had a feeling something like that was going on. There are so many wierd gremlins going on with this truck that I finally figured I should get an ECM from the right vehicle at least and see what all corrects itself after the swap. Might save me alot of times chasing ghosts. When I scan the ECM currently installed, it says it's a 2006 Chevy C-series with a 6.0L. Different VIN and all, obviously. It kind of amazes me that it runs as well as it does. If my scanner is correct, I can't imagine all the system inputs and outputs that are being processed incorrectly because the computer is mapped for a 6.0l.
Here's the end result of this issue. Same write up as my other thread:
So I put in the ECM I got online from the '04 Suburban with a 5.3l. I did the 3 cycles of 15 minutes with the key on. The truck started right up the next time. I let it idle for about 15 minutes. Since the ECM came from a different vehicle with all it's own learned info, I disconnected the battery for 15 minutes. Reconnected the battery and took it for a test drive. So far I've found that just the change of ECM corrected all of the throttle problems, passing gears, speedometer accuracy, fuek range calculation and a couple others I can't think of at the moment.
Thanks for the help.
So I put in the ECM I got online from the '04 Suburban with a 5.3l. I did the 3 cycles of 15 minutes with the key on. The truck started right up the next time. I let it idle for about 15 minutes. Since the ECM came from a different vehicle with all it's own learned info, I disconnected the battery for 15 minutes. Reconnected the battery and took it for a test drive. So far I've found that just the change of ECM corrected all of the throttle problems, passing gears, speedometer accuracy, fuek range calculation and a couple others I can't think of at the moment.
Thanks for the help.
Mudsport96
Full Access Member
Glad to hear it sounds like the serious issue is fixed. Still may want to find someone that can do a VIN flash for you and get all the vehicle specific software correct
