Tow Haul problems

hugegunnz · September 16, 2015

So I need some help, I have an 08 GMC Sierra 3500. My T/H will not engage at higher speeds. If driving around town under 30 mph it WILL engage. Is there a limit switch of solenoid that is not engaging? It seems that after 50 MPH it WONT engage. Local mechanics are stumped.

Can I please get some help.

Thank you

Scott

Formulabruce · September 16, 2015

The tow haul is just an interrupter to the lock up wire on the tranny. Have to put a meter probe on that pin on the 4L80E and then drive it and see if the +12v drops out when the tow haul is pressed, which should put you in 3rd gear with no lock up. If the wire goes dead when the tow haul is set, probably a transmission controller issue as the lock up solenoid rarely can stick engaged. If its locking up in every gear it will shift jumpy. Good luck!

hugegunnz · September 16, 2015

Thank you it shifts just fine. But its funny that it shifts into T/H mode at lower speeds. Is there something to be said about reaching 50 MPH and not going to haul mode?

Chevor · September 16, 2015

Normal. All tow haul does is lower shift points, keep torque converter locked.

newdude · September 16, 2015

Your question doesn't make sense to me. Tow Haul (the button on the shift lever) can be activated at any speeds. If you are cruising along at 60mph lets say, and hit the button, you may not see a down shift to 5th or anything like that (especially if you are empty) and it will just drive along fine. This would really only be seen driving with no load.

hugegunnz · September 17, 2015

15HDriver,

That's what I am unclear on is why it won't shift at higher speeds. But I do know it's not working because I was pulling a full load and noticed the low rpm's at high speed also it overheated. Now, with the T/h button Activated it does however operate going downhill. The transmission brake worked as it should.

One thing I should mention is I had a aftermarket fuel pump to "pump" fuel Instead of relying on the injector pump to draw fuel from the tank. Would that be causing the issues???