Transmission problems :(

RyanZ71 · July 8, 2004

I've noticed this happening a few times. After driving at highway speeds (70-80 MPH) for about 30 min or so, and after having gone up slight inclines and then while going down the hills, i'll have my foot off the gas and instead of the truck remaining in gear it seems like it goes into nutural as the RPMs drop but when I put my foot back onto the gas pedal it jumps back up as if it is in gear, but again the same thing when I take my foot off of the gas. It will do this a few times and then it will drive normally. It doesn't happen always but it has happend a few times now. Sometimes while driving and holding a steady speed the RPMs might jump around just a bit kinda feeling like it's slipping in and out of overdrive too from the feel of the truck. Any ideas? The truck has about 88,000 miles on it and it is the original tranny to the best of my knowledge. Tranny fluid is at proper level. Haven't had the tranny serviced yet was going to have it flushed and a new filter put on but haven't gotton around to doing it yet.

snoman · July 8, 2004

First of all it has been my experiance that both GM trucks I own with OD (a 4L80 and a 700R4) it does unlock converter at speed when you removed your foot from the gas from cruise. My "guess" here is that your gearing is less than optimal for your tire and conditions and when you first step back into it the tranny/ECM cannot make up its mind on want combo to use (locked or unlocked) based on road speed and load (based on TPS and MAP reading) and hunts around for a bit sometimes. There is not a problem with tranny per say, it lies in the overall setup. That fact that is is more noticable after driving for a while is just that your gearing is marginail and engine output for TBS and MAP data has dropped enough in a hot engine to make it more noticable. Drop down to at least one gear ratio step lower (or smaller tires) and the problem will go away.

Jeremy WI · July 8, 2004

Mine does the same on downhill, it happens more often in third I think than in OD. Its called deceleration mode, if the decel is rapid or for a long period the PCM may shut off the injectors. As for your torque converter that is fairly normal too for trannies with the TCC PWM solenoid as the PCM will only apply enough pressure to the TCC to keep the slippage in acceptable parameters and when you press the accelerator it slips a bit until the PCM raises the pressure

Sometimes while driving and holding a steady speed the RPMs might jump around just a bit kinda feeling like it's slipping in and out of overdrive too from the feel of the truck. Any ideas?

Do the rpms continue to jump around if you stay at a steady speed with slight pressure on the brake pedal(just enough to disengage the TCC)?