I been driving truck everywhere since i purchased it. I put a new radio and speakers in truck and now it wont shift into 3rd gear. I can manually put it into the gears but not while driving. It holds 1st gear and then shift extremely hard into 2nd gear. I still have all my gears, including reverse. It just wont shift past 2nd. Ive replaced TPS, flushed transmission and lines. Ive put new trans filter in pan. And still nothing. HELP!!

 

if the cable is not hooked up correctly it can keep your trans in second gear as the valve body is getting a wide open throttle signal from the improperly attached cable. You could try disconnecting the cable completely, if it is in fact the TV cable causing the problem, then it should shift from low to second to high gear very quickly, it may happen so fast that you might not even feel the shifts, that is what they would call stacked shifting. then you could command shift using the gear selector, start off in low then upshift to second then third, you should be in third gear right at 30mph, light steady throttle, remember your just diagnosing a problem, you don't want to blow a tranny doing full throttle shifts

 

Welcome to the forum from Oregon.

Yes, suspect the shift cable. Start with basics first.