My truck runs great...moves and spins the wheels...the only thing is that i think it may not shift into the final gear. At 55 or 60 when i left off the gas it seems the truck is "tugging" as it slows down.

Its a 1976 C6 transmission. That means its a 3 speed auto right? What do you do when its not shifting into the third gear? Rebuild or shift kit? Could the valve body be clogged? I just need a direction to move it. If the shift kit will fix it I want to do it but if it calls for a rebuild i dont want to waste my money on it. Thanks for your help!

Josh
1965 F100 Custom Cab SWB, '76 460, C6

 

Hmmm 2nd gear at freeway speeds is usually pretty buzzy experience, what RPM do you think you are turning? If it is something like 4,000 RPM then yup you are probably stuck in 2nd. If you are turning between 2 and 3KRPM then depending on your gear ratios, you are probably all right. In other words, if you are stuck in 2nd, you should be realtively uncomfortable due to noise, buzzyness etc.

I am pretty sure the C6 was a 3 speed. Don't know if it had a locking torque converter. My guess is that everything is normal and that when you let off the gas it is just decelerating noticably due to the high gearing of most older trucks with 3 speeds.

Not an expert yadda yadda, so I recommend someone who is used to your model to drive it and tell you what they think. If you really do have a problem it could be many differnt things, sticky valve body, dirty ATF, sticky Posi rear end, maybe even different size rear tires on a posi.

Also most tranny shops will do a free test drive, but be warned some shops will always find a reason for a rebuild, you you have to sort out the advice you get.

Good Luck,

Jim Henderson

 

C6 is a 3 speed, no locking anything. Like Jim says, if it really is in second, it would really be revving at 60 MPH.
From a standing stop, accelerate moderately, and count the shifts, should just be two of them, the 1-2 and the 2-3.
Third gear is a 1:1 straight through ratio. If you have been driving vehicles with a 4 speed auto, 4th gear is an overdrive roughly .7:1, and then get in an old 3 speed auto, the coast down drag of the three speed seems to pull the speed off fast. But the 4 speed coasts a lot more with the overdrive, a lot less engine braking effect.
Now if you find that it IS in second at 60 MPH, you will have to figure out if your low on vacuum, throttle opened too far for that speed. If you have a large throttle opening, intake vac will be low, which tells the vacuum modulator on the trans to keep the fluid pressure on the throttled side of the 2-3 shift valve up high. That then requires higher ouput shaft speed to spin the governor faster, to increase the governed fluid pressure on the other side of the 2-3 shift valve to overcome the throttled pressure. Delaying the upshift.
When the governed pressure exceeds the throttled pressure, the shift valve will move to allow the upshift.
An easy gentle acceleration keeps the throttle closed down more, which makes the throttled fluid pressure lower, so the governed pressure overcomes it at a lower output shaft speed. That's what makes the upshifts happen at a lower speed when accelerating easy.