C6 won't shift by itself
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Tuned
Joined: Nov 2016
Posts: 269
Likes: 13
From: Shelbyville
C6 won't shift by itself
79 F-150 460 C6 4x4
My transmission will not shift on its own. It will stay in 1st all the way up to at least 3000rpms, I haven't tried to take it higher. I can manually shift into 2nd and it hits hard when it goes to 2nd then back to D and it will stay in 2nd. My truck was down for about 3 months to repair some minor engine fire damage and my joy of having it back up and running was immediately squashed by my faulty transmission.
Does anyone have ideas on what would cause a C6 to do this?
My transmission will not shift on its own. It will stay in 1st all the way up to at least 3000rpms, I haven't tried to take it higher. I can manually shift into 2nd and it hits hard when it goes to 2nd then back to D and it will stay in 2nd. My truck was down for about 3 months to repair some minor engine fire damage and my joy of having it back up and running was immediately squashed by my faulty transmission.
Does anyone have ideas on what would cause a C6 to do this?
Hotshot
Joined: Apr 2009
Posts: 14,333
Likes: 247
From: Stanley, VA
Either the vacuum modulator is bad or the vacuum line going to it is bad.
With fire damage, my bet would be the vacuum line at the rear of the engine took a hit. Check that first. Then follow the line all the way down and check for kinks or other damage.
Pull the line off at the modulator, if trans fluid leaks out, its bad. If no fluid leaks out it could still be bad.
Replacing the modulator is simple. One bolt holds a clamp on that clamps the modulator to the trans case, it just slips in and uses an o-ring seal. There is a small pin in the modulator. Be careful when you remove the modulator, the pin may or may not come out with it. If it does come out, don't lose it, you reuse that pin.
Also, the modulators are identified by the color of the band on them. The most common and the "go to" modulators are either green or black stripe.
With fire damage, my bet would be the vacuum line at the rear of the engine took a hit. Check that first. Then follow the line all the way down and check for kinks or other damage.
Pull the line off at the modulator, if trans fluid leaks out, its bad. If no fluid leaks out it could still be bad.
Replacing the modulator is simple. One bolt holds a clamp on that clamps the modulator to the trans case, it just slips in and uses an o-ring seal. There is a small pin in the modulator. Be careful when you remove the modulator, the pin may or may not come out with it. If it does come out, don't lose it, you reuse that pin.
Also, the modulators are identified by the color of the band on them. The most common and the "go to" modulators are either green or black stripe.
Cargo Master
Joined: Nov 2011
Posts: 3,354
Likes: 48
From: Great American SouthWest
I did a similar thing. When chasing down a vacuum leak, I capped ALL the lines. Took it for a drive and wondered, at first, why the brakes were hard. Then I wondered why it was revving so high. Then I wondered why is was still in first. Then the light came on...
Cargo Master
Joined: Nov 2011
Posts: 3,354
Likes: 48
From: Great American SouthWest
Glad you got it fixed! No greater disappointment than to fix one problem then have another, potentially bigger, problem rear its ugly head
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