I have a low milage 1989 f250 (30,000 original miles) that does not want to shift into 3rd until 55 or so. Even under light throttle it does not want to shift up. 1st to 2nd shift point feels fine. I checked the vacuum line and all is good and there was no oil in the hose going to the modulator and the vacuum hose to the modulator is in good shape and a tight fit.

 

I would think about possibly replacing the vacuum modulator anyway, the C6 in my 94 was shifting late and when I checked the hose going to the modulator I did not find any oil, but replaced the modulator anyway because I had already bought one and they are cheap. The problem went away. If the modulator is an adjustable one (non-stock), I would maybe look into adjusting it, but since the 1 to 2 shift is good thats probably not the problem. Also, if it is a Fram, I would replace it, the first modulator I put in my truck was a Fram and it caused my shifts to be really eratic.

 

Just an update on this. The reason it would not upshift is 3rd gear was shot and 2nd was on the way out. The truck sat idle for so long the seals deteriorated. Rebuilt the tranny and now it shifts like a dream.

 

Did you rebuild the tranny yourself or have someone do it. I've been contemplating a rebuild on a C6 and thinking about trying to do it myself.

 

I had it done by a transmission shop.