I driving an 88 F250 5.8L W 4WD. I am pretty certain that I have a C6 (3 speed, no OD, L-shaped cover). I am having some trouble with the tranny shifting and I was wondering if someone could straighten me out on adjustments.

I bought the truck in Feb of this year, and was told the tranny was reciently rebuilt. My old mechanic checked over the truck and pointed out that the tranny was shifting late (as if I didn't notice), but that the fluid was new and clean.

So, I have been trying to figure out if there were any quick adjustments/checks I could do to try to get my shifting a little smoother. I have narrowed down to two areas that I was thinking of checking out.

The first is the kick-down cable (or whatever it is that downshifts when you give it more gas). If this weren't properly aligned, is it possible that I would be shifting late because the truck thinks I am trying to accelerate too rapidly? Is the adjustment of this fairly easy/straightforward? Do the symptoms meet this diagnosis?

The second thing I was thinking was a vacuum problem. I checked out the lines in the engine compartment and they all seem ok. My brake booster was leaking, so I switched that, but it didn't improve anything. I was wondering if I had the right transmission modulator valve. Advance has four different types for a C6, black stripe, green stripe, brown stripe, and purple stripe. How do I know which modulator to get? If I have the wrong one in there, would it make me shift late?

Thanks for any help guys,

Dave

 

Sounds like the vacuum modulator is probably the culprit. If the vacuum line is in good condition like you say and doesn't collapse, then pick up a new one. As far as which one, the color codes indicate the diaphragm size, and are based on vehicle weight. The black is supposedly for trucks while green and purple are for the cars. Not sure about brown. I would go with the black first, but if this doesn't fix it, people have used the green in trucks and say it works fine. If none of this works, then i'd say it was the governor. Also the intermediate band on the c6 is adjustable, this would change the shift firmness mostly, but may help with shift timing. The procedure is on the autozone website, they basically have the chilton's manual available online. Lastly, some companies such as TCI sell an adjustable modulator too.

 

someone in the tranny forum said that I can either adjust a screw in the modulator, or shorten the pin behind the modulator. What do you think about that?

 

ok, I checked, and I do have the adjustable modulator. At first it didn't look like it was the adjustable kind, but then I looked closer. So, I backed the screw out a full turn and it's shifting earlier.

I still feel that I need to let up off the gas too much to get it to shift. I think I need to adjust the kick-down cable too.

But, so people know, the adjustments to the modulator seemed to have worked.