Im looking at picking up a '88 F350 as a donor truck and the only real issue is the C6 tranny. When i put it into drive, it has a really difficult time shifting through the gears. I can manually take it from first to second with no issue, but it wont shift into third when i go into drive. What could be causing this? Worst case scenario, it needs a rebuild, what can I expect to pay for that? Thanks guys

 

Probably a vacuum problem. If the vacuum pump was bad you would probably feel the brakes a bit hard along with the shifts. If the vacuum pump is good then could be a defective vacuum pipe, bad transmission vacuum modulator or bad/out of adjustment vacuum regulatory valve (on side of injector pump).
Could also be that the kickdown rod is stuck "on".
Those would be my guesses.

 

The brakes are fine and when it does shift the shifts are not hard, so it doesnt sound like the pump. I assume that if it were a bad vacuum pipe, all that would be involved in finding that leak and fixing it, correct? What about that vacuum modulator? I'm not familiar with that, can you elaborate at all?

 

Ive read up on that vacuum modulator and alot of the symptoms other people have described seem to fit me. If it is bad, how difficult is it to replace, can I do it myself of do you need a shop to do it?

 

For the modulator the first thing you need to do is pull the little vacuum pipe off of it and look for signs of transmission fluid in the pipe. If fluid is found then you will need to replace the modulator. If no fluid found then the modulator may not be bad. The modulator is easy to replace, undo the bolt holding it and pull it out and replace with new one but make sure the little rod is in place when you fit the new one.
However I would check the vacuum supply to the modulator first. It seems that the vacuum pump is good so with the engine running at idle, transmission in Park, pull the vacuum pipe off of the vacuum modulator at the transmission and put your finger over the end of the pipe, it should feel like your finger is being sucked into the pipe. If not then you have broken/kinked vacuum pipe/s or the VRV on the injector pump is bad/out of adjustment.
Make sure the kickdown rod is connected and operating properly also.

 

Got the tranny all working. The vacuum line that came from the modulator was disconnected up by the injection pump, and the kickdown was way out of adjustment. Thanks for the help guys