I took the truck on an 80 mile drive around town today. 3 speed auto with a built 460. It was rebuilt a few years ago and only has about 1200 miles on it since the rebuild. The transmission seems to shift around 18-20mph into second gear, and then around 28-30 into third gear. This seems like a low speed for shift into third? Also I attached 2 videos. The transmission shifts into second gear just fine. Going from second to third it has a small slip (video #1) almost every time. Sometimes it is a longer slip (like in video #2) like it can't find the gear. I did a little digging on this forum and I am hoping its the modulator valve that's bad. Is this a sign of a bad trans? Should I change the fluid? Also should I mess with the modulator valve to get the shift point a little higher speed? Where should I start with diagnosis?

Thanks for the help!

 

That’s fairly normal and yes the vacuum modulator on the trans is what you can change to affect this. Someone else will know exactly but some are adjustable by disconnecting the vacuum hose and turning an adjustment screw to affect the shift points. Some are non adjustable and there were various ones tuned for different vehicles.

as far as the slip if that’s what it is, modulator won’t help. Sounds like a flare from 2-3

 

Could the modulator valve be the cause of the flare as well or probably unrelated? Should I just flush the fluid?

 

Modulator Valve wasn't leaking fluid so I think its ok.

Transmission fluid seemed a little low based off of the dip stick. All of the youtube videos show guys adding fluid and it gets all over everything. Is there a secret or just pull the plug, wipe up the mess, add more fluid?

Thanks for the help

 

I don't understand how fluid is getting all over everything? Use the appropriate transmission fluid funnel. Add as needed. Job done.