Guys I am at a loss now what to do! I have an 86 f-150 4.9 I6 and I have the c6 3 speed transmission. The truck has 64,000 miles on it, and the original owner passed away so I was not able to ask what had been done to the truck when I bought it from his wife. So here is what is happening. When the truck is cold, the transmission shift like a dream, but when it gets warm, even after only 5 minutes of driving, the 2nd to 3rd gear change is kind of sluggish. After the transmission gets completely warmed up it seems to lose some power. Not a lot of power, just enough to notice. So far I have tried the following to fix it.
Transmission filter and fluid with Lucas transmission
New modulator valve and I adjusted it counterclockwise because it was shifting way too early
Differential oil change
Adjusted the band and I followed directions exactly using a torque wrench

I was thinking maybe the governor but it wouldn't make sense to me that it shifts so good while it is cold. The fluid looks fine it doesn't smell burned or anything. I have searched other forums and this seems to be a complaint by many but nobody knows the fix. I was hoping someone here could steer me in the right direction of what to try next. Or if someone could tell me if this is a bad thing or if it is just a normal thing for this particular transmission? Any help would be sincerely appreciated.

 

It sounds crazy but try another tranny flush. Some times the old fluid gets thick and chunky. Then the new clean fluid breaks the tarnish loose and gets in the new fluid and causes to do the same thing. It sucks but thats the best idea I got

 

You know its funny but that is the exact thing I was thinking but not the chunky part. I thought if the man that had it before me never changed it which is a possibility, then I would need to do it a couple of times to really get most of it out of there. For some reason with the c6 only about 3 quarts comes out of the pan. I way over filled it and had to use a pump to get about 4 quarts out of it. But I think you might be on to something there, and I am going to try this. Thank you so much for the reply. I greatly appreciate it.

 

Well... I found out that I was adjusting the modulator valve the wrong way. I was turning it counterclockwise, and it made the shift way too soft. I turned it clockwise, and now it shift correctly except for some reason I have to slow down to around 5 MPH before it will kick down and it seems to only kick down to first. There is no 2nd gear kick down. Not even when I am struggling to go up a hill. It shifts to second if I do it manually though. I have a feeling this is being caused by the valves in the governor.

 

Heh... maybe fine tuning is in order. Or valve body adjustment. Or just try to run the guts outta it. Also check you leakage

 

Change your vacuum line to the modulator . When they get old they do get weak . When they warm up they become soft and collapse blocking off alot of the vacuum . And then try resetting your screw back to where it was .

 

I think I will replace the vacuum line. It is very old 25 years old. Also the valve body is something I would like to do. I have no experience with transmissions but I am a backyard mechanic, is the valve body something that I could do without special tools? I do have a torque wrench, craftsman click type. Is there like special adjustments or anything? Does anyone know if there are solenoids inside of the transmission, that maybe could be going bad? I know that there are on newer vehicles, but I know nothing about the 80's transmissions. I did not even know about the vacuum modulator until my transmission wouldn't change out of first gear. Then I changed it, and it still wouldn't because the check ball had worked its way up the vacuum line and got stuck at the vacuum separator on the firewall. took me a month to figure that one out Any info you guys could give me would be really awesome. Thanks guys!

 

Thr valve body is pretty simple. I think all you neef it a screwdriver and a 3/8 socket. And I dont thing theres any selonoids in that old of a tranny