I managed a short shake down run in my truck the other day and had a load of issues to work out, but the concerning one is my transmission. So the truck is a 66 f100 with a thunder jet 429 and a C6 transmission. I installed a transgo stage 3 Manual shift kit and a B&M Unimatic shifter. The C6 will go through all the gears under light acceleration. Once I get after it it won’t go to third. I can’t tell if it’s an issue with the Unimatic (which I am not exactly happy with) or if there is something wrong with how I installed the kit. I do have the vacuum modulator hooked up, but no kick down linkage because it’s full manual now. I was thinking of plugging the vacuum modulator to see if it made it do it all the time. I also ordered a B&M Megashifter, mostly because I don’t like the Unimatic. Anything I can check that would cause this?

 

ADjustble modulator? If so, start by backing screw out 1 turn at a time. Report back.

 

I will check. Won’t be able to get back to the shop for 4 days. I will start there.

 

check the lever angle on the trans in neutral. The c6 needs to have the lever aimed vertical in neutral or it effects length of cable movement.

 

I will check that as well. Thank you.

 

I was able to get under the truck today. I checked the shift lever arm first and it was not vertical in neutral. I picked up a ratchet shifter so I decided to swap out the Unimatic first. I made sure the lever arm was vertical in neutral and the new ratchet shifter is adjusted So it goes smoothly through the gears. I went for a drive and still couldn’t get 3rd at full throttle. So I checked the vacuum modulator. It isn’t adjustable, so I disconnected it and went for a short spin. Now I can’t get third at all. As I understand it, the vacuum modulator decreases line pressure when shifting under light acceleration. Is this correct? That makes me think I did something wrong installing the Transgo shift kit. Any suggestions?

 

Could be wrong modulator or pin for the application. It has been a long time since I messed with a c6, but you could try shortening the pin a little at a time, or just get an adjustable modulator. I am not sure which color you need.

 

The transmission came out of a 1977 F150 with a 351M and 3:25 gears. Now I’m running a 429 and 3:43 gears. Maybe I need the vacuum modulator for the car the 429 came out of?

 

Every full manual kit ive ever seen , eliminated the modulator and the governor.
If it is full manual valve body and you have it in D, it should take off in 3rd and never go thru any gears.

The modulator has influence on the governor. Mostly when the engine is in vacuum. Some when it is not. Meaning WOT.
With vac. mod. unhooked from vacuum, shifter in D, it should ,and has always in my experience, made the trans think you are a WOT leading the shifts to happen at the rpm the governor is calibrated for in every gear. That might mean 5-6k. That being said, a c6 will manual shift in stock form. It will completely ignore the governor if you manually shift.

 

The vacuum modulator is still hooked up. This is the factory valve body with a Transgo 67-3 manual kit. I still have the vacuum modulator hooked up. What I understood is the modulator can still adjust line pressure based on vacuum. Is this correct? I’m going to try a new modulator, what and where is the governor? How would I, or can I, adjust the governor? I didn’t try shifting directly into 3rd from park with the modulator disconnected. I was also confused as to why it won’t shift into third with this manual kit. I thought it should shift into third regardless of anything going on with the motor. If the modulator doesn’t work, I may pull the valve body and check my work. I had to have screwed something up.

 

Yes the modulator can alter line pressure under vacuum. But, you are at zero vacuum at WOT. The governor is attached to the output shaft. It is under the tail shaft housing.
Sorry im not a trans guy. Ive done valvebody work. And I understand how the c6 works to a point just cuz Ive had one for years.

 

I’m not a trans guy either.I was thinking the same thing about the modulator. I’m thinking that I’m not full manual somehow. Like I screwed up the install.