So I have a 1977 F250 2wd with a c6 tranny. After changing to a straight up timing chain and getting more power, Iwas talking to my brother-in-law about it shifting too early and he suggested adjusting the vacuum modulator. But when I pulled the vacuum hose off of the modulator there was NO adjustment pin in it(the small adjustment screw) I was really surprised, but decided to purchase a new modulator and install. After installing it it seemed great, but I noticed right away that it wouldn't downshift into 1st on its own sometimes. So I dealt with it for a few days.
Today a very scary thing happened.
I was coming up to a stoplight at about 35 MPH in drive, no brakes applied, and all of a sudden it dropped down to 1st on its own, virtually locking up the tires!!..sent the truck left and right abit and scared the bejezzus out of me. I quickly threw it into neutral and it idled great and was free wheeling, so it wasn't the axle or locked brakes. I put it into first and it moved about 100ft then seemed to act as if someone was putting there foot on the brake, but "not revving up" or feel like it was slipping(ive felt that before in other vehicles) it would come almost to a complete stop. Put it back into neutral, it idled fine, then into drive, would go about another 100ft then again seemed as if the brakes were being applied. Motor doesn't rev up and giving it gas doesn't make it move faster, but it also doesn't seem as if its slipping....Im totally freaking out as to what it could be....any ideas. The modulator I put in is a black stripped one. I pulled the modulator, and it wasn't full of fluid, the pin is in place...It seems fine in reverse, fyi. Thanks for any thoughts!

 

um could be a bad valve body. I can't remember but does the c6 have planetary gears( I will look- did look it up after typing and the do) if it poped it wont shift to the next gear. either way you may need to rebuild your tranny or buy a new one, or find a junkyard one, what motor? I will assume 351m/400
is there fluid pukeing out? did you mean that when you give it gas it doesn't rev?

 

Every automatic transmission uses planetary gearsets (except CVTs, but those are another animal entirely).

sony: have you tried going back to your old modulator and seeing if that changes the behavior?

 

I think its time for a rebuild.. A bad modulator usually gives hard shifts but not quite what you are experiencing

 

Its a 460 cu. in. I pulled the vacuum modulator today and removed the adjustment screw, It cant be right but the one I replaced originally didnt have one in it. I took it for a very, very short run up my driveway and reversed back down, it seemed better, yesterday the same run and it was trying to lock up. the best way to describe it is that it has a mechanical slamming which feels like the torque converter, then immediatley acts as if your power braking, until you let off on it. the motor runs great, no hesitation or anything. I know the trans is probably shot, today I drained a little bit of fluid from it, and is very dark, almost like bad oil, and its smells burnt. Im thinking Im done!...Anyone know a real good transmission shop in southern maryland?

 

Sounds like its defiantly time for a fluid/filter change at the least... You could try that and the modulator you took off, its possible the new one is faulty.

 

Yeah, Im sitting here thinking it through. The old modulator didnt have the adjustment screw in it at all, which certainly doesnt seem right, but then were did it go? Im going to go outside and blow the line through, and also put the screw back in the new one, this is a black striped modulator. Id like to add that the truck has a Holley Avenger Carb 670CFM and a edelbrock peformer 460 intake. the Modulator is connected to the full vacuum port on the front of the carb, next to the "timed" port. Its been that way for a while so not sure that is a problem. Typically, it seems to me that it takes two mistakes to create a problem, so going back and looking at what I had done.
To much info in my head going at once, haha. Im just afraid to take it for a harder test drive, that hard "locking up, or downshift was scary". Replaying it in my mind, and letting off the gas at the high speed would have created a large vacuum, thereby creating a downshift?. The truck doesnt have a kickdown on it, got it that way. It always seem to be something stupid, hopefully Ill stumble upon it.

 

the modulator line is hooked to the carb vac? well 460 trannys grow on trees the 351m/400 trans is the same bell as well.

 

Yep, it was hooked to the full vacuum connection, next to the pcv valve connection on the front of the holley avenger carb 670cfm. I blocked it off and hooked it to the intake vacuum the rear of the intake, but it didn't change anything..I checked the vacuum coming from this port on the intake and it was at 18 hg(that's good right?) at idle. Im at a loss.

 

For my own sake, to clarify... Your trans was fine, working OK, you wanted a firmer shift, changed modulators, now have a harsh downshift?

No other parts changed? And have you gone back to the old modulator or not?

BTW, turning the screw counter-clockwise softens shifts. and 18" of vacuum is appropriate.

 

The modulator that was removed was tossed, but I think it was the original, which I think is green strip. It did not have an adjustment screw in it. I tried a purple one and a black one with no improvement. I've only driven it a few hundred yards to test changes, it moves ok in 1st & reverse but if in drive it starts in first, and when it tries to upshift, it binds up and I hear a loud slam in the trans area toward the front. The fluid appears to be burnt and smells also.

 

But was the trans working properly other than the early shift?

Have you adjusted the intermediate band at all?

 

If the fluid is burnt then the transmission is likely toast.. That is the number one sign when servicing them or buying a used one that trouble is ahead.. You can try the fluid and filter if you have money to toss but when dealing with a transmission when its too late its too late..

 

well today I pulled the lines off and blew them out, but I didn't see much come out, so I decided to remove the lines from the transmission and, well, when I had the headers and stainless exhaust done, the guy was "nice" enough to make new stainless lines that ran around the transmission pans. But when he put the lines on the transmission one of the lines must have been turning as he tightened it up, so he affectively pinched it off. So no fluid was moving. I pulled the pan and the filter screen was blown down, and the pan has a small amount of shavings in it. Im figuring the screen blew out because it was clogged. So new trans for me I guess. ANY THOUGHTS ON WHO TO GET IT FROM?...Minus the carb, intake and straight-up timing, and exhaust(2.5 inch), it is stock and don't see me doing anything else. Thanks for the help guys.

 

If you don't have to drive it to work, I'd suggest seriously consider doing it yourself. It's really a very easy trans to work with.

That said, there's no real tips or tricks to them, so any local guy you're familiar or comfortable with should be able to handle it.