c6 adjustment?

[1972hiboy]

Hey guys, last night I pickup up a 72 f250 ranger xlt camper special. 390 c6 combo. My 71 and 72 hiboys are both manual trans so this is slightly differant for me. Ive noticed that driving around town that the rpms need to be kinda up there for it to shift out of first gear...then its a hard shift to second then seems to shift normally into third. The fluid acually looks really good...bright red. Just wasnt sure if this was a adjusment I could make to the vacuum modulator or related?? Any help would be appreciated.

[cdeal28078]

It should make some difference. You might have a shift kit in there. That is the way my c6 shifts after I added one. I am not happy with it at all but it did firm up the shifts. I usually shift it manually because of this.
Clint

[1972hiboy]

That is a possibility i pondered. it does shift very firm. When you had the kit installed on yours did it change when it shifted?

[fordman]

the modulator does change the shift times of the transmission. now a hard shift to me would indicate a worn transmission.

[cep62]

If it has a kick down rod from the carb to the tranny ,that can be adjusted to let it shift sooner.
also a sticky governor can cause late shifts.

[1972hiboy]

If it has a kick down rod from the carb to the tranny ,that can be adjusted to let it shift sooner.
also a sticky governor can cause late shifts.

It does have that kickdown rod and linkage, I know it needs to be adjusted. at cruising speed WOP it does not downshift....I was going to be tackling that sometime this week. Sticky governor? hmm I guess I shouldnt have shy-ed away from tranny work when it became available. Im not sure where or how that works. I did check to make sure the vacuum modulator was hooked up. Is it sapposed to be timed vacuum like the distributor? I was going to start there....thanks for everyones input...keep it coming!

[70_F100]

You didn't say what speeds your shift points are at. 1-2 shift is usually around 12 MPH and it's in 3rd by 25-30 MPH.

The kickdown rod is pretty much just that, a kickdown rod. It forces the transmission to downshift when the throttle is fully depressed. I've run C6's with the rod totally removed and I saw no difference in normal operation of the transmission, other than a failure to downshift to "passing gear" (if you want to call it that). Adjust it so that it is at full stop when the carb is at WOT.

A sticky governor would (most likely) cause a failure to downshift when slowing down. As output shaft speed increases, the weights in the governor are "slung" outward against light spring pressure. It's possible that they could hang in either direction (out or in), but most likely that they would hang in the outward location (centrifugal force on the weights is much greater than the spring pressure pushing them back in). This would send a signal to the transmission that the speed is already above the engineered shift points, causing it to fail to downshift as output shaft speed decreases. If, however, they were stuck in the inward position, shift points would be raised to a MUCH higher speed.

The most likely causes of your symptoms, IMHO, are that the modulator may need to be adjusted somewhat, or, the MOST likely bet is that someone has installed a shift kit, as Clint (cdeal28078) posted.

Try adjusting the modulator slightly and see if this helps. If it doesn't change things, I would bet on the shift kit, because LOTS of the shift kits available over the years had the greatest effect on the 1-2 shift.

[1972hiboy]

Thank you 70_f100. all of that makes tons of sence. I can tell when its downshifting and it does downshift. The 1-2 shift seems to happen around 17-20mph. then 2-3 happens at or right around, 28-32mph. If it does have a shift kit in it, it would make sence that it would have the hard shift. and possibly have a delayed shift point for first gear for anticipating a heavy load? I was going to try adjusting the vacuum modulator first. Im going to verify it has a vacuum signal with my gauge first. but I dont know what kind of vac signal im looking for. Last night when I was messing under the hood I found that the vacuum line was tied into the distributor vac advance diaphram via plastic "T" fiting at the timed vac port on the carburetor. Is that correct? Then my next question is, which way do you turn that allen/hex head screw in the modulator to effect shift points...like, turn outwards to shift sooner or vise versa?

[70_F100]

You may have something there, but I'm skeptical. On a stock setup, the heavier the engine load, the lower the manifold vacuum is, and the higher the shift points are. With the modulator connected to ported vacuum, the vacuum increases (to a point) as the throttle is opened. That would, in a perfect world, signal the transmission that the engine is under very light load, causing the governor to control the shift points.

The vacuum source should be full manifold vacuum. On my 70, it was connected to a brass fitting in the intake runner behind the carb.

I can't recall which way to turn the screw in the modulator. I'd probably try it about 1/8-1/4 turn at a time, then test drive it to see how it reacts to the change. If it changes it negatively, go back the other way. Be sure to keep track of how much you turn it, so that you can go back to the original setting, if need be.

[1972hiboy]

Hmmm! We may have something there then...there is that brass fitting on the intake runner behind the carb. it also has the big line going from the fitting to the brake booster coming off of it. there are a couple small ports on that brass fitting with caps on them. So if the trans is sapposed to be hooked up to full manifold vacuum its obviously hooked up wrong being plumbed into the timed port for the distributor. I'll re route that line to full manifold vac and try a test drive and see what the results are. after that then maybe i can then assess if I need to try adjusting the vacuum modulator...I sho9uld probably try putting the vacuum gun on the modulator to make sure it holds and doesnt have a blown diaphram huh?....

[1972hiboy]

Update: After work I pulled the truck into the shop and did a tune up. I found that the vacuum advance diaphram was completely blown. I replaced it with a new one and re-routed the shift modulator to a full time intake manifold vacuum source..( that brass fitting) and ran a new hose to the distributor advance diaphram since it was crunchy.....HOLY COW if shifts smooth as butter and at what seems to be the absolute right time.... not to mention runs as smooth as silk now and no more flat spot at half throttle...I think were making progress...Thanks for the tips fordification crew!

[Bigcalhoun]

I have a c4 and know nothing about transmissions. Mine was sticking out of first and second about half the time. This thread lead me to take a look at that vacuum connections just behind the carb. I found that the 40-year-old hose was slightly dry rotted and had a small hole at the point of connection. I cut a half inch out of the hose and reconnected. The trany, rebuilt two years ago, shifts perfectly now. What an amazing, easy fix.

[fordman]

that hose probably went to the transmission modulator. and that is what helps shift the transmsision. and when it will shift.

[1972hiboy]

You are correct. The PO had the trans vacuum modulator plumbed into a timed vacuum source. Once I re-plumbed the trans to a full intake vacuum it worked the way it was sapposed to. I think without the correct vacuum signal the transmission was just shifting with its governor.

[cdeal28078]

Great job 1972highboy. Glad they could help. Thanks for letting us know what the outcome was. So many ask a question and we never know what the outcome was.
Clint