I have a 86 f250 460/C6.
I just picked it up and I'm trying to figure out what is going on with the transmission.
It doesn't feel like it is shifting thru all the gears automatically.
I can shift it manually just fine, it goes thru the gears nice and strong with no noises.
I can't really tell if it's skipping 2nd gear and going right to 3rd or not shifting to 3rd.
I didn't want to test it much driving and damage the transmission. So I just manually shift it.
Had a buddy who is more mechanical than me ride in it and he had a hard time telling too.
We basically think it's not shifting into 3rd from 2nd. Also doesn't feel like it downshifts as you slow down either.
It appears the vacuum line is good and connected, figured I'd do a vacuum test on the modulator.
Do I just test to see if it holds vacuum or do I try for a certain vacuum?
Can you test the valve body and how? Or is that a specialty tool/transmission garage job?
Fluid looks good, smells good.

 

The modulator should hold whatever vacuum you put on it or it's leaking.Check you kickdown lever. Connected at both ends? Adjusted properly.

 

connected yes, adjusted properly, good question.
It has kicked down on me before I haven't really tried much though.
From what I read and I could very well be wrong, the kickdown lever doesn't affect the normal shifting as much as the other transmissions.
example: it could be totally disconnected but it would still shift normal, just no kickdown..
please correct me if I'm wrong, my other truck is a np-435 so I have no experience with a c6.

 

the only effect kickdown lever would have on a trans is if it was stuck in full throttle mode. then the trans will upshift late, and as soon as the throttle is released it will try to downshift.
it sounds to me like the modulator is bad. if you pull the vacuum line off and trans fluid comes out, the modulator is bad.

 

I will do that, so it makes no point in trying to put a vacuum to it than?

 

not really if it is bad, it will have fluid in the line.
check to see if vacuum is actually getting to the modulator though, there may be a bad line causing low vacuum down there.

 

good info, I'll pull it at the modulator to check for fluid, than I'll hook vacuum to it up on the T to see if it holds vacuum.
That is if I don't find fluid.
Thanks!

 

at idle the modulator should have the same vacuum as the intake has. if it is lower by more than 1 or 2 inches, i would start looking for leaks in the vacuum lines.
also check the "coffee can" on the fender for rot/leaks.

 

well no fluid in the line, tested vacuum on the line directly to transmission,
It held, than I tested the line feeding the T, that held so I suppose the lines are good.
That line comes off the intake manifold, I'm not sure if that is correct or not but just for disclosure..
Now for that canister..
it looks cylinderish on the drivers fender, I suppose that is coffee can looking.
than there is another one down below the battery that is squareish, not coffee can looking.
I haven't tested vacuum to that yet.
The square looking box I'm guessing is the purge canister? nothing is connected to that at all. Some cut hoses stuck to nipples.
The cylinder is connected but, I need to study and attempt to understand what I'm really looking at.

 

the square one should be the vapor recovery charcoal canister for gas tank.
no vacuum to that one.

if you have vacuum at the modulator than that side of the system sounds to be working properly.

if it is not shifting into third, you will have a top speed of about 45 MPH.
if it is going from first to third, it will feel like it has a lot of power, then when it shifts it will feel like a real dog.

it is very possible there is nothing wrong with it and it is shifting properly, and that you just don't feel it shifting.

 

You may be very well right.
I'll take it for a ride and feel it out a little more.

So you don't think it's the valve body than?

 

without driving it, it is hard to say.
maybe a video of dead stop to 65 mph hard acceleration, and another of regular acceleration will help diagnose.

 

You were right, I stomped on it and it shifted thru the gears.
When I test drove it before buying it, I did the same thing but never felt that 3rd shift.
It is kind of noticeable so maybe me taking the line off and on and snugging it on helped.
I don't know but I'm glad to know it is shifting.
Appreciate the insight on this.

 

no problem. my 88 diesel is the same way. easy drive and sometimes it will slip into next gear and you will not even feel it shift. ya gotta drive it like ya stole it to really feel it shift.

 

I think I narrowed this down a little more. I've been driving it in (D)
here the last week and if I take off it seems to shift fine.
If I ease into it and just take it easy it shifts into 2nd and than directly into 3rd.
So I'm in 3rd at about 27 mph, is that normal? Doesn't seem like it is to me.
Seems to shift into 2nd about 20-25 and than right into 3rd
So 1st seems to take a little longer to shift out.
Than shifts into 2nd and almost immediately into 3rd.