automatic 3 speed not shifting

[Blue1971FordTruck]

1971 f100 rwd automatic 3 speed c4 not seeming to shift. what should i do?
i haven't had it faster than like 25 mph

[70f10rat]

Check vacuum lines

[Donnie]

Did you find a problem with your vac. lines, as someone suggested?..............Donnie

[Blue1971FordTruck]

NO IT WASNT THE VACUME LINES

[tac]

Modulator? Check for any trans fluid in the vacuum line coming off the modulator. If there is any replace it.

[Donnie]

How about a little more info?..Did you just buy this truck , or just install this trans? Has it been driving & shifting OK before & this just happened, all of a sudden? When you checked your vacuum lines, how did you check them? just a visual check or with a gauge? Are you sure that the line to the modulator has the same vacuum at the trans as it does at the intake manifold? Are there any other symptom's that we should know about in the way the truck operated before this issue? It is difficult to diagnose your problem with the small amount of info. that you provided.
Give us a lot more detail, answer the above question's.........then you may get more help...or you can just start throwing parts at it
Donnie

[Blue1971FordTruck]

DONNIE REPLY ANSWERS TO YOUR QUESTIONS

p.s. im 16 years old

no the truck has been in the family since new (grandfather-dad-uncle-me). when my uncle got it he drove the piss out of it and it, overheated once, and the old 302 started to knock and he parked it for 10 years, and i came along got it running found out the original motor was done for, and then i installed a mildly built 302 out of a 1972 f100 with the original c4 transmission. i never noticed if it was shifting or not i didn't get it fast enough to notice if it was shifting or not


i need a diagram of the vacuum lines and a walk through step by step on where each one goes and how to check them visually or with a gauge

im not sure what a modulator is exactly

in park or neutral the truck idles and revs fine the second i put it in gear (drive, reverse, 1st, 2nd) the engine bogs down until the it shuts of and it wont stay running in gear but runs fine in park/ neutral so i don't know if that is from the transmission or what it is...


i hope this is enough info if not post more questions and i will answer them for you as best as i can

[Donnie]

Way to go young fella, I'm glad to see that you are keeping it in the family....In all reality, you will have to correct the engine stalling condition
before we can diagnose the trans. I would make DOUBLE sure that ALL ports on the intake manifold & carb are capped or connected....
PCV valve in the valve cover....power brake booster, if it has one.......there are MANY reasons that the engine is stalling when put in gear.
Does the engine, when sitting there at idle , seem to be running too fast or too rough? Dropping it in gear slows down the engine & puts a load on it........Get the engine problem corrected, then we can talk trans........BTW, the modulator valve is the valve at the rear of the trans. where the vacuum line connects coming down from the engine to the trans...there is a rubber hose "elbow" at the trans end.......check this, it is a small rubber connector, you can actually pull it off & make sure that there is NO fluid in it .........post back
SOMEONE ELSE MAY BE ABLE TO SEND YOU A VACUUM SCHEMATIC..................Donnie

[Blue1971FordTruck]

okay that makes sense... so if the trans had too much fluid in it could that over fill in to the line

[Donnie]

No, only if the modulator is faulty will fluid be sucked into the engine...The engine creates a vacuum when running & can suck the fluid from the trans. if the modulator is faulty...Work on getting your engine running well & not stalling----then we we will discuss trans..
Try posting your engine symptoms on the engine forum...see if someone will send you a schematic on your vacuum lines...Donnie

[Blue1971FordTruck]

the vacuum line to the side of the transmission in missing can someone please post a clear picture of the line and a diagram of where it connects to thank you guys

[lee_ford]

tubeclipV8.gif

[Stolen from Mustang Monthly forum.]

[alphawave7]

the vacuum line to the side of the transmission in missing can someone please post a clear picture of the line and a diagram of where it connects to thank you guys

At least on my 390/C6, there is a vacuum line from the intake manifold (top rear engine, roughly) that follows down to the tranny and runs to the rear of the C6...you should see an empty 'hard' line hanging near the roundish modulator on the C4. Attach about 6-7 inches of hose between them, and you're golden. Good luck!