I have a 82 F-250 with a 351W and a C-6 that has been machined for extra clutch pacs. My truck shifts at around 2800 to 3000 rpm first at 25 then at 50. I know the tranny itself is good, i would like to know how to adjust the modulator and where to adjust please.

 

Get under your truck and pull the vacuum line off the modulator valve. Look inside and you should see a little flat-blade screw inside the opening. I believe you turn the screw one way or the other to adjust shift points. I've never adjusted one myself though, so I can't say which way adjusts shift points one way or the other.

If you don't get satisfactory results with the current mod valve, you might try out the different C6 valves that are sold. Each style has a different colored band painted on it, I think the difference is in the shift points.

 

In shifts higher and softer, out shifts lower and harder. Unfortunate it works this way since most people want a higher and firmer shift.

 

Could a vacuum leak cause it to do this? I had to clamp the vacuum line in the manifold tree and it started to shift at 15 25 and 40, then it slowly crept back to the 25 and 50.

 

It could...try running a new line from another connection on the vac tree to the mod valve and see if it helps.

 

A vacuum leak gives the same effect as having the accelerator down harder (low vacuum in the modulator line) so causes the modulator to give a higher-speed upshift......or maybe no upshift if bad enough.

 

I don't think that is right? When you adjust the modulator, you are adjusting the oil pressure in the tranny. Higher pressure means later firmer shifts while lower pressure means early lazy shifts.

Holding the pedal to the floor means you are demanding a lot of power from the engine. It also means low to no vacuum. This tells the modulator to delay the shift to let the engine wind up, and also gives greater clamping force to the clutches so they won't slip under full throttle. Mechanically adjusting the modulator just varies the "range" this all takes affect.

 

I appreciate all the advice! Im going to check for a vacuum leak today. Its got to be a very small leak because the engine is running really smooth. Ill let you know what i find.

 

Just throwing this out there, but are you certain that the modifications that were done to the transmission in order for it to fit extra clutches doesn't have something to do with the higher shift points? Could they have put a shift kit in it?

 

Low vacuum signal will cause the shifts to delay. So a vacuum leak between the engine and mod certainly can cause the shifts to come in much later.
Even without vacuum hooked up to the mod, it will shift, but very late. The governor will force the trans to shift once it reaches a certain rpm.
I don't know how, or if, adjusting the mod affects shift firmness. But turning the adjustment screw in does delay the shift, while turning it out makes it shift sooner.
Just out of curiosity, what kind of manifold vacuum does the engine pull? An older worn engine doesn't pull as strong vacuum as a newer/fresher engine. I have seen many cases of people thinking they had transmission issues, when the actual cause was a well worn engine that had a weak vacuum signal. This really is a situation where the 2 items are truely inter-dependant.

 

I dont know how much vacuum it pulls because i dont have a gauge, but when i put my finger over the manifold port it feels like it pulls pretty good vacuum IMO, Im a diesel mechanic so it feels pretty good to me, but i could be wrong.

 

HAHA! fixed it! My manifold vacuum port wasnt tightend all the way. The PO only tightened it a half turn, I fixed it and it shifts perfectly now. Thank you for all the advice

 

Glad the Op got this fixed.

Franklin2, that is how every modulator and valve body on C4 and C6 trans I've ever touched has worked. With a stock valve body you can't have a higher firmer shift. If you pull the valve body apart there is one hole in the plate you can drill larger that will firm up the 2-3 shift a little but this is independant of the modulator.

 

JNM76, glad you fixed it! Don't you love it when fixes don't cost anything? lol I love those...

I bet your truck runs a little better now too, doesn't it? Having that vac tree loose would cause a pretty bad vacuum leak!

FWIW I was flipping through one of my JEGS catalogs awhile back and found a TCI-built C6 for under $850...had a bunch of improvements, manual/auto valve body, plus some other stuff I can't remember. Seemed pretty nice!

 

It runs alot better now thanks! From what the previous owner told me the C-6 thats in it right now has been rebuilt and machined for extra clutch pacs. I installed a trans temp gauge on it and pulling a 7k trailer the temp on it never got above 165 f.