Ford C6 transmission question (Shift flare)

[The_Eric]

I just acquired a 1979 Ford F150 2wd with the 351M and C6. It currently has 74,200 miles on it and drives pretty much like a new truck of that era. It is in exceptional shape for a truck of this vintage in this area.

My question is this: What would cause a light 1-2 and a noticeable 2-3 shift flare? The fluid level is correct but likely in need of changing. I have a filter and enough Valvoline Dex/Merc as well as Valvoline Max Life to do the job. I plan on changing as soon as time allows, but want to know what would cause this issue, if the fluid change doesn't resolve it.

The Max Life appears to be the LV formula, is this acceptable for use in the old C6?

 

[gregk24]

Change fluid and filter and see how it acts then. If still flaring probably time for a rebuild.

 

[MrMeeks]

try a trans. additive

 

[LargeCarManX2]

Originally Posted By: MrMeeks
try a trans. additive


I would agree before the change. Try Trans X and at least a quart. You can suction out a quart through the fill tube with a cheap pump kit. I would not use the LV formula and stick with a DexIII.

 

[DuckRyder]

I would replace the servo seals and adjust the band.

Does it do it regardless of throttle opening or only at WOT?

 

[The_Eric]

It is worse with increased throttle opening/rpm, but always there no matter how light the throttle input.

 

[michaelluscher]

Kickdown cable issue?
Maxlife F+F change + an additive like Trans-x or LubeGard Red.
Take it from there.

 

[panthermike]

FWIW, Lubegard doesn't recommend using their trans additives in transmissions that call for Type F. I would drop a bottle of LG trans flush in and change the fluid/filter. If problem persists, rebuild it!

 

[Chris142]

Originally Posted By: DuckRyder
I would replace the servo seals
this. The rubber seals have hardened and are not holding pressure correctly. Does this trans call for f or dex?

 

[The_Eric]

That's a good question. I assumed Dex/Merc, but after another poster mentioned F, I'm not so sure. I'll check it tomorrow and cross it to a modern spec (it's some oddball Ford number).

 

[demarpaint]

1966-76 called for Type F. 1977-96 called for MERCON. A band adjustment won't hurt either.

 

[The_Eric]

An update: I'm doing things one at a time to see what changes occur. As of now, I've replaced the old fluid (factory?) with the Valvoline Dex/Merc with no perceivable difference. Next is to adjust the bands and if that fails, I'll do the servo seals if it doesn't require removal of the transmission or special tools.

 

[Highline9]

tractor fluid

 

[The_Eric]

Originally Posted By: Highline9
tractor fluid


I don't think tractor fluid will fix this...




I've got it to slip a couple of times going from 2nd to 3rd (hot), and it's almost nonexistent cold so I'm sure it's the servo seal.