Hello all,
I have a 1976 F150, 390 cu. in., automatic transmission, 4 bbl. holley carb w/electric choke.
It's been sittin' for a couple of years while I've done some work on it; (timing chain & gears, rebuild carb, replace rear brake cylinders, rewire igition and replace vacuum hoses replace ditributor wires, etc,) and now after I fire it up I find that the tranny slips. I've added fluid, (maybe a little too much, but still it's right up there). I can put it in drive 1 and it seems o.k.; I can put it in drive 2 and get up to 50 mph; but when I have it in DRIVE, it gets up to about 30 - 35 mph and then it slips. It tries to shift into the next higher gear but slips back out all the time; sometimes with a nasty clunking noise as it slips in and out. Please help. Thanks. John.

 

Check out the vacuum servo. When was the last time the bands were adjusted? I have had the same "clunk" problem. It ended up being loose/worn motor mounts that were reaking havoc on my pinion and ring gear. A pressure test never hurts.

 

I pulled the vacuum hose off the transmission modulator and some tranny fluid ran out of the hose. I still have to figure out if it's a C5 or a C6 so I can order the correct modulator. John.

 

Well hopefully it is the modulator that would be a relief wouldnt it ?

 

If its mated to a 390 its definitely a C6. to my knowledge an FE never came with a C5.

If theres ATF running out of the hose to the modulator, the Diaphram inside is torn. with a torn diaphram theres no way the modulator is working, this is why it wont shift normally.
The reason it feels like it wants to shift is because the kickdown rod is operating properly and moving the manual valve.

John,
GM writes my paycheck, but all I drive is fords

 

Good news. I deceided to change the tranny fluid from both the pan and the torque converter. I removed the torque converter cover and pulled its plug and while it was draining, I proceeded to clean the cover. Lo and behold, beneath 25 years of accummulated crud was the inscription C6AP-7986A.
Therefore, I must conclude that this is a C6.
Good call mrgoodwrench.

tetraruby- I don't think the bands have ever been adjusted.
This is another item I will have to address.

otto- yes, it would be very nice if it was to turn out to be the modulator. (which might just be the case).

mrgoodwrench- I believe your assessment of the broken diaphram in the modulator to be correct. I performed a benchtop test on it by pushing the plunger all the way in and then holding my thumb on the vacuum port. My thought was that if the diaphram was good, the plunger would be held in due to the vacuum port being closed off and not permitting an inrush of air allowing the modulator to return to its normal state. The plunger did not hold, so I concluded the the diaphram has been comprimized. Chock another one up for mrgoodwrench.

My thanks to all for their input. John.

 

I'm glad I could help.

Although I'm sure the tranny is a C6, the part number is Just a coincidence. Actually most of the parts on your truck will have part numbers that start with C6, if they are original to the truck. This is because first 2 letters in the part number refer to the year they were made. C6=1976, C5=1975, D6=1966.

I just thought you might like to know.


John,
GM writes my paycheck, but all I drive is fords

1976 F150 Ranger XLT, 390, C6

 

Sorry to anyone who read my last post I must've not been thinking clearly, the numbers you found are casting numbers. the first two letters refer to the year that the part was designed. C6 actually eqauls 1966

 

John I guess that explains the C3 and C5 that came along later, right?

John
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