I have a '69 F100. My husband and I rebuilt a 390 for it and had the C-6 transmission rebuilt soon after. We had a problem with the transmission before it was rebuilt and it's still there. It shifts into 2nd at 5 MPH and into high at about 15MPH. Vacuum is excellent and line is clean and connected. We even completely unhooked the vacuum line and there was no change either way. Replaced the modulator valve, valve body, torque converter and governor, Added a mild shift kit. Replaced the pin in the modulator valve with a longer one and screwed it in all the way. It shifts fine manually. We did not put the passing gear linkage back on when we rebuilt...with a 389 rear, felt it wasn't necessary but we could be wrong. Have we forgotten anything? What else can we check or replace?

 

I'm no transmission expert, but it sounds like the modulator is out of whack. And no you don't need the kickdown linkage for the transmission to function normally.

 

Could be out of whack...but the old modulator did the same thing and this one is brand new.

 

Beth, Turning the screw inward should have made the tranny shift later. The early shifting is a trate of the C6, and the short gearing you have isn't helping. If you were running a 3.00 it may not shift as fast. Try one more adjustable valve. You may just have to shift manually when racing.
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John

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John: Thank you for responding! We had a 3.00 gear in it before the rebuild and it shifted early then too. This modulator is the third, actually, and since the problem still exists even when we disconnect the vacuum line, we're stumped! and tired of changing stuff out. Oh, and I'm not racing it, just driving it to work and enjoying an occasional hole shot! (with a manual downshift of course) It also slips in between shifts and if you really nail it...it revs. I am really hoping you guys can help us figure this thing out.
Beth

 

Well having the same thing here, Aftear installing new engine and tranny the same thing, Just got the thick tranny book out of the truck and from what it is reading so far its sounding like the line pressure is going to be problem on this one, It looking like I will have to spend a little time on this myself, lot of thing must be
consider here going by the book,
GWB

 

Beth I am going to repost, something that Jon (flatbed63) posted on our NCFTO last night about the same thing. As far as I know it doesn't harm anything.

Edited

First off there is no mention of different years using different fluid. It states to use fluid Mercon Multi-Purpose Automatic Transmission Fluid XT-2-QDX or DDX (ESP-M2C166-H) or equilivalent. The capacity is 11 3/4 quarts with drained torque converter.

As to your shifting problem here goes. I looked in the troubleshooting section and matched the conditions.

You state early shifting. Per the manual:

Condition - Mushy/early all upshifts/pile up upshifts

Possible Cause
1. Low main control pressure.
2. Valve body bolts loose/too tight.
3. Valve body valve sticking.
4. Govenor valve sticking.

Condition - Mushy/early/soft/slipping 1-2 upshift

Possible Cause
1. Improper fluid level.
2. Incorrect engine performance.
3. Intermediate band out of adjustment.
4. Low main control pressure.
5. Valve Body bolts loose/too tight.
6. Valve body dirty/sticking.
7. Govenor valve sticking.
8. Damaged intermediate servo or band.
9. Polished, glazed band or drum.

Condition - Starts up in 2nd or 3rd

Possible Cause
1. Improper band and/or clutch application, or oil pressure control system.
2. Damaged or worn govenor. Sticking govenor.
3. Valve body loose.
4. Dirty or sticking valve body.
5. Cross leaks between valve body and case mating surface.

That is about it. Sounds like the govenor to me but could be any of the above.

Jon
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John

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Thanks for the posts. Since we aren't transmission experts it was nice to have the list to go down and check. We have replaced the valve body valve with no improvement. Someone has suggested drilling a hole in the govenor. That will be the next thing we try. Will post the results when we get that done.
Beth

 

Beth, drilling a hole in your governor is only going to hide the symptom, not solve the problem--if it works. I have never heard of that before, but I am guessing it is a way for drag racers to get higher RPM's between automatic shifts. They need this because the engines they use have extremely modified power bands. Why do something that may cause you other problems in the future? You say you had this problem before the C6 was rebuilt. Did it happen all of a sudden, gradually, or for as long as you owned the vehicle? You also mentioned that it slips in between gears; does this happen with all shifts or only 2-3. If it is all gears, then the early-shift symptom is probably related. Either way, if the transmision slips as bad as you say, an early up shift can actually be a blessing. If you modify the governor to shift at higher RPM and it still slips, you are going to wear out your clutch packs very quickly.

If it was me, I would remove the tranny from the truck, get a C6-specific rebuild manual, and completely tear it apart from front pump to parking gear. Take lots of pictures, make lots of drawings, check sizes and clearences, look for wear, and check for cracks/clogs in any oil passages. Basically blueprint it like you would an engine. If all else fails and you can't find anything wrong, pick up another C6 at a wrecking yard, rebuild it with a good kit, and try it out.

I would recommend the B&M Transkit part #40230. It has a good instruction book with it that gives some good modification pointers/ideas.

 

Posting to let everyone know what we came up with on the TX. With all respect to Like M. Fords I was apprehensive about the hole in the govenor. But, we eventually decided to try it as a last resort. If it didn't work we would rebuild another transmission. It worked like a charm. It shifts sharp and strong, there is no slippage at all. It shifts into 2nd at about 10mph and into 3rd at about +/-30mph. As I don't have a tach, don't know the RPM's for sure, but it sounds like it's shifting at the proper places. Thanks everybody, for all the suggestions and help. I've got a couple of other questions I'll get posted as I get to them.
Beth

 

As early shifts are pretty common on the C6, could you elaborate on where you drill the hole and what size? Probably will want to try this on my C6 as well, it shifts about where yours did.

 

I would like to know more details also. Although I have never noticed an early shift problem in the C6-equiped vehicles I have been around, it may be possible that this is common practice for tranny shops to perform on a particular year or series of C6 that may have an early shifting flaw. Was this the only modification you did to it this time, or did you change something else to fix the slipping between shifts problem you also had?

Thanks for replying back, we need to do more of that here AFTER the problem gets fixed so that we all can learn something.

 

I need to check with the guy that rebuilt my C6. It shifts very nicely. It will hold on to first gear while rounding a corner even at low throttle. I always hate it when an automatic shifts to second just as you're getting the truck straightend out and ready to get in the throttle. I'm guessing mine holds on until about 10 MPH with even low throttle. I'm wondering if he made this mod to the govenor. I paid dearly for this rebuild but it shifts very well, quick but not rough and I got 12,000 miles worth of warranty. I'll get back to the forum if I speak with the mechanic.

 

I'm not avoiding you all on the specifics about the hole. The one who suggested it was a tranny guy and since he knew what to do...we had him do it. I have called him to get him to e-mail me with the specifics and as soon as he responds, I'll post.
Beth