It's a 79 f-100 w/460 and c6 tranny. I just rebuilt the motor it runs great. The problem is the transmission. The truck probably sat for about 5 years before I bought it. The reason it was sitting was because the no. 8 connecting rod bent. When I bought the truck the guy said that the only thing wrong with it was the motor and that the transmission shifted great. The truck has only rolled over one time and it now registers 45,000 which means 145,000 miles. Now that I have the motor back in the truck It won't shift. By saying that I mean it doesn't shift automatically. If I put it in reverse it feels like a real positive engagement. But when I put it in drive it feels sluggish and I can barely feel it engage. When I'm driving it down the road it wont shift out of first gear. I can manually shift from first to second, but when it try to put it in d next it just shifts back into first. And also when I'm in D going down the road if I let off of the gas and put my foot back on the gas it feels like it's slipping and then it will pick back up in first gear. And all of this happen's whether the motor it is cold of hot, it doesn't matter. Things that I have done to the transmission: changed fluid and filter, fluid it dextron 3 and is at the correct level, I also changed the modulator valve with the purple ring around it, the old one had the purple ring around it. I just attempted a band adjustment today on the one on the drivers side. I'm not sure that I did it correctly (loosen nut, tighten 10 ft/lbs back off 1.5 turns and retighten nut) but it didn't seem to change anything. I know this is long and I'm sorry but I just feel like I need to list everything I can about the problem and maybe someone that has had a similar problem can enlighten me a little. Thanks in advance.

 

Try the transmission forum.

 

and also try the vacuum modulator.
BTW, Is Mercon 3 the correct fluid?

 

I had the EXACT same problem not too long ago, after having my motor rebuilt. I tried the Vac Modulator and that didn't fix it.
Here's the thread:
https://www.ford-trucks.com/forums/5...rd-on-own.html

Between the transmission shop, and the shop that rebuilt my motor, they both told me that a lot of times when you install a rebuilt motor into an old beat up tranny (especially one that's been sitting for a long time), it will eat the tranny. If you pull the tranny pan, and find a lot of metal shavings, that's what's going on. The shavings build up and clog everything, they've probably clogged up your governer (that's what happened in mine)
Sorry to say, but you'll probably need a rebuild. It sucked for me...

Oh, and you should be using Type F fluid in a 79, not D3...

 

C6's take Dextron or Mercur.

F type is for Ford plastic power steering pumps

 

C-6's are very cheap and easy to rebuild.

 

That's odd, because my C6 dipstick says to use Type F only on it...

 

Type "F" was specified for ford autos from the 50's untill 77 for all autos. Then a greater spec was used, and ford called it Mercon.
Funny about all of the posts regarding transmission fluid, but one would think with all of the "true blue" guys here that they always make reference to Dexron, and
Dexron III. (forgetting about Dexron II, or Dexron IIE). Dexron, and Mercon are two different specifications from two different manufacturers. Mercon is a ford spec, and Dex is a G.M. spec.
They are not the same fluids.
Type "F" is just an early fluid that had no friction modifier in it. Shifts were pretty firm, and a little harsh by todays standards. Back then the alternative was type "A", (G.M. )and chrysler had something like ATF-2 or something funny like that. The early fluids were / are mineral based, and oxidized rather quickly. Newer synthetics, and blends last longer, and foam less, and have friction modifier seal swell additives, man the list goes on, and on, but nowhere is it stated that dex, and merc are the same like I have seen it used together on this site.
Now, some manufacturers have their own fluid that meets or exceeds the Dex , or Merc specs, so I can understand the use of the two in the same sentance.
As for the use of "F" and DEX, or MERC, in a C6, well some C6's after 77 up to about 81 still used "F" even without administrative approval from ford that actually expired in 77. Anyway, the type "F" is not compatible with any other fluid, and quick failure will result if they are introduced in the same transmissions.

 

My literature states that beginning in 1976 the C6 required Dexron II fluid.

The C4 used F-Type.

This yet another reason this thread should be in it is proper forum....

The drivetrain forum.

 

Well if you think it is appropriate, have it moved. I'm fairly new to this site and haven't learned all of the different forums yet. Sorry about that.

 

Dang, I've got half a million miles on my '78 with original C6, and it has run Type F since forever... It shifts like a new transmission, so, ummm... Just lucky? Should I change it to something else now? Haha.

One thing you will want to check is for vacuum coming to your modulator. I have experienced it. Before I was into changing my own transmissions, I had a performance shop install a built C6 for me. It didn't shift, same as what you are describing. Check for vacuum, and found very little. Turns out the steel vacuum line from the engine was crushed during the installation, stopping proper vacuum signal. A length of vacuum hose was installed in place of the steel line, and all was well.

 

as I already stated, I have great vacuum at the modulator valve.

 

If ya got Dex in a C6 ya already ruined it. Sorry but it takes Type F till replaced by the E4OD and AOD.

 

Where are you guys getting this info about C6's talking F type ?

It distinctly states that the C6 trans requires Ford # ESPM2C138-CJ or Dexron II


F type was used in the 1975 C6

Then 76 they chanmged to Dexron



C4's take F type.

 

What I've read about fluid is that the F fluid was for the type of clutch material used by Ford ubtil 1970.They stil recomended F type but the change in clutch material made other fluids OK to use.The F fluid was used by a lot of drag racers and performance shops for the firmer shifts.