I purchased a 76 F100 and with it inherited a rebuilt C6 transmission on a crate 302 and can't seem to find an answer as to what tansmission fluid I should use. I believe it's either Type F or DEX/MERC. Right now I just need to top off because I changed the cooling lines an lost some fluid in the process. With that transmission being rebuilt which fluid should I go with and will it matter if there's one in already and I top off with the other? I'm not particularly concerned with hard or soft shifts I just dont want to jeopardize the life of my transmission because if something as simple as the wrong fluid. Thanks for the help.

 

Look at the dipstick. The fluid spec will be on it. M2C33-F = TYPE FA M2C138CJ=DEXRON

 

Type F would be correct, but you can use DEX/MERC and it will be OK. The only diff is Type F is a little less "slippery". Type F is getting hard to find, I just run the stuff at Walmart called "Automatic Transmission Fluid".

 

Depends on year of the c-6 and what the rebuilder used as initial fill. You can rebuild a '76 or older C-6 and used the later fluid as initial fill and from now on.

I have always been told that you should not put later fluids like Mercon or Dex/Merc or Mercon V in a transmission that was started out on Type-F .... nor vice versa as the fluids are not compatible. I know Type-F is not as "slick" as the others.

Up through 1976, the C-6 originally used Type - F

From 1977 on the C-6 used the CJ fluid which was discontinued in the '80s and substituted with Mercon, later to be substituted by Mercon V.
Mercon is also no longer made as just Mercon and it along with Dexron were replaced with a single fluid "Dex/Merc"

Look on the dipstick. The fluid spec will be there but being rebuilt ..... that won't tell you for sure. Do you know who rebuilt the transmission, maybe they'd know what they put in it?

 

1964/76 C4 & 1966/76 C6 = TYPE F ATF

1977 and later C4 & C6 = Dexron II

 

Bought a used C6 that worked fine, but it leaked and after finding the leak... I refilled it. Now, it won't shift down into 1st unless I do so manually.
I cleaned up the governor, and it looked ok... i was hoping the type of fluid may be an issue. Or should I look elsewhere for the solution?
Oh, i worry about the amount of fluid as well, due to using the old dipstick off the old c6 i had. The tubes are different lengths. Why? I don't know.

 

As far as I know, use the dip stick that goes with the tube.

I have no idea what fluid you'd use now. Is this dipstick original to this transmission? What does it say to use if so?

 

Don't know for sure. I have put c6 autos in that were full of rust and totally unknown year, filled them with anything red and ran them for years, they all failed eventually but got me home every time. One failed up in the mountains of CO on a closed seasonal road in winter and all I had was 2nd gear all the way to the nearest town where I put hardware store fluid in it and let it cool then I had gears to home, but the fluid was black and trans was HOT. I rebuilt and had a bad (rotten rubber) piston seal. the local tranny shops say a c6 is a "modern day rock crusher; will tolerate absolutely anything and fluid means nothing to one of those" apparently meaning all c6 trans units must just need something that builds pressure. I blew a cooler line about 500 yards from my house back in 2017 or 2018 one morning and it would not move at all from the stop sign because all the fluid ran out. I repaired line on side of road Not wanting to walk I had a gallon of autozone universal ATF of some sort in truck, dumped it in, it moved out of intersection and had to rev wayyyy up to move, still low, so I had a gallon of 15w-40 motor oil, added entire gallon, drove fine. Ran it ever since until it got totaled last month, NOT because of a trans issue. fluid was dark, but worked fine, likely had 5 different fluids in there at least and made it 20000+ miles of our steep mountain roads, towing, hauling, behind a tmyer 400. I feel that if you have a fully rebuilt expensive c6, put good fluid in it and don't get it too hot. If you just want to drive it, the rubber seals inside will rot after only so long and will not matter, put clean fluid and keep it clean. These are good units and easily repaired if you make a mistake. I would pour anything in there and figure that if It gets me home, its cheaper than a tow bill. I made it home once with a c6 that was roughly half water half oil, broke the case crossing a river, fluid ran out quit engaging, put a few quarts in, worked a few miles, quit, put water in worked a few more miles, quit, added another gallon water, worked a few more miles. got home. Never tried to examine damage. JB welded break in aluminum and refilled with atf and ran another year.

 

I don't have the original dipstick for this C6, and couldn't believe the tubes were different lengths. This tube is a bit shorter. So I wonder if I'm a bit low on fluid.
When it was leaking. I noticed it would slip when it was running out.
It shifts manually just fine. And i feel it shift down when i put it into drive, but not completely into first. It jus barely pulls. Then when I pull it down into first. Ya feel it really grab.
. Sorry to bother ya all, and appreciate any tips.

 

Do you know how much shorter the tube is than the one the stick belongs in? Sounds like it's low.