I've got a 86 f250 with a 7.3 diesel. I bought it yesterday for a grand. It's a great running truck but the trans sucks. It shifts hard into second and only has 3spd. I'm wondering what automatic trans do I have, and what's the easiest way to install a trans with at least a 4spd or maybe even something with overdrive. I'm doing 2800 rpm @ 60 mph in 3rd? All I want to do is drive down the hwy at less than 2000 rpm. Thanks

 

you have the c6, a bulletproof old monster, but not the best on fuel. i would guess your axle probably has 4.10 gears, so it'll happily pull a house straight up a cliff, but it'll be screaming on the highway.
if you want to stay with an automatic, your only overdrive option is the e4od, which usually cost about $3k for a rebuilt one, and require a computer to control it and a few sensors. they tend to wear out roughly every 100k miles, so while they do pay for themselves in fuel, it can be painful.
your other option is to go with a manual tranny, which still isn't cheap, but are bulletproof.
if you don't have a ton of budget but want your truck a little more highway friendly, swap the tires out for something around 35" tall, figure out the difference in the speedo, and just drive it as is
come down here Pre-Power Stroke Diesel (7.3L IDI & 6.9L) - Ford Truck Enthusiasts Forums for and diesel-specific help

 

TCI has a 6 speed auto tranny that fits Fords. It's not cheap.
6X Six Speed Transmission Package; Big Block Ford Bellhousing - TCI® Auto

 

Ill have to make do for now but thanks for the tips, any guesses of why it shifts hard into second. It gives a pretty good jolt. But shifts into third fine and down shifts fine. Sometimes it will shift into second smooth after its warmed up, but 99% of the time it's a hard shift.

 

Your tranny still uses vacuum to control it's shifts. But guess what? A diesel doesn't generate vacuum. You do have a belt driven vacuum pump though, that runs the power brakes, the cruise control if you have it, and the HVAC controls under the dash. So what Ford did was rig(yes I called it a rig) a variable vacuum control onto the throttle of your truck. It's located right on the side of the injection pump where the throttle cable hooks to. If this thing breaks or gets out of wack, it can make the tranny have delayed hard shifting, or early soft shifting. They are trying to simulate the vacuum of a gas engine with this thing. And then you also have the vacuum modulator on the rear pass side of the tranny, and it can also be adjusted like it always could in the old days with the regular gas engines. This modulator is what the vacuum line feeds that comes from the throttle rig. Before you mess with anything though, I would mark it so you can put it back where it was if it doesn't help.

I would also pull the dipstick and check the color of the fluid. If it's dark brown or dark maroon, I would change it. It should be bright pink/red. You would be surprised what a fluid change will do if it looks dark. Like someone said, you can't hardly kill these things if the fluid is good in it.

 

My 89 diesel had this tranny, and I had the same problems you did, high rpms at highway speeds and 12 mpg is all I could get out of it. I swapped in a zf 5 speed, and it's 500% better than the c6 as far as performance from the diesel. I also can get 15mpg out of it now.

 

Thanks for all the great tips. Ill look into that vacuum simulator and the modulator, that sounds like it might do the trick. I've been reading about the ZF5 and I think that's something I'll try to do long term. I'm brand new to diesel, I bought this truck for $900 knowing it had the shift problem, but what sold me was the power it had. I want to keep it around long term so I'm happy to hear that people love these motor and trans. Sounds like it'll last a very long time.

 

Watch Craigslist for ZF5's, but obviously you want one for a diesel as the bell housing patterns are different for the gas engines. I paid $500 for my ZF5 that came from a 460 and included the transfer case (it was 4wd), clutch, flywheel, pressure plate, throwout bearing, hydraulic linkage and pedals.

As for the C6, if you are going to drain/refill it you'll want to also drain the torque converter as it holds quite a bit of the fluid. There's a plug on it that you can get to with the inspection cover off, but you'll have to turn the engine over until the plug stops down where you can get to it. And, I would suggest installing a drain plug in the trans pan while you have it out. Kits are available many places, and that dramatically improves the odds of you changing the fluid later since, as you will learn, draining it w/o a plug is MESSY.

 

When I was running a C6 on a gas engine I replaced the pan with one of these.

B&M Pan

Helps with cooling as well.

Cheers
Ross

 

I think I'll put all these tips on my weekend agenda. Thanks for the help fellas, they helped!