I have a 1985 F250 with the 6.9L diesel engine 4x4 with a c6 transmission. The transmission is shifting hard from first to second gear. I also have a 1987 F350 2wd with a c6 transmission. I would like to put the c6 transmission from the 1987 F350 into the 85 f250 4x4. I would like to know how difficult this will be. I would also like to know if the transfer case from the 1985 will bolt onto the transmission from the 1987. I don't know if this will matter but the F350 is a crew cab and the F250 is an extended cab.

 

My guess on the hard shifting is the vacuum modulator or whatever the source of vacuum, such as a pump, is on the diesel. A blown modulator causes bad shifting.

As for swapping the transmissions, the 4wd tranny is different on the aft end than the 2wd. A good tranny man can take them apart and swap the parts, but it isn't for the faint of heart. It isn't something I'd do, and I'd tear an engine down in a heart beat.

 

Like Gary said, check the vacuum controls. I do not know what you mean by shifting hard, possibly someone put a shift kit in it?

If the controls are out of wack, it will shift early and soft, or late and hard. If it's winding out before it's shifting, and then shifts hard, there are a couple of things you can do. You can adjust the vacuum regulator valve on the throttle linkage at the injection pump, or you can adjust the modulator on the tranny.

What Ford did was rig the c6 to work with the diesel engine. The c6 was designed to work with a gas engine. The c6 shifts according to road speed(the governor in the tranny) and the load on the engine(the vacuum level of the engine). The c6 normally had a vacuum line hooked to the gasoline engine intake manifold, and this ran down the backbone of the tranny and hooked with a short hose into the vacuum modulator on the pass side rear of the tranny. The modulator has a diaphragm inside and actuates a pin that goes up in the tranny. The length of this pin can be changed by taking the vacuum hose off, and taking a small screwdriver and putting it up in the hose connection and turning the adjustment. This will vary the shift points. The gas engine loses vacuum under load, and gains vacuum with no load, so that's how the tranny decides what to do.

There is no vacuum on a diesel engine. So the first thing they had to do was put a belt driven vacuum pump on it. This works the HVAC controls, the brake booster, the cruise control if you have one, and also supplies vacuum to the throttle control on the injection pump. Since they do not have any way to sense how much load is on a diesel engine, they put a variable vacuum valve thing on the throttle, so it varies the level of vacuum according to how much the throttle is opened. This thing can go bad, and it can also be adjusted to how much vacuum it sends down the line to the modulator on the tranny.

 

Thank you Gary and Dave. I will look into the vacuum system for the transmission.