I have a 1994 GMC 6.5 3500HD dually 4X4.

The transmission that was in it went bad... and I had a 1993 4L80E that I put in it...all bolted up but the when I drive it, its shift points are all wrong, shifts into overdrive at about 15-20 mph. boggs the engine down..then give it more throttle and it will speed right out of it and drive like it should...

What do I need to do to make this swap work...

 

Sounds like a TCC problem more than an overdrive issue.

 

did you unhook both batteries to reset the ecu? or you can pull the glovebox and unplug the ecu itself for a minute.

 

I switched the force motor and the PWM out of the 94 4L80E transmission cause they say the 94 run of a different wavelength than a 93, that didnt change anything,

would it make a difference if the turck has a chip in it.

 

I wouldnt think so. Try unplugging the TCM for a minute or two, plug it back in, and try it

 

I unhooked the batteries and the computer and left them off for awhile, and it didn't make any difference.

kinda stumped right now, and I need my truck next week...dang!..

 

maybe its the output spped sensor. try unplugging it and see what happens.

 

Which one is the output speed sensor?

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So this is what its doing now after I've have changed the force motor, PMW, unhooked the batteries, unhooked the computer, disconnected the input speed sensor, changed the input speed sensor from the 94 transmission.

the truck still does what it did from the start, I can manually shift the transmission and it works like it should. it will go through all the gears, but if I put it in overdrive and go from there it shifts real fast and goes into overdrive at 20 mph.

 

The input speed sensor is directly above the shift rod on the drivers side of the trans and the output speed sensor is slightly lower and about 12" toward the tail of the trans

 

On the 1994 GMC the output speed sensor was not hooked up, doesn't have any wires to hook up to that.