I have an 87' F-350 with a 6.9L engine and a C-6 trans. I actually have 2 transmissions from different trucks. I am having minor problems with both of the transmissions though. One of them is acting "lazy", and doesn't give the full amount of power to the wheels. The other which i believe is the better of the 2 gives plenty of power, but has to be manually shifted (its an automatic) through first and second gear, and when you put it in O.D. it shifts back to first gear by its self! I don't have alot of experience with C-6 transmissions because i am a Cummins Engine Technician. I would apreciate any know how, and or advice as to what it is/ might be and how to fix it!

 

I am thinking that it may be either the nuetral safty switch, and/ or the vacuum actuator needs to be adjusted (turned). If anyone has any input on what it might be ide greatly appreciate it, and could use the help.

 

i would say flush ur transmission and and replace the filter.