my 1977 F150 4x4 is experiencing some transmission difficulties. It does not slip and seems to work fine, other than it has a rough shift between 1st and 2nd, and again from 2nd to 3rd. It seems to be better when I left off the gas pedal, but still is noticably rough. I am thinking it is probably something vacuum related.

any ideas?

thanks

 

Maybe the modulator valve is going bad. How long has it been
since you changed your tranmission fluid. If it has been a while
I would change the filter and the fluid.

Glenn

 

sounds to me like you might have too much line pressure since all gears are shifting rough, could be a pressure relief valve stuck. You could do the fluid and filter, but most likely it won't fix your problem. I seem to remember a bad modulator will cause it to not shift out of 1st. If the obvious stuff isn't the problem you should hook a gauge to the test port, go from there.

 

I am no expert but I did buy a tech manual as I need to redo my C6 - In the diagnosis guide it states possible cause for harsh engagements (warm engine) as being
1) Improper fluid level,
2) TV cable adjusted short (I think this is for AOD only, but do not know),
3) Engine curb idle too high,
4) Valve body bolts too loose or too tight, and
5) Valve body dirty/sticking valves.

Under the heading "All upshifts harsh/delayed or no upshifts" they list
1) Improper fluid level,
2) ADO only, TV cable too short,
3) Manual linkage misadjusted or damaged,
4) Governor sticking,
5) Main control pressure too high,
6) Valve body bolts too loose or too tight,
7) Valve body dirty/sticking valves,
8) Vacuum leak to diaphragm unit.

All this is from Automatic Transmission Service Group Ford C6 Service Manual - I would not recomend this manual to anyone who is not already well versed in repairing automatic transmissions.

For myself, I would be looking at the vacuum line to the trans, maybe even adjust the vacuum modulator just to insure I could (there is a screw inside the tube where the vacuum line connects, adjusting it changes your shift points.)

And, of course, it never hurts to change the fluid. I changed the fluid and filter on mine when it had about 50k on it. At that time I installed a deeper pan with a drain plug, as well as a transmission cooler and a oil temp gauge. I have never changed the filter since then, but have changed the fluid several times. I now have over 260K on the transmission and am just now thinking I should go through it as I will be rebuilding the engine.

 

TV linkage is AOD only.