1999 150 4.6 4 speed auto with overdrive 94,000 miles
I was towing a 1500lb boat at about 70 trying to catch up with the rest of my party when i noticed my tranny couldnt seem to find a gear, i turned off the OD and that sent the RPM's a lil high but it would at least stay in gear. figured it wasnt an issue i was probably just pushing it too much so i dnt mind taking OD off, it was probably better anyway. however driving my truck two days later around 40 mph the tranny tries to shift but only revs like its in neutral. this was not a hard accel just normal traffic and not towing anything. if i take the OD off the tranny grabs what i think is 3rd gear and drives fine, but everytime i try to go past 40 mph with the OD on it "misses" the gear. Im probably going to take her in to the shop soon (1700 miles left on warrante phew hope they cover it) but wanted to get some info before walking in blind to the dealership. i have checked the fluid level its not too high/low. Im gna guess that its a schyro but not sure. i had read something about a solenoid that could get dirty and prevent shifts, but i also realise i probably overheated the tranny pushin 70 on the highway for about an hour, oops thanks guys for any feedback

 

That would be VERY interesting since there are no synchros in an automatic transmission.

I think the overdrive servo has developed a massive internal leak. When it tries to shift to overdrive the leak dumps the line pressure and the trans goes to nuetral.