FMX Issue - 70 351C-4V Erratic or Won't Shift

Been working through an engine rebuild, got those things about done. The FMX has run well for years, it ran well a month ago. The engine rebuild just finished a 20 minute 2000 rpm cam break in. Took the car out for a drive. Shifting into first was erratic. Going down the road it wouldn’t shift out of 1st. After stopping it disengaged as if I’d shifted to neutral. Shifted to neutral, rev’d the engine, shifted back to drive and it engaged. Back to the house and backed into the garage. Trans fluid level has been checked and rechecked it is correct. Looked for the Control Pressure Take-off plug to be able to hook-up a pressure gauge. Not adjacent to the modulator as the service manual indicates. Short of dropping the transmission for rebuild, anyone have a suggestion?

A side issue, the modulator has a yellow stripe not the blue stripe as expected, it’s been like that and run fine for years. Don’t think that has much to do with the current issue but I don’t know if the transmission has been modified.

When they get close to full a lot of times one more quart of fluid is needed to make it operate. It’s already got like 11 quarts in there so it looks full on the dip stick. It’s probably just needing one more quart. Or maybe the vacuum modulator is disconnected. Or the kickdown rod is not installed right.

It’s run well before at “add a quart”. I’ve been cautious about overfilling. The modulator vacuum connection is sound and tight. It had long standing slight leak that I fixed when this condition first showed, no difference. The kickdown is good but shouldn’t effect going in to first from a stop. It doesn’t seem to be as crisp going into gear from neutral as it once was.

Likely the governor valves stuck on the output shaft. It’s governor pressure that causes upshift.

Long shot………check vacuum lines to the trans, and check the oil cooler lines haven’t got a kink.

been there and got that tee shirt…….

I know the vacuum line is good but will check the cooler lines. They were disconnected and hanging free before the failure

My fmx was doing the same thing on a 351 2v. The vacuum modulator had a tear in the diaphragm. Atf was shooting up the vac line. $12 part fixed it.

Thanks BillyC76
No ATF in the vacuum line. Always wanted overdrive the Tremec T5 arrives Wednesday. A stick shift should be fun.