727TF - lost 3rd and reverse - what happened?

[471_Magnum]

In route to Mopar Nationals, the Roadrunner suddenly dropped out of third gear into second.

After a ride home on the roll back, discovered reverse was gone to.

No loss of fluid. Nothing burnt. Supposedly recent rebuild. Shifts from 1st to 2nd normally.

What broke?

Going to pull the transmission this week, but I'd like to know what to look for before just sending it off for repair.

[Ghoste]

Front clutch.

[471_Magnum]

Front clutch.

Had someone else tell me rear clutch.

[Ghoste]

Rear clutch is engaged for forward gears but not reverse.  To my knowledge the only apply device common to 3rd and reverse is the front clutch.

[471_Magnum]

Yeah. Just did a little of my own research on the subject and I concur.

Wonder if it's time for me to rebuild my first transmission....

Edit- There was no slippage before this occurred, so I'm wondering if it wasn't the actual clutches that failed.

[John_Kunkel]

The failure is in the front clutch as a whole, could be seals, sealing rings or discs.

[NMike]

usually you find the inner seal on the drum has shrunk and is the source of the failure.

[471_Magnum]

So it actually turned out to be a broken sealing ring. Might have been broken during the last rebuild.

Anyway, it's all back together now.

[375instroke]

I'm confused as to how second was engaged.  How could a bad front clutch cause the front band to apply?

[Ghoste]

The front clutch isn't applied in second and the band operates through a different circuit.

[375instroke]

That's my point.  If the front clutch went out, wouldn't it just freewheel, and not drop down into second?

[John_Kunkel]

It wouldn't "drop down into second", second is already engaged.

If the pressure leak at the front clutch is large enough there won't be enough pressure to release the front servo so the front band will stay engaged.

The thing that confuses a lot of people about the operation of the front servo is that the apply pressure isn't reduced when the release pressure is introduced to the opposite side of the servo piston, the differential surface area of the piston allows the piston to move in the release direction even though the pressure on both sides of the piston is the same.

[375instroke]

I just looked at the schematic, and you're totally right.  Pressure does stay on the apply side of the servo when in third.