<VV> 66 PG Trans dropping out of high

[Brent Covey]

Hi John,

> We're also trying to diagnose pesky downshift from high to low at freeway
> speeds (lots of noise and rapid slowing down..). Have replaced modulator,
fluid
> including removing small gunky bits attributed to internal clutch, and
> rechecked  linkages.

High gear is achieved by the rear pump building up pressure from the rear
wheels beginning to roll- about 12 mph the pump output is sufficient to
pressurize the transmission, which shuts down the front pump and initiates
the shift to second.

The shift to second is moderated by a couple things-

Throttle position (via Thottle Valve- the 'TV" lever)

Governor position (axle speed)

Rear Pump Pressure (under 20 mph)

Vacuum Modulator pressure

The balancing of these factors will operate the low-drive shift valve. The
Low drive shift valve is weighted to prefer second gear, as well, to reduce
hunting between ranges, it will favour staying in 2nd if its on the verge of
shifting.

The most natural place to start looking with your symptoms is the governor.
The governor is driven by the pinion shaft and is the bell shaped thing just
ahead of the starter. It has flyweights inside driven via nylon gear that
operate a speed sensing valve to initiate a shift or force one at various
speeds, 140's use a different one for higher shift speeds as example. The
nylon gears sometimes slip or wear the teeth down and might 'stall' when
they are not being driven positively, which would dump you into first gear
in a hurry. If the teeth catch again it will be back to normal, pull the
trans to axle bolt that holds the governor on and inspect the teeth or
substitute a different one and see if that works, as a first try. I would
imagine theres a governor issue on your Powerglide with a worn-concave gear.

If there is a huge leak or restriction in the rear pump circuit or a large
internal leak in the transmission high clutch there could also be an
intermittent drop from high but this is very unlikely and I hitherto havent
encountered a Powerglide with a similar problem. If you like, you could test
the trans pressures at the front cover plugs and see if the pressures are in
the ballpark, this could be done with the rear wheels off the ground or you
could rig a guage and line to check while actually driving the car. You need
a guage that is calibrated to 200psi for this task, a compression tester may
work with adaptation.

See what you can discover and let us know if that helps,

Brent Covey
Vancouver BC