I am a prospective GMC motorhome owner and have followed
your mailing list information with interest for some time. After
following the discussions on power trains, it occurred to me that due to
the unique and discontinued design of the transmission, it suffers from
the lack of a lock up converter; a design innovation that began in the
early 80's when downswing of power trains began. With an overdrive
lockup convertor and 3.66 gearing, there would be plenty of power in the
standard gears, and with the .75 transmission gearing it would offer as
opposed to the 1.00 of third gear, my calculations say that you would be
running about 2400 rpm at 70 mph. This is right on the max torque rpm to
provide best mileage, and of course, the convertor slippage is
eliminated as in modern transmissions.
After searching web resources and library material on GM
Oldsmobile transmissions, I found that the TH425 transmission was a
modification of the conventional TH400 transmission, and that many parts
were interchangeable. The torque convertor is interchangeable with the
TH400 and the TH350, a smaller series of GM transmissions, and there was
a TH350C variant with a lockup convertor. There was also a switch-pitch
torque convertor configuration used in the '66-'67 Toronado that allows
a selection of two different stall speeds (you have recently had
listings that referred to this configuration as a six speed
transmission).
It appears that the torque convertor requires a matching
input shaft and pump with valve to reverse torque convertor fluid flow
for engagement, and the valve is solenoid controlled, allowing simple
external activation. With this in mind, I posted the question to
alt.autos.gm to see if such a parts can be interchanged with related and
similar systems for the switch pitch Toronado. Even though the reply was
not encouraging, this information may be of interest to the group, and
maybe even though as Gary says...'that dog won't hunt', there may be
another way found to 'skin the cat' with group involvement.
I posted this question to alt.autos.gm:
> > Will a lockup torque convertor from a TH350C fit the input
> >shaft of a switch-pitch equipped TH400? From what info I have, early
> >TH400's used a hydraulically actuated switch-pitch convertor that
> >required a special input shaft, pump, and solenoid valve; a similar
> >configuration to the 1983 TH350C. These parts perform the same
function
> >as those operating the lockup convertor in the TH350C; namely,
> >switching modes of operation by reversing the oil flow through the
> >convertor.
> > If a lockup convertor would fit the switch-pitch input
shaft
> >configuration, could it not then be engaged by the electric solenoid
> >valve through a manual external circuit (eliminating TCC) with
brake,
> >speed, and shifter lockout switches? Interchangeability of parts
> >between the TH350 and TH400 series suggests to me that this just
might
> >work; and similar manual circuits have been used to adapt
switch-pitch
> >convertors for street use.
> > Would someone more familiar with TH transmissions please
tell
> >me if I am on the wrong track with this idea? In case you might
think
> >it easier to just replace the transmission with a newer 200-4R or
> >700-4R, I
> >must add that my application is the front drive variant of the TH400,
> >namely the TH425 from a pre-1978 Toronado ( Toronado also had the
> >switch-pitch convertor in 1966-67 ).
- -Bill
I recieved this reply to my question from a transmission specialist:
> Will it fit...yes, partly.
>
> Will it work...no.
>
> After an hour of searching, I finally located a v/p 400 converter and
the
> pump that matches it. Results:
>
> Distance from end of input shaft recess to rear of pump hub: 400 v/p
=
> approx. 6 inches. 350 l/u = approx. 5.5 inches. That means the 350C
> converter will bottom on the input shaft before the pump lugs engage
fully.
>
> The oil passage that controls the variable pitch in the 400 is located
on the
> pump stator shaft immediately behind the stator splines. The passage
for
> the 350 lock-up is in the end of the input shaft.
>
> Sorry to be the bearer of bad news, it sounded fine in theory, but in
the
> real world, (as Ross Perot would say) ..that dog won't hunt!.
- -Gary
your mailing list information with interest for some time. After
following the discussions on power trains, it occurred to me that due to
the unique and discontinued design of the transmission, it suffers from
the lack of a lock up converter; a design innovation that began in the
early 80's when downswing of power trains began. With an overdrive
lockup convertor and 3.66 gearing, there would be plenty of power in the
standard gears, and with the .75 transmission gearing it would offer as
opposed to the 1.00 of third gear, my calculations say that you would be
running about 2400 rpm at 70 mph. This is right on the max torque rpm to
provide best mileage, and of course, the convertor slippage is
eliminated as in modern transmissions.
After searching web resources and library material on GM
Oldsmobile transmissions, I found that the TH425 transmission was a
modification of the conventional TH400 transmission, and that many parts
were interchangeable. The torque convertor is interchangeable with the
TH400 and the TH350, a smaller series of GM transmissions, and there was
a TH350C variant with a lockup convertor. There was also a switch-pitch
torque convertor configuration used in the '66-'67 Toronado that allows
a selection of two different stall speeds (you have recently had
listings that referred to this configuration as a six speed
transmission).
It appears that the torque convertor requires a matching
input shaft and pump with valve to reverse torque convertor fluid flow
for engagement, and the valve is solenoid controlled, allowing simple
external activation. With this in mind, I posted the question to
alt.autos.gm to see if such a parts can be interchanged with related and
similar systems for the switch pitch Toronado. Even though the reply was
not encouraging, this information may be of interest to the group, and
maybe even though as Gary says...'that dog won't hunt', there may be
another way found to 'skin the cat' with group involvement.
I posted this question to alt.autos.gm:
> > Will a lockup torque convertor from a TH350C fit the input
> >shaft of a switch-pitch equipped TH400? From what info I have, early
> >TH400's used a hydraulically actuated switch-pitch convertor that
> >required a special input shaft, pump, and solenoid valve; a similar
> >configuration to the 1983 TH350C. These parts perform the same
function
> >as those operating the lockup convertor in the TH350C; namely,
> >switching modes of operation by reversing the oil flow through the
> >convertor.
> > If a lockup convertor would fit the switch-pitch input
shaft
> >configuration, could it not then be engaged by the electric solenoid
> >valve through a manual external circuit (eliminating TCC) with
brake,
> >speed, and shifter lockout switches? Interchangeability of parts
> >between the TH350 and TH400 series suggests to me that this just
might
> >work; and similar manual circuits have been used to adapt
switch-pitch
> >convertors for street use.
> > Would someone more familiar with TH transmissions please
tell
> >me if I am on the wrong track with this idea? In case you might
think
> >it easier to just replace the transmission with a newer 200-4R or
> >700-4R, I
> >must add that my application is the front drive variant of the TH400,
> >namely the TH425 from a pre-1978 Toronado ( Toronado also had the
> >switch-pitch convertor in 1966-67 ).
- -Bill
I recieved this reply to my question from a transmission specialist:
> Will it fit...yes, partly.
>
> Will it work...no.
>
> After an hour of searching, I finally located a v/p 400 converter and
the
> pump that matches it. Results:
>
> Distance from end of input shaft recess to rear of pump hub: 400 v/p
=
> approx. 6 inches. 350 l/u = approx. 5.5 inches. That means the 350C
> converter will bottom on the input shaft before the pump lugs engage
fully.
>
> The oil passage that controls the variable pitch in the 400 is located
on the
> pump stator shaft immediately behind the stator splines. The passage
for
> the 350 lock-up is in the end of the input shaft.
>
> Sorry to be the bearer of bad news, it sounded fine in theory, but in
the
> real world, (as Ross Perot would say) ..that dog won't hunt!.
- -Gary
