66 TH400: Variable pitch?

[wbdeford]

Is a '66 TH400 variable pitch?

[dplotkin]

Variable pitch through 67.

Dan

[Julien Abrahams]

Weren't all TH 400's variable pitch?

[TJ Hopland]

I think it was 64-67.   I think it was common on Cadillac and and option on some Buick and Olds.

[cadillactim]

Variable pitch was 65-67 except commercial chassis. With larger engine in 68, variable pitch was discontinued.

Tim

[wbdeford]

Thanks, guys!

[bcroe]

Is a '66 TH400 variable pitch? 

All 65-67 Buick, Olds, and Cad big block cars had switch pitch.  Its not really "variable", you
can choose between low stall (economy) and high stall (performance) modes.  Properly
controlled, it can smooth the car, increase performance, and raise mileage a bit.  All 5 of
my cars have been converted to switch pitch.  Lot of info on the internet, including some
albums on my PHOTOBUCKET.  Bruce Roe

[Coupe Deville]

I have heard of this before. Is there a actual switch to choose the stall speed?

[savemy67]

Gavin,

There is an electric switch mounted on the intake manifold near the throttle linkage that is activated by the position of the throttle.  When the switch is activated, it in turn activates a switch inside the transmission that is affixed to the transmission pump.  This switch actuates a valve that redirects the flow of transmission fluid to the inside of the torque converter, where the stator is located.  The redirected transmission fluid hydraulically changes the angle of the stator blades in relation to the torque converter's impeller and turbine.  As Bruce Roe noted, the switch in the pitch of the stator is designed to be either high or low, and not continuously variable.  Considering that this transmission is half a century old, I think GM devised a reasonable solution to the issues of performance and economy in fluid coupling/torque converter automatic transmissions.  And, as one would expect from a Cadillac, drivers did not have to give the process a second thought.  Just step on the gas pedal and the throttle and the transmission would do the rest.

Christopher Winter

[dplotkin]

For the curious, note that Buick's Dynaflow, from 56 through 63 had switch pitch too, but it was activated by a throttle rod & not a switch. Dynaflows did not downshift to low, that had to be done manually so switch pitch is what you got when you matted the pedal.

Dan

[Jon S]

Gavin,

There is an electric switch mounted on the intake manifold near the throttle linkage that is activated by the position of the throttle.  When the switch is activated, it in turn activates a switch inside the transmission that is affixed to the transmission pump.  This switch actuates a valve that redirects the flow of transmission fluid to the inside of the torque converter, where the stator is located.  The redirected transmission fluid hydraulically changes the angle of the stator blades in relation to the torque converter's impeller and turbine.  As Bruce Roe noted, the switch in the pitch of the stator is designed to be either high or low, and not continuously variable.  Considering that this transmission is half a century old, I think GM devised a reasonable solution to the issues of performance and economy in fluid coupling/torque converter automatic transmissions.  And, as one would expect from a Cadillac, drivers did not have to give the process a second thought.  Just step on the gas pedal and the throttle and the transmission would do the rest.

Christopher Winter

That switch on the intake manifold is a detent solenoid and was used to activate downshifting; not any switch in pitch of the stator as you imply.

[Scot Minesinger]

bcroe,

You mentioned that all five of your collector cars are switch pitch.  Is that because you changed them from non-switch to switch pitch?

The switch pitch seemed like a better transmission, although I do not know for sure.  The 1966 year seemed to be the pinnacle of GM innovation (Olds Toranado for example), and after that it was more about cost cutting, keeping up with emissions/govt regs, and then better mileage thru underpowered engines.  Perhaps the non switch pitch was just cost cutting in 1968.  I better stop typing about this now or will deviate way too much from original topic.

I noticed that the drive shaft of my 1970 Cadillac looks like it has a different size yoke that feeds into splines of trans shaft than a 67 I worked on.  How did you convert, was it simple bolt in operation?  How did you set it up to trigger pitch to switch?

[savemy67]

Jon S.,

Per my 1967 Cadillac shop manual, Cadillac used three versions of the TH400 transmission (not including the TH425 used in Eldorados) in 1967,  The TH400 transmissions were designated AA, AB, and AC.  The AB and AC transmissions were used on commercial chassis and limousine models.  The AA transmission, with the switch pitch feature, was used on everything else except Eldorados as noted above.

Attached are two pictures of my 67 Sedan DeVille showing the drivers side of the carburetor.  If you look closely to the right of the throttle dashpot you will see a switch that controls both the throttle kick-down (aka detent) and the stator switch.  There are several references in the manual discussing operation and adjustment of the switch which controls both the kick-down function and the stator switch-pitch function (pages 6-87, 7-1, 7-3, 7-30, 7-31, 7-36 in the 67 manual).  Notice the connectors at the switch.  One connector is a single pole and the other connector is a dual pole.  That is because the entire switch enclosure houses two switches - the kick-down and the stator pitch - that are actuated by the movement of the same throttle linkage.

I have also attached a picture of a 1968 Eldorado carburetor.  To the right of the dashpot (which is broken) notice that there are two single pole connectors attached to the switch housing.  Compare this to my 67.  The difference is because in 1968, the switch pitch feature was no longer offered and the switch functioned only to kick down the transmission to a lower gear when the throttle was mashed.  Respectfully,

Christopher Winter

[bcroe]

bcroe,    You mentioned that all five of your collector cars are
switch pitch.  Is that because you changed them from non-switch to switch pitch?

The switch pitch seemed like a better transmission, although I do not know for sure.  The 1966 year seemed to be the pinnacle of GM innovation (Olds Toranado for example), and after that it was more about cost cutting, keeping up with emissions/govt regs, and then better mileage thru underpowered engines.  Perhaps the non switch pitch was just cost cutting in 1968. 

I noticed that the drive shaft of my 1970 Cadillac looks like it has a different size yoke that feeds into splines of trans shaft than a 67 I worked on.  How did you convert, was it simple bolt in operation?  How did you set it up to trigger pitch to switch?   

My cars are 70s but the RWD have a TH400 switch pitch and my 79 Eldo has been converted from
a TH325 trans to a 67 Eldo TH425 switch pitch trans.  In 68 GM decided it was cheaper to bore all
the engines than build switch pitch.  Some early TH400s had a yoke with smaller 1.69" dis, about
69 they all went to 1.89" probably using cheaper metal.  The trans output shaft is identical, and
you can convert to the other yoke by swapping the tail piece containing the outer bearing. 

The TH400 or TH425 switch pitch had a different bell housing for Cad 429 and some Buicks; otherwise
they were interchangeable if the tail was the same length (4", 9", or 13").  All the internals will
interchange, so my cars use a newer trans with the needed switch pitch internals.  The factory
switch only used the feature at idle or near full throttle.  I experimented a couple years, 60,000
miles, and came up with an electronic control.  It used the feature in the mid range where most
driving occurs, smoothing shifts, adding some zip.  My Delta 88 with 2.41:1 axle felt like it was
starting in the wrong gear; the switch pitch took a whole second off 0-60.  Bruce Roe CLC 14630

[Scot Minesinger]

bcore,

There was an article a while ago about 67 - 70 Cadillacs and it mentioned that the switch pitch made the car faster accelerating than fixed pitch.  Ingenious the way you switched the tail pieces out.  What is the control to switch pitch?  You mentioned electronic, but what is trigger to switch the pitch, fluid pressure, vacuum, throttle position, or etc.?

Many will go to MTS on their 472/500, where you used the trans to shave a second or two of your 0-75 mph time - very cool.

[bcroe]

bcore,
There was an article a while ago about 67 - 70 Cadillacs and it mentioned that the switch pitch made the car faster accelerating than fixed pitch.  Ingenious the way you switched the tail pieces out.  What is the control to switch pitch?  You mentioned electronic, but what is trigger to switch the pitch, fluid pressure, vacuum, throttle position, or etc.?
Many will go to MTS on their 472/500, where you used the trans to shave a second or two of your 0-75 mph time - very cool.   

12V to the trans sets the high stall, or performance mode.  It lets the engine rev to its power
range much sooner, with more torque multiplication.  Race cars use high stall, useless on the
street.  Remove 12V and get low stall, economy mode.  I believe its even more efficient than
a fixed "compromise" converter.  Does not affect the gears. 

I use torque sensing (manifold vacuum) and brake release with timers to anticipate what the
engine is doing.  I'll attach the flow diagram, but it makes more sense driving than just reading
about it.  Bruce