1951 Pontiac Chieftain

[PhilA]

Bought a couple of pieces to adapt the gauge to the gearbox. Got it all put in and checked to see what it was going to do.

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Cranked over on the starter for a few seconds sees the pressure up to 60psi. Starting the engine shows that's definitely the regulated idle pressure. Engaging a gear makes the pressure dip as expected (shifting to Lo dumps main line pressure temporarily before engaging any gear, a clever bit of design to make it so Reverse always starts to engage from a zero engagement state). Dr to N dips briefly to 45psi before rising back up to 60.

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Engaging R causes the high pressure valve to engage and sit, shuttling back and forth as it's meant to "somewhere above 120psi", in this case between 130 and 160psi.

Driving, the pressure modulates with throttle position from 60-100psi, this is an I8 variant behavior, causing the clutches and bands to clamp a little tighter the harder the throttle is pressed, to counter the additional torque the straight 8 has compared to the six.

It did misbehave one time tonight. The line pressure remained perfectly regulated as it should be, which means the pump, non-return, regulator and TV system are operating correctly. That means the problem is in the main valve block, the 1-2 valve is expanding and getting stuck. It's a cold day today and the gearbox being colder than usual it behaved beautifully for a few minutes. I need to pull the valve block apart again. Ideally it needs replacement but that's expensive so I'll try massage a few more miles out of this one.

Phil

[mercrocker]

Problems aside, it does illustrate how bloody advanced the early post-war American production car was. I know we had Hobbs transmissions and the like but essentially this Pontiac is a an upmarket Vauxhall in American market terms - we were still double-declutching 3 speeds here. No wonder R-R went shopping to GM for gearboxes....

[Hooli]

Shows how little actual progress has been made too. That Saab I had off SiC had sticking valves in the control block too.

[PhilA]

Shows how little actual progress has been made too. That Saab I had off SiC had sticking valves in the control block too.

This is why the industry moved away towards computer controlled electrical solenoid valves, instead of spring loaded balanced hydraulic systems. With age and abuse the electrical ones are more reliable and also can be designed to be replaced individually.

Problems aside, it does illustrate how bloody advanced the early post-war American production car was. I know we had Hobbs transmissions and the like but essentially this Pontiac is a an upmarket Vauxhall in American market terms - we were still double-declutching 3 speeds here. No wonder R-R went shopping to GM for gearboxes....

In terms of what it does yes. The ideas behind it are straightforward and had been about for decades. What they developed heavily was the hydraulic control system to fully automate it.

It gives you the basic automatic gear selection, based upon road speed, this idea had been in about since about 1915.
What it did over that is why people went to Detroit Gear:-

+ It changes gear progressively later the harder you push the throttle
+ It clamps the bands and clutches harder the more you push the throttle
+ It changes gear softly under light throttle, snappy under hard acceleration to reduce slippage
+ It has a parking lock (you park it in R)
+ It has a basic kickdown- it'll drop from 4th to 3rd if you floor the gas, later models introduced a 4-3-2 kickdown
+ It has a basic low-range which sets the shift point of 1 to 2 much higher than usual, and will force the gearbox into 2nd if the road speed is low enough to do so, for hill descent
+ It has a hydraulic mechanical interlock to prevent reverse from being engaged above about 8mph
+ It is remarkably efficient, in top gear (1:1) the fluid slippage loss is about 2%, compared to a regular torque converter at nearly 20%

So yeah, for a 30's core design is pretty damn advanced for a mass produced consumer item.

Rolls Royce had issues with their ones initially, they made the valve system to a much higher tolerance than the American ones, because RR- they found that the things behaved terribly. They made the tolerances more slack and got them working properly. The metallurgy just wasn't there, the valves would hang up as the valve block changed shape as it heated up, similar to the issues I'm having now with mine. This is why later valve blocks are bigger- they circulate more fluid around to keep the metal a consistent temperature all over and stay in shape to stop the valves getting stuck. This block sits on the side of the gearbox, not the bottom and is overall quite small.

Phil

[Hooli]

Shows how little actual progress has been made too. That Saab I had off SiC had sticking valves in the control block too.

This is why the industry moved away towards computer controlled electrical solenoid valves, instead of spring loaded balanced hydraulic systems. With age and abuse the electrical ones are more reliable and also can be designed to be replaced individually.

I'm pretty sure the Saab was ECU controlled, but suffered sticky pistons in the valve block anyway.

[paulplom]

Fascinating, seriously.

[mercrocker]

Yes, a very concise explanation of the developments....I still don't have much of a grasp of autobox technicalities!

[PhilA]

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The book helps. Parts in question are 36 37 and 38, with the middle one of 42.

Phil

[DodgeRover]

So this is a 4 speed auto? That's way way in advance of most UK autos right through to the 80s!

[PhilA]

So this is a 4 speed auto? That's way way in advance of most UK autos right through to the 80s!

Yup, 4 speed.

3 was adopted as the norm because a torque converter plus 3 is adequate for most driving. It also allows for a significantly more compact gearbox because you can put two epicyclic gears end to end sharing the same carrier, rather than two separate ones driving each other as this one is.

It's only in recent years that they've begun stacking nested epicyclics, the gearbox in my car has 8 ratios via a number of combined gear sets. That is complicated.

Phil