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1951 Pontiac Chieftain - Troubleshooting gearbox

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Took this bloody thing completely apart again.


Thoroughly cleaned it


Found a high spot on the 1-2 piston, clearanced that better. Did the valve swapsies as suggested by the manual to find the best matched one to the bore.


Will put it back on after lunch and see what it does.



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The more I think about it, the more I think I might be suffering low oil pressure at hot oil temperatures.


Looking at possibly getting a gauge for it; failing that in the interim putting some glop in to see if it makes a difference.



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Keep it up Phil.  You'll get the thing licked eventually then it'll hopefully give you years of trouble free service.

Refreshing to see someone who's not afraid of taking an auto box apart (well, kinda...but it's near as dammit an automatic from a mechanical perspective).  Can't see me trying the same sort of "I'll just take that apart and clean it..." approach on the GM400 in the Jag!  Admittedly the fact I'd need to probably dismantle the car down to it's individual atoms first may have a lot to do with that.

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It's a fully automatic box. It even has some of the features yours has! I just viewed this as a learning experience. There's nothing special about epicyclic gears (look at a certain Lanchester on here, you can make them work like a regular gearbox), I think it's the hydraulics that tend to give people the ooeeer about automatics. If you study them their principles are sane, just you can't often easily see what they do because the moving bits are hidden and arcane.

The valve control block is poorly documented in the book for this one, so I'm still trying to figure that out completely but the principles are sound. There are a few things that can cause this to do what it's doing, just a case of finding what's happening (and likely jamming) when it does.

Also, there's a quiet list being built up in the background here.

The rear axle needs new seals, that I'm going to try and hold off until I can get a lower ratio diff and pinion; the master cylinder is now liking itself a bit of a drink and the steering really needs a rebuild.



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Just been chided by the rivet counters for gently massaging the high spots on the valve block, being advised that is wholly inexcusable to do that.

I nod and smile and reflect that on a valve block that hasn't done 100000+ miles and had monkeys working on it previous I might agree.


I think I'm possibly due a slightly less worn valve block and pressure regulator. But money isn't gonna allow that right now so I'll settle on "slightly poorer clearances than in the specifications".



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So, with regard to the gearbox, the valves do slide out under their own weight on both sides. I matched the governor plungers to the best bore as suggested in the book; the 1-2 bore has a couple of high points as evidenced by the witness marks. They are helical so part of the casting rather than being scored.

I'm surprised the damage isn't worse, considering how much in the way of metal particles were inside the system.


The failure is 1st gear holds for much higher road speed than it should per throttle position after the gearbox is put into either N or R; it will then change violently into 3rd and then be reluctant to engage 4th.

If you slow down before it engages 4th, it'll change back down 3-2-1 and then replay the high speed (15mph or thereabouts) 1-3 gear thump over and over. 

The speed at which it changes 1-3 is critical, and throttle position has no bearing on it. Snapping the throttle shut will not make it change up, only road speed.


Once it's engaged 4th, so long as the shifter remains in Dr, it'll cycle through the gears smoothly at the speed you'd expect from the throttle position.


4-3 kickdown operates correctly and does not impact gear changes after it's been used.


No gears slip. It'll happily squat the back end down and leave 2 pale tire tracks up the street if you stamp on the gas from a standstill, through 1st and 2nd.


Occasionally R will engage the parking pawl if you aren't at a complete halt, rather than the cone only engaging, then the pawl coming in once the gearbox has become stationary. By general habit I try not to change the position of the shifter until the car is fully at a rest, was just taught that way.


Some of the symptoms feel like a sticking gear selection valve, but some hark of low line pressure. I'm not 100% sure the compensator valve isn't getting stuck.


Gauge should be here by close of work tomorrow, I'll have to get an extension piece to fit it. I will run the road tests as outlined in the book and see if it's regulating within spec.


Front pump is NOS. Rear pump is original to the engine but wasn't in bad shape. Regulator is existing to the gearbox, TV valve shuttle and pressure relief plunger move smoothly in the pump bore.



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Gauge installed on an extension and adapter.

It regulates nicely to 60psi in N, Dr and Lo as it should, there's the correct dips and wobbles shifting between positions.


Reverse, the high pressure valve opens as it should and shuttles to and fro as per the manual's description, keeping between 130-160psi, as per spec.

Driving, the TV valve modulates the line pressure between 60 and 100psi, as it should.

It decided to misbehave one time, with the line pressure perfectly regulated, which leaves the fault within the main valve block, particularly the 1-2 main shuttle. There's not a great deal I can do about that so between getting a new one and now I'm take it apart again and try massage the divot out of the valve again.




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  • PhilA changed the title to 1951 Pontiac Chieftain - Troubleshooting gearbox

Oh, and one giggle- I did the book test on the rear pump last night.

Drive up to about 20, put it in neutral, kill the engine then put the ignition back on then drop it into Dr and see if it builds up enough pressure to engage.

Happy to say I can bump-start my car. It makes about 30psi at 15-20 mph which is enough to get it to lazily engage a gear and spin up the torus with fluid.



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1 hour ago, PhilA said:

Oh, and one giggle- I did the book test on the rear pump last night.

Drive up to about 20, put it in neutral, kill the engine then put the ignition back on then drop it into Dr and see if it builds up enough pressure to engage.

Happy to say I can bump-start my car. It makes about 30psi at 15-20 mph which is enough to get it to lazily engage a gear and spin up the torus with fluid.



Is safe towing / bump-starting the sole purpose of the rear pump?

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6 minutes ago, Asimo said:

Is safe towing / bump-starting the sole purpose of the rear pump?

That's one reason. The other reason is the front pump, driven by the engine is a variable pitch pump designed to produce high flow at low to medium RPM. At high RPM it can begin to run out of capability so the rear pump, driven by the output shaft is a standard meshed gear like an engine oil pump. That one is poor at low speed but very effective at high speed and bolsters the main pump at high roadspeed.



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7 minutes ago, hairnet said:

4k pornhub?

how small are the parts?? 😮

The parts aren't that small, but the edges have to be accurate and those are tiny.

I just can't see that well up close any more.


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