Push-start with automatic transmission 2

[ivymike]

Hey guys,

An idea crossed my mind the other day, and I was wondering if somebody could poke some holes in it (the idea, not my mind)...

Typically one cannot push-start a car with an automatic transmission, because the tranny can spin freely when the engine isn't running. (Torque converter slip % would be close to 100%, but the trans input shaft doesn't spin anyway)

Are there automatic transmissions where the tranny input shaft will spin when torque is input to the output shaft, even when the engine isn't running (and thus the trans fluid pump isn't pumping)? If such a tranny exists, are they common? Further, if such a tranny exists, and was coupled to a torque converter with electronic lockup, could one rig a cheater switch to goose the lockup while attempting to push-start the vehicle?

Thanks for any info
Isaac

 

[GregLocock]

Last bit first, yes, definitely. Our lockup clutch is strong enough to stall the engine. When we are testing we often use an override box to drive the auto manually, and if you leave the lockup switch engaged the engine stops when you stop the car.

The main problem is that to get the bands to engage you need oil pressure, and it would be a brave designer who put the oil pump anywhere but on the input side of things.

You could design a box in which the default position was, say, direct drive, but I think it would not be common, for safety reasons.

Cheers

Greg Locock

 

[Chumley]

In older GM Powerglide 2 speed automatic transmissions, they used two oil pumps. One in the front powered by the engine, and one in the back powered by the driveshaft. Both were capable of generating system pressure. I could start my 63 Chevy Powerglide automatic by push starting. At about 20 mph all you did is shift from neutral into drive. It was a little difficult for one person to manually push the car at that speed but what were friends for? Many GM and Chrysler automatic transmissions of that era were similar.

Chumley

 

[evelrod]

Virtually ALL automatics from 1939 on had two pumps and could be push started. You had to build up a little speed to bring the oil presure up, as low as 25 mph on some of the GM stuff. The LAST car I push started I think was a 1960 Plymouth. A couple of other oddities---The early Hydromatics(pre '50) could be started by pushing in reverse. They all had a mechanical reverse gear as in a manual gearbox! Shift pattern was N-D-L-R and THAT got hairy on a couple of occasions.
The early Dynaflows had as many upshifts in reverse as forward. We use to get that old Buick up to about 60+ in REVERSE!!! Great fun???


Rod

 

[ivymike]

Well that brings up another question then... if virtually all automatics have output shaft driven oil pumps, then what's the harm in towing them with the drive wheels on the ground?

 

[evelrod]

HAD, Isaac. HAD! The bean counters took care of the redundant pump YEARS ago!

Rod

 

[evelrod]

I must add, that in the dawn of automatics during the reign of the 6V electrical system, push starts were VERY common. My guess is that if the gurus of GM had made a tranny that COULD NOT be push started in a pinch, we would still be shifting gears with a long stick!

Rod

PS---Re the current IVT and CVT with their 'simulated' gears. It is hard to change the publics perception of what is "better".

 

[ivymike]

re: HAD

That makes more sense... I was starting to worry about whether I really knew a thing about auto trannies. I can state confidently that the THM400 used in at least one 1978 Jeep Cherokee had no rear pump, but that would be about the limit of my first-hand experience with auto trannies (rebuilt one under careful supervision).

 

[Chumley]

ivymike: Only the older automatics had rear pumps. They generally went away in about the mid 60's if I remember correctly. The one problem I had with my Chevy was when I got the bright idea to shift it into neutral and shut engine off while going down a long 6% grade at about 70 mph. Saves gas ya know! (Well here's a self inflicted dope slap for ya, ya dumb kid!) About 30 seconds later, I smelled burning transmission fluid on the exhaust pipes. Seems the rear pump somehow backed up some excess pressure and promptly jettisoned several quarts of ATF out the filler tube. What a mess. No damage though.

I suspect a towing situation would generate the same "objection" except that you might not see it if your in the tow vehicle. That could be a problem.

Chumley

 

[ivymike]

I was driving a '65 mustang from Santa Barbara up to Davis one sunny day about 6 years ago, and I noticed that if I pushed it over 75mph, tranny fluid smoke would start blowing from the rear. It wasn't my car, so I just made a note of it and kept the speed down to 70. I wonder if it was a similar situation.

 

[TheBlacksmith]

The old (60's) Ford automatics had a rear pump also, I believe it was an FM3 automatic. I lived in a hilly area with a '65 T-Bird and it saved me a couple of times - just like everyone said - at about 20 mph, put it in gear and fire it up!

Blacksmith

 

[romke]

Mercedes-Benz automatics used to have a secondary oil pump driven by the output shaft. They even advertised with it in the seventies - you could push start the vehicle when you had a flat battery while this was not possible with most vehicles. I am not sure if their modern designs still have a secondary oil pump.

 

[callaghan]

The older Hydramatics had fluid pumps on both the input and output shafts, so the engine could be turned over by pushing the car. I think this stopped in the mid-50's, and now, with the single pump on the input shaft, there's no pressure to work the clutches unless the engine's turning.