• We’re currently investigating an issue related to the forum theme and styling that is impacting page layout and visual formatting. The problem has been identified, and we are actively working on a resolution. There is no impact to user data or functionality, this is strictly a front-end display issue. We’ll post an update once the fix has been deployed. Thanks for your patience while we get this sorted.

Push/roll starting a car

Status
Not open for further replies.
Z

zoiks

Lifer
A simple question. Why is it that only a manual transmission car can be roll started and not an auto trans one?
 
Because the hydraulic pump that powers an automatic transmission is turned by the engine and not the drive wheels. No pressure = no means to turn the engine over. The drive wheels are turning parts within the transmission, but not the pump.

Edit: This is a bit of an oversimplification.
 
boomerang is all over it, but let me try to clear it up:

An automatic transmission isn't engaged without the engine running.....meaning, the tranny/driveshaft/axles aren't connected to the engine unless the engine is running. So the car is essentially in neutral in every position but Park, if the engine is not running.

This is due to lack of pressure boomerang was talking about. If you could ever sit in a car that has no neutral safety switch and start it up in gear, you'd feel what we're talking about when the pump pressurizes the tranny.
 
yep.

and the reason the building of line pressure is important is because it enables the different clutches and bands to be applied hydraulically, which is what engages your gears. the gears inside an auto are always in contact, but certain parts must be either held or turned (former done by bands, latter clutches) to complete the flow of power through a tranny's internals.

you also wouldn't be able to generate enough speed while pushing to cause the innards of the torque converter (splined to the trans input shaft) to transmit any motion to the engine. which, in turn would not send any power to the oil pump in the trans (which spins at the same speed as the flexplate and outer shell of the converter). it's a chicken or the egg kinda thing, really.
 
And you can push start some automatics but you have to get them up to 25mph or so to do it. My 67 Ford Galaxie with a FMX 3 speed auto had a pump driven off the drive shaft so you could build pressure just by pushing it. Went over push starting it in the manual but I never tried to do it.
 
Status
Not open for further replies.

TRENDING THREADS

Back
Top