Push-starting, which is also called bump-starting, works because in a moving car with a clutch, letting the clutch out causes the transmission to lock the spinning wheels to the engine’s flywheel, which causes the crankshaft to move. When the engine starts spinning fast enough, the motion will kick off the standard combustion cycle.
It’s a handy fix if you’re dealing with a dead battery, or if you left your lights on and drained it. Unfortunately, it won’t work with an automatic transmission. An automatic transmission has a torque converter that’s driven by the engine. If the engine’s not already on, the converter won’t function. The hydraulic pump also won’t work, meaning you can’t mechanically connect the wheels back to the engine. Rolling a car with an automatic transmission will just lead to sloshing transmission fluid around against the impeller on the transmission, and it won’t generate the force necessary to turn the engine and get the vehicle started.
How an automatic transmission works and why it prevents bump starting
In a standard manual transmission, the clutch is appropriately named, because releasing it physically clamps the engine to the wheels. That’s not how an automatic transmission works. An automatic uses a torque converter, filled with hydraulic fluid, as a medium to transfer power. When the engine is running, an impeller spins fast enough to drive transmission fluid into a turbine with acute force, causing the turbine to also spin. Spinning the turbine turns the gears; the gears turn the driveshaft, and turning the driveshaft ultimately turns the wheels.
Some older automatics were an exception, as they used a rear pump driven by an output shaft. This setup could produce hydraulic pressure if you got the vehicle moving fast enough, but it’s a design almost completely extinct in modern cars. If you find yourself with a dead battery in an automatic, your best bet is a jump start. You’ll need a set of jumper cables (a great accessory to keep in your car at all times) and another vehicle or a charged jump pack, but unlike a bump-start, a jump can quickly resurrect your automatic and get you back on the road.
