Push starting an automatic

[Chuggy]

I know it's not possible to do this, but would somebody like to explain, in simple terms, why not???

[finlo]

Because the hydraulic pressure required for the box to do anything is driven from the input shaft meaning the engine has to be turning to get any connection between the engine and driven wheels or vice versa.

[Miserablegit]

I’m going to try…

A traditional auto had a torque converter- a fluid-filled coupling which allowed the engine to move without engaging the transmission - when the engine speed increased the pressure of the fluid then caused the torque converter to start to rotate and drive the transmission.

Before we even get to the transmission, it’s not possible to spin the torque converter from the other side to move the engine without borking the torque converter. If you try to pedal a bike from stationary in top gear you’ll have a heck of a job and might break the chain - it’s not quite the same but near enough for my purposes.
Then add in the complexities of an auto gearbox needing oil pumps etc which won’t be working if the engine is off and you’ve got a whole world of hurt waiting for you as you try to power the system in reverse.
Somebody will be here shortly with a much better explanation

[liner33]

In the simplest of terms - You cant bump start an auto because without the engine running there is no connection to the driven wheels

[SkodaIan]

Would it be technically possible on a DCT auto if the battery was fine, but the starter motor was not working?

There's probably something in the control electronics to stop you doing it, but if with electrical power you could get it to engage the clutch by shifting from N to D whilst being towed along it should then start the engine in the same way as a manual car.

I'm not recommending anyone try this out though ... DCT gearboxes seem perfectly capable of destroying themselves in normal use without doing anything like that!!

[Krikkit]

SkodaIan said:

Would it be technically possible on a DCT auto if the battery was fine, but the starter motor was not working?

There's probably something in the control electronics to stop you doing it, but if with electrical power you could get it to engage the clutch by shifting from N to D whilst being towed along it should then start the engine in the same way as a manual car.

I'm not recommending anyone try this out though ... DCT gearboxes seem perfectly capable of destroying themselves in normal use without doing anything like that!!

Yes, assuming you could get it to play ball, that would work

[Register1]

You can tow start the older BW auto boxes.
Need to tow up to 30 - 40 mph.
So can be done.

[GreenV8S]

Register1 said:

You can tow start the older BW auto boxes.
Need to tow up to 30 - 40 mph.
So can be done.

Any idea how that works?

I get that 30 - 40 mph is enough speed difference across the torque converter to transfer some torque, but how is the 'box engaging any gears? I thought the band clutches were typically hydraulically actuated with hydraulic pressure generated from the input shaft.

[hellorent]

Seem to recall some of the old porsche's could be bump started as well, thinking around 924/944 models

[Stiggolas]

i once had an old 1972 Merc 280e auto. That was regularly bump started. Get it moving down hill in neutral, drop into drive at about 15mph. No idea if that damaged the box but it went on forever...

[hairy v]

Reminds me of a holiday I had in Amsterdam with some mates years ago. We came across a car that was struggling to start so we offered the owner a push start. They readily accepted so we rolled up our sleeves and got the thing moving.

We shouted "Let out the clutch!"

To which we got the response from the dutch driver "I can't, it's an automatic"

Oh...

[Register1]

hellorent said:

Seem to recall some of the old porsche's could be bump started as well, thinking around 924/944 models

Need a bit more than a bump.
But can be done on a decent long downhill,
Keep in neutral, then drop it into drive

[tr7v8]

GreenV8S said:

Register1 said:

You can tow start the older BW auto boxes.
Need to tow up to 30 - 40 mph.
So can be done.

Any idea how that works?

I get that 30 - 40 mph is enough speed difference across the torque converter to transfer some torque, but how is the 'box engaging any gears? I thought the band clutches were typically hydraulically actuated with hydraulic pressure generated from the input shaft.

Nothing to do with the torque converter. Simply put of it only has a single pump on the box that will be on the input shaft. Some boxes have dual pumps so there will be one driven on the output shaft. So when you tow it you can get enough pressure in the box to engage one of the servos & then engage the brake band & it'll then go into a gear.
My father ran a fleet of Mothers Pride Morris vans with 2.2 BMC taxi diesels & BW 35 boxes. The boxes were all dual pumped so could be tow started. On a nice icy morning dragging a loaded van around a yard at 30+MPH with a tow truck was pretty hairy. As a 14 year old I learnt lots of skid control pretty quick!

[donkmeister]

finlo said:

Because the hydraulic pressure required for the box to do anything is driven from the input shaft meaning the engine has to be turning to get any connection between the engine and driven wheels or vice versa.

This. Which is why the 4 speed AP auto used in Minis (which had a hydraulic pump on the output shaft too) was one of the few autos that CAN be push started.

Without hydraulic pressure the box is in neutral, and with only a pump on the input shaft to the gearbox then the only way to take it out of neutral is to have the engine spinning.

[GreenV8S]

tr7v8 said:

Some boxes have dual pumps so there will be one driven on the output shaft. So when you tow it you can get enough pressure in the box to engage one of the servos & then engage the brake band & it'll then go into a gear.

Interesting - I haven't encountered those. I guess cost cutting may have lead to the single pump systems that are common today.

Does it mean those dual pump boxes were also safe to tow? Usually, modern auto boxes can't be towed long distance or at any significant speed due to lack of lubrication.

[tr7v8]

GreenV8S said:

tr7v8 said:

Some boxes have dual pumps so there will be one driven on the output shaft. So when you tow it you can get enough pressure in the box to engage one of the servos & then engage the brake band & it'll then go into a gear.

Interesting - I haven't encountered those. I guess cost cutting may have lead to the single pump systems that are common today.

Does it mean those dual pump boxes were also safe to tow? Usually, modern auto boxes can't be towed long distance or at any significant speed due to lack of lubrication.

Yes they could be towed, I think there were max speed & distance limitations although these things struggled to 45mph anyway. And Dad's tow truck was a manual, crash box 2.2 van so typical tow speed was 35MPH max. They had crap brakes so you wouldn't want to go faster!