Coasting with stalled engine, automatic transmission in gear

cavalier
chevrolet
transmissions

#1

The car is 1985 Chevrolet Cavalier with automatic transmission.
I had the engine stall several times and I coasted sometimes for a quarter of a mile with the transmission in gear. Can this cause damage to transmission?
Later on I wised up and moved the shifter to neutral when the engine stalled.
When I coasted in gear, I was getting some vibration that smoothed out when I put it in neutral.


#2

I think it unlikely this would cause much in the way of damage to the transmission.


#3

The pump for the transmission is mounted behind the torque converter which supplies pressure as long as the engine is running. When the engine stops running, the pump in the transmission stops pumping. And this can damage the transmission.

This why you can’t tow an automatic transmissioned vehicle by the drive wheels. Because if the drive wheels are turning without the engine running, the pump isn’t providing pressure as the internals of the transmission spins.

Tester


#4

Thanks. So in my situation, with the engine stalled, is it better to coast in neutral or in gear? Why was I getting some vibration when coasting in gear?


#5

You can’t coast in either.

As long as the drive wheels are turning, the internals of the transmission are still spinning without lubrication.

Tester


#6

I see. But there are times when you have no choice. So which is the least destructive way, in gear or in neutral?


#7

Sure! There are times where you have no choice. But to do it constantly will wreck the transmission.

Tester


#8

@Mike123 wrote:

But there are times when you have no choice.

You always have a choice - which is to solve your immediate need of getting to your destination, or to risk damaging your transmission.

Car manufacturers stopped using a 2nd pump in the output shaft of transmissions over 45 years ago (when it was easy to push start cars with automatic transmissions).

Take Tester’s advice seriously - especially given his experience.


#9

When the motor stalls you should not coast very far, just enough to safely get off the road. With a stalled motor you have no power steering and the power brakes will work only for one stop after that you need a lot more leg pressure on the brake petal to stop. The amount of “coast” necessary in an emergency is not enough to damage the transmission. Now, if you stall at the top of a mountain and coast a few miles to the bottom, you will do damage.

I think the OP should deal with the poor running stalling engine. No stalling means no worries about the transmission.


#10

Tester . . . and in a manual the lubrication is constant now matter if the engine is running or not, right? How is a manual lubed? Splash? (assuming that there are no dumb questions) Rocketman


#11

@rocketman, yes, manual shift transmissions have no pump. Just like a rear differential, the gears and bearings moving through the gear lube bath provide all the lubrication needed.


#12

Thanks BK! Rocketman


#13

If you have to coast, coast in gear, that will turn the engine and in turn will operate the pump some. FYI, modern manual transmissions do have pumps in them.


#14

@keith

Did you ever notice that you can’t push start an automatic transmission vehicle? That’s because the torque converter can’t transfer the rotation of the transmission back to the engine because of the turbine inside the torque converter.

No matter if it’s in drive or neutral. the engine isn’t rotating to operate the pump in the transmission.

Tester


#15

Tester is correct.Lets say you are going 40 mph and the engine dies ( stops turning while in drive ).

The transmission pump is no longer being turned by the engine so no more pressure and the tranny neutrals out no matter what gear its in. Now things get a little different like on my 59 Thunderbird…this car has front and REAR transmission pumps. The rear pump was driven by the output shaft on the tranny.

When my fuel filter clogged the engine continued to rotate until the car dropped to about 10mph. This car can be roll started. Get it up to 20 mph turn on ignition and drop it into “L”. The rear pump creates enough pressure to engage the clutches and spins the engine. Most rear pumps were removed around 1966 as they were redundant once the engine was running and basically allowed an automatic to be push started.


#16

Modern FI engines (including my '85 Accord SEi automatic) cut the fuel off above idle with the throttle closed.
So what keeps the engine spinning when coasting in gear when there’s no fuel injected?


#17

@circuitsmith:
This has been tossed around, and I’m thinking the answer to your question is “the lockup torque converter.”

I had a 88 Dakota that you could turn the engine off, in D, and you could descend a hill and keep the AT turning for over a minute…then turn the ignition back on and drive away. I also owned a Olds Ciera that had the engine quit at highway speeds…and the engine quit spinning in 2-3 seconds. Duplicating this purposely, same result.

I’m inclined to believe the difference is the Olds was designed to unlock the TC if engine off, and the Dodge wasn’t. I think if you ovverrode the TC, and left it in lockup, the Olds would perform much like the Dodge.


#18

The car will pull the engine along through the torque converter. I think what makes the difference is that the engine is turning. When you try to push start an auto, the engine isn’t turning at all, thus the tranny pump is not running. If you put an auto in 1st gear with the engine idling and drift down a hill from a stop, the torque converter will pull the engine RPM up with the lockup disengaged.


#19

@meanjoe75fan wrote: “I’m thinking the answer to your question is ‘the lockup torque converter.’”

On my Accord the fuel cutoff still activated below converter lockup speed (~40mph); above 40mph the converter unlocked when coasting anyway (rpms drop a little when it unlocks).

I think @doubleclutch has it right.


#20

The turbine and impeller blades (vanes?) are probably slightly angled in such a way that would allow a bit more slip when the turbine is driving the impeller than when the impeller is driving the turbine, but as things speed up, the amount of slip decreases pretty quickly either way. That fluid is spinning around in there, and either member will drive the other whether the lockup is engaged or not.