How does engine braking work on an auto?

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Wed 25 Nov 2009 10:38
How does engine braking work on an auto? - Pondlife
On an auto, if you lift off the accelerator and coast then the revs drop down. So the torque converter allows the wheels to turn faster than the engine. But if I select a low gear when going downhill, I get engine braking, so the TC must be forcing the engine to turn with the wheels just like a manual would. I've seen this behaviour on all autos I can recall, so it seems pretty standard.

So what is happening? Does selecting a lower gear engage the TC lock-up clutch, or is there something else going on?
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Wed 25 Nov 2009 11:04
How does engine braking work on an auto? - Number_Cruncher
It's not the torque convertor.

When you're in drive, in some ratios there are overrun clutches in the gearbox which allow this freewheeling behaviour because it leads to a more relaxed jerk free drive.

When you select a low gear hold, an extra clutch is activated in the gearbox which locks the overrun clutch solid, removing the freewheel effect, giving better downhill speed control.

There are rare faults where the overrun clutch fails completely, and these faults can be diagnosed by having no drive in that gear while in D, but, having drive in that ratio while in the low gear hold selector position.

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