Driving down a muddy field this morning, I locked the wheels (no hill descent). After releasing the brakes there was a whirring like an electric motor which ended after a second or so. It was replicated every time.

I’ve never noticed it before but the only other time I’ve locked the wheels there was tyre-squeals and my heartbeat / swearing to drown it out.

Normal? Something to be investigated?

Thanks

Diesel?

Yep.

I could be wrong, but since diesels don’t generate a vacuum when running like petrols do they need an additional pump to create a vacuum reservoir. I thought I read somewhere that this could be electric – which might be on yours. What car is it?

Or it could be (perhaps more likely) a mechanically driven pump with a clutch that’s cutting in?

Isuzu D-Max pick up. 3.0L TDI. Auto.

I was driving with the button pressed which starts the truck in 3rd (not first).

From the general tone of your reply it’s nothing to worry about. Correct? I know little to nothing about mechanical issues!

Totally normal. The ABS system has a large electric motor to drive the pump that has to return the released fluid back into the high pressure side of the system. Without that, everytime it released a bit of fluid to “let off the brakes” the brake pedal would sink a bit towards the floor. After a few secs you would run out of brake travel and you’d not be able to re-apply the brakes. The characteristic “pulsing” you feel on the pedal as ABS operates is caused by the pressure fluctuations from the return pump pushing the pedal back up a bit.

You can see the big motor on the ABS modulator in this pic:

it’s the black cylinder sticking out from the LHS of the valve block.

Thanks Max.

Better safe than sorry 🙂