cjflynnnh

when I hit brakes car brakes thump. felt like abs in ice and snow

iolmaster

iTrader: (1)

Does the steering wheel shake also? It may be rotors that need to be resurfaced.

NateJG

Is this a recent instantaneous condition; or did it develop over hundreds or thousands of miles?

Absent something that has recently "broken", thumping or pulsating felt while braking is frequently due to dimensional abnormalities (thickness inconsistencies or "hot spots" in the metal of the brake rotor (or drum - in other/older vehicles).

Hot spots can occur - contrary to what one might think - during LIGHT brake applications -- where high-spots on the rotor (or drum) share a high share of the load of braking -- thereby overheating -- and developing hard spots due to high-temperature changed in the chemistry of the cast iron or steel from which they're made.

Hot spots will not normally "wear down" to match the surrounding material - and the best way to eliminate them is by grinding (as opposed to lathe-turning).

If/Once hot-spots (hard spots) form on the rotor/drum, the torque of braking changes as the pads (shoes) rub past the high/hard/"hot" spot. You'd feel that in the seat of your pants and/or the shake of the steering wheel.

Again - contrary to common sense - light applications of braking can lead to exceptionally localized heating on as little as one of the brake positions on the vehicle.

To prevent such an occurrence, a firm application of brake (likened to stopping for a yellow light on a 45 MPH highway) will ensure that an even pressure is applied to the brakes of each wheel - further ensuring that each brake does it's share - and each rotor/drum is subjected to a firm, even amount of friction.

Burning off a layer of "crud" on the braking surface is a topic for another discussion; but here's one simple thing to remember that can often "repair" slightly-pulsating brakes -- or, better yet, prevent the occurrence:

When coming to a stop after a hard application (like from a downhill drag or stop, or while heavily loaded, or merely that red light that you didn't notice until the last minute), simply roll forward 6" and stop, roll another 6" and stop, roll another 6" and stop -- while waiting for the light to change. And especially after driving the horse hard before putting it into the barn, allow the brakes to cool WHILE ROLLING so as to avoid letting the car sit with hot brakes. The pads (or shoes) against the hot rotor(drum) will not allow the rotor(drum) to cool at the same rate - and can set up the situation for dimensional differences around the circumference of the rotor(drum) and lead to hot-spotting (described at the start of this narrative).

So, that's a long way around the barn to agree with iolmaster that the rotors might simply need to be resurfaced -- with my 'value add' to help you understand the underlying causes and to avoid the situation from recurring.

Happy Labor Day!

NateJG

ukrkoz

brakes are by wire. No direct connection between pedal and actual brakes. hence, something tells ECM to "slam on brakes" for you. That can be anything from faulty brake pedal sensor>pedal feel simulator>brake actuator>any given ABS sensor and anything inbetween.
What you need to do is to find ABS CAN BUS capable scanner and scan system. Regular scanner will not show ABS codes. That's starting point. Or, find someone who has Techstream and scan with that.