Does ABS Work If Emergency Brakes Come On?

Topic 26753 | Page 2

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Rick S.'s Comment
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Hey Errol, I asked this question originally because one of the teachers said the ABS would work if the emergency brakes came on but I didn't understand how the ABS could work in that situation. So I hoped someone could explain why it could or couldn't happen to me more clearly.

I do understand why it's called the tractor protection valve, just makes for some tricky questions ( stops/protects the tractor from loosing air if there is an air leak in the trailer air system eg. the emergency air line gets disconnected) I can't seem to find any right now but I know at first I was getting really confused as to what they were talking about in some questions but through asking some questions in class I got that straighten out.

Thank you for your time and support!

ABS WILL NOT WORK if the spring brakes are applied (loss of pressure). Pressure from the SUPPLY VALVE, pressurizes the diaphragm that overcomes the pressure from the SPRING. If there's not enough air pressure to OVERCOME SPRING PRESSURE - the springs will push the pads against the drum - or in the rare case of disc brakes on a trailer the pads against the disc (I haven't seen one yet, though many tractors are coming with them in the steer axle now).

Likewise - if a diaphragm on the spring brake cylinder goes bad (or the supply line to that particular cylinder goes bad) only that cylinder will activate the spring brake ****il you lose enough pressure to pop the protection valve).

Typically - ONLY ONE AXLE ON THE TRACTOR will have a spring brake.

ABS functions by PULSING THE PRESSURE to the brakes (hydraulic or air) in order to prevent (hopefully) lockups and loss of control. It (if working properly), keeps all the wheels at the same (or close to the same) speed during a hard brake application. Conversely - TRACTION CONTROL will apply brakes (or reduce power) to wheels that are spinning faster than others (all detected by sensors on each wheel for both ABS & TC).

Prior to ABS, if you panic braked, you would lock the wheels and then momentum (and GOD) takes over - and where you end up is ANYBODY'S GUESS. ABS (when it's working properly), allows you to decelerate rapidly, without locking up, and maintain a straight line when braking (which SHOULD keep the trailer straight and in line with the tractor, should you have to stand on the brakes).

So - since the spring brake operates INDEPENDENTLY of the normal air system of braking, and ABS is a function of the AIR SYSTEM of braking - if they spring brakes are applied (for whatever reason) - ABS doesn't function.


(technical explanation, from the technical advisor - LOL)


When a violation by either a driver or company is confirmed, an out-of-service order removes either the driver or the vehicle from the roadway until the violation is corrected.

Amber L.'s Comment
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Thanks Rick!! It's nice to know how things really work! That is how I thought they worked but I've always stayed away from mechanics before this so I wanted to know for sure.

A different teacher explained how the clutch has three areas and how the clutch brake works. So nice to get a picture of how things really work. Helped me shift better understanding that better for sure.

Really appreciate all the help on here!! I feel more way more prepared for what we will need to do to be successful in trucking than really any of my classmates. I keep telling them about TT but I can't make them come and read.

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