As part of your CDL air brake inspection, you must test the tractor protection valve & system.
How to CDL Airbrake Inspection | Tractor Protection System
Updated Jan 2021
Hi there smart drivers, Rick with Smart Drive Test talking to you today about how to test the tractor protection system.
Any truck designed to pull a trailer with air brakes is going to be equipped with tractor protection system, which consists of the tractor protection valve--just to confuse you [CHCUCKLING] the system and the valve are called the same thing - the tractor protection valve and the red, eight-sided trailer air supply valve on the dash.
Many people think the trailer air supply valve on the dash controls the parking brakes on the trailer, it doesn't.
That's just a subsequent action.
Today we're going to talk to you about how to test the tractor protection system on the truck and trailer.
We’ll be right back with that information.
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Hi there smart drivers, welcome back.
Rick with Smart Drive Test talking to you today about how to test the tractor protection system on a truck that's designed to pull the trailer equipped with air brakes.
All trucks pulling trailers with air brakes will have a tractor protection system.
As I said in the introduction, the tractor protection system consists of the tractor protection valve and the trailer air supply valve.
So, the first test--there's three test for testing the tractor protection system:
1) the first one is to pump down the air pressure in the system between 20 and 45psi (pounds per square inch) the trailer air supply will pop out, shutting air off to the trailer.
2) the second test is to go out to the trailer to ensure that the parking brakes have indeed applied.
3) If the trailer brakes have applied you come up to the front of the trailer and pull the glad hand off the trailer.
You come into the cab of the truck and make a service brake application.
There shouldn't be any air leaking out through the tractor protection valve.
And that is the third test.
And when the tractor protection system passes those three tests, then you know that the tractor protection system is working on the truck.
The first one here is to pump down between 20 and 45psi to ensure that the trailer air supply shuts off air to the trailer.
Now as I said in the introduction, the trailer air supply does not activate the parking brakes on the trailer - it's a subsequent action.
The Trailer Air Supply Valve
The trailer air supply valve turns air on and off to the trailer – it’s nothing more than an on/off switch.
Think of the trailer air supply like a light switch in your house.
The light switch in your house controls the flow of electricity - it doesn't turn the light off and on.
The light switch controls electricity and subsequently it turns the light off and on.
So if you have a light switch in your house that turns the garage light on and off, you turn the light switch off to turn the electricity off.
And to know that the light in the garage actually went out, you have to go to the garage and check the light went out.
The same thing with the trailer air supply valve that shuts air on and off to the trailer.
You have to go out to the trailer to ensure that the trailer brakes applied, which is the same thing as the light in the garage - you have to go to the light to make sure it actually went out.
Testing that the Trailer Air Supply Shuts Off Air
And so what we're going to do, we're going to pump down between 20 and 45psi to ensure the trailer supply valve pops out.
The low-air warning has activated and the trailer air supply has shut air off to the trailer between 20 and 45psi.
Now we're going to get out of the truck and go back to the trailer and ensure that the trailer brakes have applied.
Testing that the Trailer Brakes Are Applied
So you do is you come back here to check that the trailer brakes have applied.
You simply look under the trailer and see that the slack adjuster and pushrod are making a 90° degree angle, therefore you know the brakes are applied.
It’s an approximate 90° angle.
If the brakes are installed correctly, when applied the pushrod and slack adjuster form an approximate 90° angle.
If it’s not a 90° angle when the brakes are applied, the brakes may not be set up correctly.
There have been some people out there that have said it’s applied and that sort of thing…
When applied the pushrod & slack adjuster make an approximate 90° degrees because it isn't 90° degrees it's not good physics.
And it's just a way the brakes are set up.
So you know that the brakes are applied because the pushrod and slack adjuster form an approximate 90° angle.
Testing the Trailer Air Supply Valve
Now we’re going to go up to the front of the trailer, and we’re going to disconnect the gladhands.
You just remove the gladhands – you don’t have to take the electrical line off.
Just lay the gladhands down here on the deck like that.
Open the door and make a service brake application--you don't have to get in the truck--just with your hands push down on the brake pedal.
There isn’t any air leaking out of the gladhands, therefore I know the tractor protection valve is working properly.
All 3 Tests Indicate...
All three tests indicate that the tractor protection system is working.
The trailer supply valve on the dash shut off air to the trailer between 20 and 45.
The trailer brakes applied and I know that because the pushrod and slack adjuster make a 90° degree angle.
I took the gladhands off and made a service brake application.
There is no air escaping from the tractor protection valve and after I do that I reattach the two lines.
And most of the time, to test the tractor protection system, you're going to be doing this as part of all of your overall in-cab checks.
So you get back in the truck and do your compressor test, maximum build-up, and at maximum pressure you’ll do your leak test.
But this is just tractor protection system that you're testing.
Quick review of testing the tractor protection system.
All trucks designed to pull trailers equipped with air brakes are going to have a tractor protection system.
The first thing you do is chock the wheels on the truck, release the parking brakes, and pump the system air pressure down between 20and 45psi.
The trailer air supply should shut air off to the trailer.
And as I said, it doesn't apply the parking brake, the trailer air supply supplies air to the trailer(s).
It’s just like a light switch in your house that turns the electricity off and on.
The trailer air supply shuts air off and on to the trailer.
After it's shuts air off to the trailer between 20 and 45psi you go out to the trailer and make sure that the trailer brakes have applied.
And you know that because the slack adjusters and pushrod form an approximate 90° degree angle.
You then come up to the front of the trailer and take the gladhands off, and make a service brake application.
If there isn't any air leaking out of the gladhands there, you know the tractor protection valve is working.
And those are the three tests.
And after doing those three tests, you say to the examiner, “all three tests indicate that the tractor protection system is working.”
Question for my smart drivers:
For those CDL drivers doing trailers fitted with air brakes, do you have to test the tractor protection system as part of your license test?
Leave a comment down in the comment section there – all of that helps us out.
I'm Rick with Smart Drive Test.
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Thanks again for watching.
Good luck on your road test.
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Have a great day.
[BLOOPER – STANDING AND SMILING] so just just quick review… testing tractor protection system…tru…