21
Perform the SERVICE Brake Differential Pressure Test on a trailer or dolly:
The Differential test only applies to vehicles equipped to tow other air brake vehicles, such as
dollies, short semi-trailers (for doubles and triples) and B-train front semi-trailers. The test
requirements are specified in FMVSS-121, Paragraph S5.3.5.
The purpose of a Differential test is to verify the control signal passed through the rear SERVICE
glad-hands of a trailer being tested, varies no more than the permitted amount from the signal
controlling the brakes on the vehicle being tested. This permitted difference depends on the
SERVICE/Control input pressure to the vehicle being tested.
5 psi to less than 20 psi
1 psi difference or less
20 psi to less than 40 psi
2 psi difference or less
40 psi or greater
5% of the input pressure or less
NOTE:
The rear SERVICE glad-hands of the vehicle under test can be used to supply air to a towed
trailer or dolly. So this test is used to verify that the air supplied at the back of a trailer under test is
sufficient enough to properly provide the needed air to a towed trailer or dolly.
This test requires attachment of the double glad-hand fitting assembly, also known as the
“Differential Kit” or “Diff Kit” (PN 18115). The Diff Kit is an option which can be purchased
separately from the 1200 Certifier.
1. On the Diff Kit,
notice the “Air Flow” direction sticker.
Connect the Service Air line from the
1200 Certifier tester to the correct Diff Kit glad-hand. The correct glad-hand is identified by being
sure the Air Flow will be flowing away from the tester and into the Diff Kit.
NOTE:
Immediately after that glad-hand is a brass nipple which has an orifice inside of it to allow Air
flow into the Diff Kit at a specific rate. The size of the FMVSS-121 specified orifice is 0.0180 inches
diameter (which is about diameter of a pin). Air must flow through this orifice before getting to any
sensors being used for this test.
2.
Connect the other glad-hand on the Diff Kit to the SERVICE glad-hand
on the FRONT of the
vehicle under test.
3.
Connect the Sensor
attached to the Diff Kit to the “Sensor Port” on the right side of the tester.
4.
Connect the glad-hand on the 50 CI canister assembly (PN 18025)
to the REAR service air
connection at the back of the trailer under test.
5.
Connect a sensor and cable assembly (PN 17200) to the quick disconnect on the 50 CI
canister assembly.
Connect the other end of this sensor and cable to any Port on the HUB.
6.
Connect the glad-hand on the EMERGENCY air line output
(the coiled red hose) from the
Certifier to the EMERGENCY glad-hand on the front of the trailer under test.
Lite-Check CERTIFIER 1200 Operations Manual
This test continues on the next page.
