The Pressure Differential Test, also known as the Control Signal Pressure Differential Test, is used to measure the pressure at the control signal line at both the front and the rear of the vehicle.  A PD-730 is used to limit the pressure flow.

This test is very similar to the Air Balance Test, but is for measuring the Control Signal only on converter dollies and trailers made to tow another vehicle.
See Mode, Test and Vehicle

Specific Equipment Connection

A PD-730 (supplied with the Mini-Tractor) must be connected between the air source and the control signal line on the head of the vehicle.  The PD-730 contains an orifice designed to limit the pressure flow in accordance with test standards.  The pressure sensor connected to the PD-730 must be Sensor #1.  Sensor #2 must be connected to the 50 cu. inch tank (PD-710) installed at the rear of the vehicle.  Insure that both sensors 1 and 2 are shown on the main display of the computer.

Running the test

When the pressure at Sensor #1 reaches one of the pre-defined measurement points, the time since the air pressure was first applied is recorded at that pressure.  At the same time, the pressure is recorded at Sensor #2 (the rear of the vehicle).

If conducting a certification test, the left side of the data panel shows the requirements for the pressure differentials that may exist at different pressure points based on the type of vehicle selected.  The reference pressure points are those of Sensor #1.  A PASS or FAIL indicator is shown in the data panel above  the Apply or Release Requirements.

When running in diagnostic mode, the on-screen control panel does not display the pressure requirements or a PASS or FAIL indication.

Using the mouse, the operator can left click on either the Application or Release selection buttons to select which set of pressures to display.

The test is started by pushing the red mushroom button on the Mini-Tractor or by the movement of the accelerometer when not using the Mini-Tractor.

Example

 

Figure 26 Pressure Differential

 

Along the left-hand side of the data display are the timing measurements made for the reference sensor (Sensor #1) when it reaches the pre-determined pressure points during the test (shown in the INPUT column).

Sensor #2 pressure measurements are displayed in the OUTPUT column.  The difference between the input and output pressure is displayed in the next column.  In the example, when Sensor #1 reach 45.0 psi (0.29 seconds after the test started), Sensor #2 has recorded 44.5 psi, or a difference of 0.5 psi.  Since the reference pressure (Sensor #1) is greater than 40 psi, the allowed difference is 5%, or 2 psi.  Since the difference of 0.5 psi is less than 2 psi, the test passes.