Output Diagnostic Test Mode (DTM)

Output Diagnostic Test Mode (DTM) permits checking of the function of the control elements. The control elements are also called actuators. These are components which actuate the control module when it responds to the incoming sensor signals.

The function of the "final control elements" includes all functions and properties of the final control elements under operating conditions. This means, for example:

• An electrically operated hydraulic valve which opens and closes when activated by the control module.
• The relay of an electric motor which starts the motor when activated by the control module. Engine starts and produces the required power.

Before running a Output Diagnostic Test Mode (DTM), it is advisable to make sure that there are no electrical DTCs in the system you wish to test. This will enable you to recognize any mechanical DTCs. Perform troubleshooting procedure as described in ROAD TEST  .

Are you sure there are no electrical DTCs in the system? If yes, you will recognize the DTCs in the mechanics of the individual control elements as soon as the Output Diagnostic Test Mode (DTM) deviates from its description. In this case, replace the affected component.

Output Diagnostic Test Mode (DTM) with the Bosch ABS/EDL 5.3 system permits checking of the electrically actuated hydraulic valves and the pump motor in the hydraulic control module. You can also check that the brake lines going to the four wheels are connected correctly.

When implementing the Output Diagnostic Test Mode (DTM) function, the individual test steps are shown in the second line of the V.A.G 1551 display. The following abbreviations are used: 

I = Inlet valve

O = Outlet valve

FL = front left

FR = front right

RL = rear left

RR = rear right

VBAT = Battery voltage applied to valve

0V = No voltage at valve; 0 Volt

Locked/free = Wheel condition; must be checked by 2nd technician

Hydr-P = Hydraulic pump

• Connect Scan tool V.A.G 1551 and select "Brake electronics" with address word 03. See VAG1551 SCAN TOOL (ST), CONNECTING AND ENTERING ADDRESS WORD  .
• Check the control module version (see CHECK CONTROL MODULE VERSION  ) and press the --> button.

Indicated on display:

• Press buttons -0- and -3- for "Output Diagnostic Test Mode" function

Indicated on display:

• Confirm input with -Q- button.

Indicated on display:

ABS Return Flow Pump -V39- must run. It can be heard.

When you put your foot on the brake pedal you can feel it vibrating. The brake pedal vibrates because the hydraulic pump generates pressure pulses in the brake lines. These pressure pulses induce vibrations. These are transferred to the brake pedal and the vibrations can be detected there.

The pressure pulses in the brake lines are not sufficient to lock the wheels.

Fig 5  is a schematic diagram of the hydraulic circuit of an ABS/EDL system. This will give a better understanding of the various test steps in the Output Diagnostic Test Mode (DTM). 

• The shaded components are active.
• Thin solid lines represent hydraulic lines or hoses which are not under pressure.
• Thick broken lines represent hydraulic lines or hoses working as suction lines.
• Thick solid lines represent hydraulic lines or hoses which are under pressure.
  • Press --> button.
    Fig 5: Schematic Diagram Of Hydraulic Circuit Of ABS/EDL System (Bosch 5.3)

    Courtesy of AUDI OF AMERICA, INC.

Indicated on display:

• Press --> button.

Indicated on display:

When you press the brake pedal, brake fluid pressure will be built up in all four wheel brake cylinders. All four wheels will be locked.

This presupposes proper functioning of the mechanical and hydraulic components of the brake system. This means, that the brake master cylinder is able to build up pressure in all wheel brake cylinders and that there are no leaks in the hydraulic connections and lines. If the wheels do not lock, perform a visual inspection of the brake fluid reservoir, the master cylinder, the ABS Hydraulic Unit and the wheel cylinders.

To start with, however, concentrate on the front left wheel. The function of the control elements for this wheel is tested first.