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Output Diagnostic Test Mode (DTM) (Scan Tool Function 03)



Output Diagnostic Test Mode (DTM) (scan tool function 03)
The output Diagnostic Test Mode (DTM) enables you to check the function of the components that are actuated by the ABS control module after it processes incoming signals from the sensors.
These checks include all functions and properties under operating conditions, for example:
- An electrically-operated hydraulic valve that opens and closes when activated by the ABS control module
- The relay of an electric motor that starts the motor when activated by the ABS control module
Before initiating the output DTM, make sure that there are no electrical malfunctions in the system you wish to test. This will enable you to recognize any mechanical malfunctions. Check Diagnostic Trouble Code (DTC) memory before starting the output DTM. Checking DTC Memory (Scan Tools Function 02)
If you have made sure that there are no electrical malfunctions in the system and the results of the output DTM is not as specified, you will be able to identify any mechanical malfunctions in the individual actuators.
With the Bosch 5.3 ABS/EDL system the output Diagnostic Test Mode (DTM) enables you to check 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 calipers are connected correctly.

Notes:
- The vehicle must be raised until all wheels are free to turn. A second technician is required to check whether the wheels are locked or can be turned.
- You can stop the test sequence at any time by pressing the -C- button. To start the output Diagnostic Test Mode (DTM) again, you must first interrupt the data link using "End Output" function 06, and then restart the sequence. End Output
- To avoid overloading the components being checked, they are only actuated during the test sequence for a period of 60 (or 90) seconds. The output diagnostic test will be terminated if the -> button is not pressed within this period.
- The ABS warning light and the red brake warning symbol blink during the test.
- If the brake pedal is pressed repeatedly, this will exhaust the vacuum in the brake booster. For this reason, the brake pedal must be pressed harder in order to achieve the same braking effect. It may be advisable to start the engine briefly to build up vacuum in the booster.

During the test, the individual test steps are shown in the second line of the VAG1551 scan tool 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 at valve
OV = 0 Volts; No voltage at valve
Locked/free =Wheel condition; must be checked by 2nd technician
Hydr - P = Hydraulic pump

- Connect VAG1551 and insert "Brake Electronics" address word 03. Safety Precautions and General Troubleshooting Information
- Read control module version on scan tool display.
- Press -> button.





Indicated on display
- Press buttons -0- and -3- to select "Output Diagnostic Test Mode" function 03.





Indicated on display
- Press -Q- button to confirm input.





Indicated on display
ABS return flow pump N39- should start running audibly.





When you put your foot on the brake pedal you can feel it vibrating. This is because the hydraulic pump is producing pressure pulses in the brake lines, which induce the vibrations that can be felt at the pedal.
The pressure pulses in the brake lines are not sufficient to lock the wheels.
Illustration A01-0015 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 DTM.
- The grey-shaded components are active.
- Thin solid lines represent hydraulic lines or hoses that are not under pressure.
- Thick broken lines represent hydraulic lines or hoses working as suction lines.
- Thick solid lines represent hydraulic lines or hoses that are under pressure.
- Press -> button.





Indicated on display
- Press -> button.





Indicated on display





When you press the brake pedal, brake fluid pressure will be built up in in all four calipers. All four wheels will be locked.
This indicates that the mechanical and hydraulic components of the brake system are working properly, which means that the master cylinder is able to build up pressure in all four wheel brake cylinders and there are no leaks in the hydraulic lines, hoses and connections. If the wheels do not lock:
- Visually inspect brake fluid reservoir, master cylinder, hydraulic unit and calipers.
Concentrate on the left-front wheel first. The function of the control elements for this wheel is tested first.

Note: This test step in the output diagnostic test mode corresponds to the pressure build-up phase during ABS-controlled braking.

- Press -> button.





Indicated on display
- Keep pressing brake pedal.





The inlet valve is activated, which interrupts the brake line. However, the fluid pressure is still maintained in the calipers. The left-front wheel remains locked.

Note: This test step in the output Diagnostic Test Mode (DTM) corresponds to the "maintain pressure" phase during ABS-controlled braking.

- Press -> button.





Indicated on display
- Keep pressing brake pedal.





The inlet valve and the outlet valve for the left-front wheel are activated. The hydraulic pump starts running and reduces the fluid pressure in the caliper via the open outlet valve.
- The brake pedal must not give, and should press back harder against your foot.
- You should now be able to turn the left-front wheel.

If you cannot turn the wheel:
- Check whether brake line for left-front wheel is correctly connected.

If yes:
- This indicates a mechanical malfunction in one of the valves, providing there is no electrical malfunction in the system:
- Replace ABS hydraulic control unit.

If the brake pedal gives under your foot:
This means that the inlet valve or the non-return valve connected in parallel with the inlet valve is leaking:
- Replace ABS hydraulic control unit.

Note: This test step in the output diagnostic test mode corresponds to the "pressure reduction" phase during ABS-controlled braking.

- Press -> button.





Indicated on display
The ABS return flow pump N39- should stop running.





- Keep pressing brake pedal

Only the inlet valve is now activated, and this interrupts the brake line. If the inlet valve is not leaking you should be able to turn the front left wheel.
If you cannot turn the wheel:
- This means that the inlet valve or the non-return valve connected in parallel with the inlet valve is leaking.
- Replace ABS hydraulic control unit.
- Press -> button.





Indicated on display
- Keep pressing brake pedal.





The inlet valve is no longer activated and does not interrupt the brake line. The brake pedal will give perceptibly under your foot. Fluid pressure is built up again in the left-front wheel cylinder. The wheel will be locked.
- Press -> button.





Indicated on display
- Press -> button.
Continue the output diagnostic test mode in order to check the right-front, left-rear and right-rear wheels in turn. The test steps are the same as for the left-front wheel, so they are not described in detail.





Indicated on display
- Press -> button.





Indicated on display
- Press -> button.





Indicated on display
- Press -> button.
- ABS return flow pump -V39- should start running.
- Brake pedal must not give.





Indicated on display
- Press -> button.
ABS return flow pump N39- will stop running.





Indicated on display
- Press -> button.
Brake pedal should give perceptibly.





Indicated on display
- Press -> button.





Indicated on display
- Press -> button.





Indicated on display
- Press -> button.





Indicated on display
- Press -> button.





Indicated on display
- Press -> button.
- ABS return flow pump N39- should start running.
- Brake pedal must not give.





Indicated on display
- Press -> button.
ABS return flow pump -V39- will stop running.





Indicated on display
- Press -> button.
Brake pedal should give perceptibly.





Indicated on display
- Press -> button.





Indicated on display
- Press -> button.





Indicated on display
- Press -> button.





Indicated on display
- Press -> button.





Indicated on display
- Press -> button.
- ABS return flow pump -V39- should start running.
- Brake pedal must not give.





Indicated on display
- Press -> button.
ABS return flow pump -V39- will stop running.





Indicated on display
- Press -> button.
Brake pedal should give perceptibly.





Indicated on display
- Press -> button.





Indicated on display
- Press -> button.

m.y. 1999 > only:





Indicated on display

Note: On vehicles before 1999 model year the hydraulic pump is not activated).





The EDL change-over valve (left) and the EDL inlet valve are activated. ABS return flow pump N39- runs for one second. The pump draws brake fluid out of the fluid reservoir via the EDL inlet valve, and builds up pressure in the EDL-controlled wheel cylinders. These wheels will then be locked.
If not:
- This indicates that there is a mechanical malfunction in one of the valves, provided that there are no electrical malfunctions in the system.
- Replace ABS hydraulic control unit.
- Press-> button.





Indicated on display
The output diagnostic test mode is completed. The ABS warning light and the red warning symbol for brake system go out.
- Press -4 button.





Indicated on display

Note: If the ABS warning light does not go out, this indicates a malfunction in the system. Follow the specified test sequence exactly. Refer to Warning lamp functions.

- Press buttons -0- and -6- to select "End Output" function 06.
- Press -Q- button to confirm input.


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