Fayat Bomag BW 900-50 Service Manual page 204

Measuring fault
Measuring sensors and actuators
Troubleshooting – Troubleshooting, electrical systems
Whenever the electrical system of the vehicle behaves in an unex-
pected way, one should first consider that there may be a ground
problem! The older the electrical system and the lower the voltage,
the more likely this will be the cause.
As we all know, being able to utilize electric current requires the
existence of a closed electric circuit, the electrons leaving the bat-
tery must be able to return to the battery. In vehicles numerous
existing metal parts can be used for this purpose. The negative
pole of the battery is connected with the metal of the frame. The
current then flows through an insulated cable to e.g. a lamp should
light, and instead of a return line, the socket of the lamp is also, in
some way, connected to the frame, through which the current can
flow back to the battery. However, should the connection between
lamp and frame corrode causing a high electrical resistance at that
point, the current will try to find a different way. If no other way can
be found, the component will simply not work, even if the meter
shows that voltage arrives. Very often the current finds a different
way back to the battery, sometimes through other consumers.
Sensor signals are best checked where they are
needed, i.e. on the control unit. If the correct signal
is received you can be sure that not only the
sensor, but also the wiring to the control unit is in
good working order.
It is a common procedure to pick up the signals
with a break-out-box, the Y-cable of which is fas-
tened between control unit and control unit plug. If
no break-out-box is available, the measurement
takes place directly on the sensor, or one must try
to gain access to the back of the plug.
Trouble shooting procedure:
n First check the corresponding actuator, in case of a faulty
signal the output signal must be checked directly on the control
unit.
n If the output signal is correct, check the wiring to the actuator.
n In case of an incorrect output signal, check the corresponding
input signals after.
n In case of incorrect input signals check the signal on the
sensor itself.
If the sensor signal is correct, check the wiring to the control
n
unit for continuity and against ground (short circuit).
n If the sensor delivers a correct signal, the sensor itself may be
the cause of the problem, or the sensor may be affected by
other incorrectly working components.
n However, the voltage supply and the ground connection of the
control unit as well as the power supply for sensors and actua-
tors must also be checked. Because an incorrect voltage value
can distort the input and output signals.
n If the action steps described above do not deliver a result, the
periphery is OK and the fault must be in the control unit, but be
cautious, most faults are caused by poor contacts in the plug
connections.
BW 900-50 209