36
Electrical system
Self-diagnosis
The trailer detector control unit monitors all
components' inputs and outputs during vehicle
operation and in the workshop, e.g. during a control
element test. The reversing light and rear fog light are
the exceptions, as neither of these were legally
prescribed for trailers until recently.
The fault memory can contain five faults and is not
equipped with "self-healing". This means that static
and sporadic faults remain stored until a trailer is
connected again.
A fault may change its status from "static" to
"sporadic" if a defective bulb is exchanged whilst the
trailer is connected.
Function
Based on cold monitoring of the tail lights and turn
signals, the trailer detector control unit determines
whether a trailer is connected. This is carried out via
bulb current sensing. The minimum load which has to
be connected at the control unit outputs to achieve
this is 5W per output.
The advantage of current sensing is that the bulbs can
be monitored via the self-diagnosis function. If a bulb
is defective, an entry is made in the control unit's fault
memory.
This remains there until trailer re-connection is detec-
ted. This means that, even without the trailer, a defec-
tive turn signal or rear light can be detected in the
trailer electrics in the workshop.
J345
Fault memory
Current sensing
NOK
OK
S338_116
