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4-18
4.19 MICROPROCESSOR
4.19.1
Service guidelines
NOTE
The erasable, programmable, read only memory
(EPROM) chip (component U3 on the microprocessor
logic board) has a window on it which is covered with
a label listing the revision level of the software. The
window is used to erase the chip’s memory with the
use of ultraviolet light. the label prevents light from
entering the chip and erasing the memory. Under NO
circumstances should this label be removed.
WARNING
Under no circumstances should a
technician electrically probe the
processor at any point, other than the
connector
terminals
where
the
harness
attaches. Microprocessor
components operate at different
voltage levels and at extremely low
current levels. Improper use of
voltmeters, jumper wires, continuity
testers,
etc.
could
permanently
damage the processor.
As mentioned above, some microprocessor inputs
operate at voltage levels other than the conventional 12
vdc. Connector points and the associated approximate
voltage levels are listed below for reference only. Under
no circumstances should 12 vdc be applied at these
connection points.
Grounded wrist cuffs are available at most radio,
computer and electronic supply stores. It is
recommended that these be worn whenever handling a
microprocessor.
Table 4-2 Connection Point Voltage
Connection point
Approximate voltage
CDT, RAS, SAS, WTS
2.5 vdc (variable)
MPF1
5.0
vdc
WARNING
Most electronic components are
susceptible to damage caused by
electrical static discharge (ESD). In
certain cases, the human body can
have enough static electricity to cause
resultant damage to the components
by touch. This is especially true of the
integrated circuits found on the
truck/trailer microprocessor.
Although there is less danger of electrical static
discharge ESD damage in the outdoor environment,
where the processor is likely to be handled, proper
board handling techniques should always be stressed.
Boards should always be handled by their edges, in
much the same way one would handle a photograph.
This not only precludes the possibility of ESD damage,
but also lowers the possibility of physical damage to the
electronic components. Although the microprocessor
boards are fairly rugged when assembled, they are
more fragile when separated and should always be
handled carefully.
During emergency situations, the test board may be
used to keep a unit running and prevent a critical load
from spoiling. Since the microprocessor is totally
disconnected from the unit, it cannot monitor the
engine’s safety switches for oil pressure and coolant
temperature.
Since the engine is running unprotected
when the test board is used,
it is imperative that should
a problem develop with the microprocessor, it be
replaced immediately.
The test board is intended to be
a trouble-shooting tool only.
When using the test board to troubleshoot, the unit
should be started in low speed, unloaded cool in the
same way as the processor would start the unit.
Good
judgment should also be usedwhen cycling any unitwith
the test board. Rapid cycling should be avoided.
When welding is required on theunit frame, or on the
front area of the trailer, ALL wiring to the microprocessor
MUST be disconnected. When welding is performed on
other areas of the trailer, the welder ground connection
MUST be in close proximity to the area being welded.
It is also a good practice to remove both battery cables
before welding on either the unit frame or the truck to
prevent possible damage to other components such as
the alternator and voltage regulator.