Installation Manual

Electrical & Electronic Application And Installation Manual
Production Release Version 1.0
Page 45
Suppression resistors are a low-cost alternative and can be less stressful on
relay coils. Resistor selection should be determined by the voltage applied
across the coil, the resistance of the coil, maximum power dissipation allowed,
and the level of transient voltage to be tolerated. For example, if the coil is 50
Ohms and the voltage applied is 24V, an 82 Ohm suppression resistor would
allow the transient voltage to reach –39.6V (V = IR = -.48A x 82 Ohms, the
voltage is shown as negative since the polarity reverses once the switch
opens). Using a 330 Ohm resistor would allow a –158V transient but the
current would diminish in the loop faster. For power dissipation concerns, the
resistors power rating should be considered. For example, if 24V were
applied across a 330Ohm resistor, the power dissipated by the resistor would
be 1.75W (P = V
2
/R). Therefore a 330 Ohm resistor should be selected with
the capability of dissipating at least 2W. The heat generated by the resistor
should be considered when selecting a resistor.
Note: If the resistance of the suppression resistor is too low, the driver
circuitry in the ECM may be loaded to a point where the relay or solenoid
does not function properly. If the resistance is too high, the transient
voltage may reach undesirable levels.
There are other techniques that can be used for transient suppression.
Snubbers, Zener diodes, and varistors are all methods that have
characteristics that make them better suited for some applications. But for
simple applications of relays or solenoids, diodes or resistors should suffice.
Inductive devices such as relays or solenoids should be located as far as
possible from the components of the electronic control system. Wiring
harnesses that are installed by the customer should be routed as far as
possible from the wiring harness of the electronic control system in order to
avoid problems that are associated with electrical noise.
4.7 Direct Battery connection requirements
Some engine and aftertreatment components require direct fused battery
connection to ensure they are allowed to go through a power down sequence
after the engine ignition has been set to OFF. A list of the components
requiring direct battery connection are shown below.
• Engine ECM
• Aftertreatment DCU
• Engine Fuel Lift Pump (via relay for 4 cyl only)
The electrical power connection to these components must not be switched
other than by a machine battery disconnect or emergency shutdown switch.
Failure to meet this requirement may lead to engine and aftertreatment
diagnostic code, shutdown procedure failures for example DEF system purge,
Component failure and false tripping of engine diagnostics. The engine ECM
will record and store engine shutdown events where the post ignition OFF
process has been unable to run.