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6/30/22, 2:16 PM
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US10 Emission Engines - Diagnostic Trouble Code
( DTC ) SPN 1231 FMI 9 Troubleshooting
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SPN 1231 FMI 9 is a code indicating a disturbance on the J1939-7 datalink
which enables communication between the engine and aftertreatment
control modules, sensors, and components. There are numerous causes
for this code, but with basic fault tracing and and a logical path of
elimination, the issue can be resolved consistently. Components on
J1939-7 are as follows:
Engine Management System (EMS) MID 128
Aftertreatment Control Module (ACM) MID 233
Transmission Electronic Control Unit (TECU) MID 130 (If I-Shift
equipped)
Smart Remote Actuator (SRA, commonly but erroneously referred
to as the VGT)
Inlet and outlet Nox sensors
Further information and pinouts of the circuit can be found HERE.
The circuit also has two terminating resistors installed. One is internal in
the EMS, the other is plugged into a dedicated connector located near
the Nox sensors. These 120 ohm resistors give the datalink 60 ohms of
resistance.
The datalink can be checked with the breakout harness installed at the
ACM, EMS, or Engine Interface (EI) connector. Basic fault tracing values
are as follows:
Verify the following:
Remove, clean, and inspect the ring terminals on the ground stud
in the frame rail. Make sure the terminals are not damaged, free of
; Live UI corrosion, and securely crimped to the cable.
Remove, clean, and inspect the ring terminal on the positive
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Remove, clean, and inspect the ring terminal on the positive
battery post feeding power to the ACM. Make sure the terminal is
not damaged, free of corrosion, and is securely crimped to the
cable.
Inspect the inline fuse connector and fuse in the battery positive
feed to the ACM. Ensure the fuse is not blown, terminals secure
and not corroded, and that power is consitent during a wiggle test.
Verify battery voltage and load test
Verify proper voltage and ground to all components on J1939-7.
Ensure no high resistance on the ground circuits.
Inspect the ACM connector for corrosion and proper pin tension
on power, ground, and datalink terminals.
Ensure no signs of DEF contamination in the DEF pump, chassis
harness, or ACM connectors.
Examples of failed testing and possible causes:
Very low resistance between J1939-7 high and low, i.e. 0.5 ohms:
Datalink wires shorted together. Perform pin to pin testing to
isolate the short. EI may be disconnected to isolate short to engine
or chassis side.
Very high resistance between high and low: Missing terminating
resistor, faulty module, damaged/corroded wire. Ensure 120 ohms
on EMS directly, ensure the terminating resistor in place in the
chassis harness and functioning correctly. Disconnect each
component one at a time on J1939-7 and recheck resistance after
each one. If no change, wiring must be load tested from pin to pin.
Very high voltage on high or low side: Likely shorted to power, also
possible faulty module.
Very low voltage on high or low side: Likely shorted to ground, also
possible faulty module.
Continuity to ground low i.e. 2 ohms: Likely module shorting to
ground.
Continuity to ground high i.e. 30k ohms or OL: Poor or open
chassis/engine ground. EI may be disconnected and rechecked to
isolate issue to chassis or engine.
There is also a possibility of a bad module bringing the link down
with no apparent faults. Disconnect each module one at a time
and refresh TechTool and verify 1231 inactive.
; Live UI
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mid 128 ems
spn 1231
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