Cummins ISB CM550 (1990-94) Fault Code: 2559 PID: SPN: 1845 FMI: 4 Transmission ECU to ECM Communication – Abnormal Update Rate

Circuit Description
Normally, switches, accelerators, and other components are connected to the engine control
module (ECM) directly through individual wires. Multiplexing allows those same components to
be hard wired to an original equipment manufacturer (OEM) vehicle electronic control unit
(VECU) or transmission electronic control unit (ECU) in the cab. Component values and states
from components such as sensors, accelerators, and switches can then be transmitted from
the OEM VECU to the Cummins ECM over the data link.

Messages sent from OEM VECUs or transmission ECUs are received by the Cummins ECM
and used for controlling the engine. The Cummins Inc. ECM and OEM VECU(s) must be
configured properly so that proper operation of the multiplexed components will occur.

Component Location
The data link wiring and the devices vary by OEM options.

Conditions for Running the Diagnostics
This diagnostic runs continuously when the keyswitch is in the ON position or when the engine
is running.

Conditions for Setting the Fault Codes
The ECM did not receive a message from the multiplexed device.

Action Taken When the Fault Code is Active
The ECM illuminates the amber CHECK ENGINE lamp and/or malfunction indicator lamp (MIL)
immediately when the diagnostic runs and fails.

Conditions for Clearing the Fault Code
1) To validate the repair, start the engine and let it idle for 1 minute.
2) The fault code status displayed by your diagnostic tool will change to
INACTIVE immediately after the diagnostic runs and passes.
3) The ECM will turn off the amber CHECK ENGINE lamp after the diagnostic runs and
passes.
4) The “Reset All Faults” command in your diagnostic tool can be used to clear
active and inactive faults.

Shop Talk
When this fault code is active, some multiplexed devices may not function as desired.

This fault can occur for the following reasons:

1) When the ECM is set up properly (components enabled and OEM VECU source
addressed correctly) to receive information from an OEM VECU, but the OEM VECU is
actually transmitting a message that says that component is not available for
multiplexing. This can be caused when a multiplexed component is enabled in the
Cummins ECM, but the OEM VECU source address of the VECU transmitting the
component message is incorrect in the Cummins ECM, or the OEM VECU is not set up
to transmit the multiplexed component message.
2) This fault can also be caused by a malfunctioning data link connection between the OEM
VECU and Cummins ECM, a malfunctioning connection between the component and
the OEM VECU, a malfunctioning OEM VECU, or a malfunctioning Cummins ECM. It
may be necessary to contact the OEM for the proper multiplexing configuration.

It is possible to use your diagnostic tool to monitor multiplexed components.