Civil Engineering Reference
In-Depth Information
Fig. 4.31
Example for SPN 190 trouble code (engine speed)
another ECM that the signal is not valid. If the defaulted value is stored in a freeze
frame, then the data will not help the service technician, i.e. he/she will not know at
which temperature the malfunction was detected.
4.4.5
SAE J1939-73—Diagnostics
Diagnostic communications are based on SAE J1587 and include special PGNs
called DMs. They can be considered as equal to diagnostic services in ISO 15031-5
or ISO 14229-1. Most of the DMs are sent on request. The main exception is DM1
which is used as communication message to the instrument cluster in order to il-
luminate warning lamps. If the ECU does not support the request DM, then it will
respond with a NACK. The DTC contains the 19-bit SPN with a 5-bit failure mode
indicator (FMI) (Fig.
4.31
).
SPN is the suspect parameter number of the failed component. FMI is the fail-
ure mode indicator (comparable to failure type byte of ISO 14229-1). The CM is a
conversion method, since there has been at least four different methods on how to
represent the DTCs. At the moment there is just one method, #4 Intel format with 19
bit SPN + 5 bit FMI and CM0, which is allowed for legislated OBD communication.
The other versions shall not be used. Finally, OC is an occurrence counter, which
counts the number of times the fault has been gone from an InActive (previously
Active) state to an Active state.
This method of using the signal as the part of the DTC is useful for simple diag-
nostic function, e.g. if an ambient air temperature sensor detects an electrical fault,
then it will be clear for the service technician on which signal he should check or
read. As said, this will work fine as long as there is a one-to-one relationship, i.e.
the monitor is based on one signal/sensor. OBD II usually requires more advanced
functions, e.g. the catalyst monitor for a gasoline engine maybe utilize the intake
manifold airflow sensor and the rear oxygen sensor plus some other sensor values
to check if the conversion ratio in the catalyst is OK. This is a multiple sensors to
one DTC scenario and it is not possible to read a single sensor anymore and the SPN
is more “fictive”. The detection mechanism of a fault is usually called a monitor.
