Self-Diagnosis System - Subaru XT 1988 Service Manual

FUEL INJECTION SYSTEM 2-7
Self-diagnosis System
General
The self-diagnosis system detects and indicates a fault in
various inputs and outputs of the complex electronic con-
trol. The warning lamp (CHECK ENGINE light) on the instru-
ment panel indicates occurrence of a fault or trouble, and also
the light emitting diode (LED) in the control unit indicates
a trouble code.
Further, against such a failure of sensors as may disable the
drive, the fail-safe function is provided to ensure the minimal
driveability.
Fig. 51
SELF-DIAGNOSIS FUNCTION
The MPF I control unit executes the computational pro-
cessing on the input information received from various sensors
and produces the output information for driving the fuel
injector, fuel pump, etc.
Along with this computational processing, it reads out all the
input/output information to examine matching with the pre-
determined levels (proper values or ranges). If a predetermined
level is not satisfied, i.e., a fault is found, the warning lamp is
signaled to a driver. In this fashion, the self·diagnosis function
is performed.
FAIL·SAFE FUNCTION
For the part which has been judged faulty in the self-diagnosis,
the MPFI control unit generates the associated pseudo signal
(only when convertible to electric signal) and carries out the
computational processing. In this fashion, the fail-safe
function is performed.
Function of Self-diagnosis
The self-diagnosis function has four modes: U-check mode,
Read memory mode, D-check mode and Clear memory mode.
Two connectors (Read memory and Test mode) and two
lamps (CHECK ENGINE light and O
2
monitor) are used. The
connectors are for mode selection and the lamps monitor the
type of problem.
RELATIONSHIP BETWEEN MODES AND CONNECTORS
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Mode Engine Read memory connector Test mode connector
U-check Ignition ON DISCONNECT DISCONNECT
Read memory Ignition ON CONNECT DISCONNECT
D-check Ignition ON DISCONNECT
CONNECT
Clear memory Ignition ON (engine on)
CONNECT
CONNECT
U·CHECK MODE
The U-check is a user·oriented mode in which only the MPFI
components necessary for start-up and drive are diagnosed. On
occurrence of a fault, the warning lamp (CHECK ENGI N E
light) is lighted to indicate to the user that the dealer's inspec·
tion is necessary. The diagnosis of other parts which do not
give significant adverse effect to start-up and drive are ex-
cluded from this mode in order to avoid unnecessary un-
easiness to be taken by the user.
READ MEMORY MODE
This mode is used by the dealer to read past problems (even
33
when the vehicle's monitor lamps are off). It is most effective
in detecting poor contact or loose connections of connectors,
harnesses, etc.
D·CHECK MODE
This mode is used by the dealer to check the entire MPF I
system and detect faulty parts.
CLEAR MEMORY MODE
This mode is used by the dealer to clear the trouble code from
the memory after the affected part is repaired.