Chrysler RAM TRUCK 2500 1994 Service Manual page 334

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ELECTRICAL 8A - 19
USING ON-BOARD DIAGNOSTIC S Y S T E M
OPERATION OF ON-BOARD DIAGNOSTIC
(OBD) SYSTEM
The powertrain control module (PCM) monitors
critical input and output circuits of the charging sys­
tem making sure they are OK. Some are checked
continuously and some are only checked under cer­
tain conditions.
I f the OBD system senses that one critical circuit is
bad, i t will consider this a real problem and put a
DTC into memory. Each input and output circuit
monitored by the OBD system has its own DTC. The
DTC will stay in memory as long as the circuit con­
tinues to be bad. I f the problem does not happen
again after the code is put into memory, the PCM
will clear the memory after 50 engine starts.
DIAGNOSTIC TROUBLE CODES
Diagnostic trouble codes (DTC) are 2 digit numbers
flashed on malfunction indicator (Check Engine)
lamp that identify which circuit is bad. I n most
cases, they do not identify which component i n a cir­
cuit is bad. A DTC description can be read using the
DRB scan tool. Refer to Group 14 - Fuel Systems for
more information. Therefore, a DTC is only a symp­
tom, not necessarily the cause for the problem. I n
some cases, as a result of the design of the driveabil-
ity test procedure, a DTC can be the reason for the
problem. I t is important that the test procedure be
followed, i n order to understand what caused the
DTC of the on-board diagnostic system to be set.
HOW TO USE MALFUNCTION INDICATOR (CHECK
ENGINE) LAMP TO OBTAIN A DTC
To start this function, cycle the ignition switch ON-
OFF-ON-OFF-ON within 5 seconds. This will allow
any DTC stored i n the PCM to be displayed. The
malfunction indicator (Check Engine) lamp will dis­
play a DTC by flashing on and off. There is a short
pause between flashes and a longer pause between
digits. All codes displayed are two digit numbers with
a four second pause between codes.
An example of a code is as follows:
(1) Lamp on for 2 seconds, then turns off.
(2) Lamp flashes 4 times pauses and then flashes 1
time.
(3) Lamp pauses for 4 seconds, flashes 4 times,
pauses and then flashes 7 times.
The two codes are 41 and 47. Any number of codes
can be displayed, as long as they are i n memory. The
lamp will flash until all are displayed (55 = end of
test).
CHARGING SYSTEM DIAGNOSTIC TROUBLE
CODES
See Generator Diagnostic Trouble Code chart for
DTC which apply to the charging system. Refer to
the Powertrain Diagnostic Procedures manual to di­
agnose an On-Board Diagnostic System, Trouble
Code.
GENERATOR DIAGNOSTIC TROUBLE CODE (DTC)
Diagnostic
Trouble
Code
DRB Scan Tool
Disc-by
Description of Diagnostic Trouble Code
12* Battery Disconnect
41 ** Generator Field Not
Switching Properly
46**......
Charging System
Voltage Too High
47** Charging System
Voltage T o © Low
55*.
N/A
Direct battery input to PCM was disconnected within the last 50 Key-on cycles.
An open or shorted condition detected in the generator field control circuit.
Battery voltage sense input above target charging voltage during engine operation.
Battery voltage sense input below target charging durina engine operation.
Also, no significant change detected in battery voltage during active test of
generator output.
Completion of fault code display on Check Engine lamp.
* Check Engine Lamp will not illuminate at all times if this Diagnostic Trouble Code was recorded. Cycle Ignition key as
describeain manual and observe code flashed by Check Engine lamp.
** Check Engine Lamp will illuminate during engine operation if this Diagnostic Trouble Code was recorded.
J948A-8