Hummer H1 Service Manual page 1139

74 PCM/Tech 1 Scan Tool
DTC P0251 Injection Pump Cam System
C5
C27-D14
Circuit Description
The optical sensor provides a pump cam signal to the PCM by
counting pulses on the sensor disk located in the injection
pump. The pump cam is one of the most important inputs by
the PCM for fuel control and timing. This test monitors the
number of crankshaft position pulses that have occurred since
the last cam pulse. The physical one to one correspondence be-
tween the pump cam and the crankshaft implies if more crank
pulses are detected than cam pulses, cam pulses have been
missed. This is a type A DTC.
Conditions for Setting the DTC
• RPM less than 300.
• 8 consecutive cam pulses missing for 8 #1 cylinder.
or
• RPM greater than or equal to 300.
• 8 consecutive cam pulses missing for 32 #1 cylinder.
Action Taken When the DTC Sets
Backup fuel.
_____________________________________________________
A
5 VOLT
REFERENCE
PUMP CAM HIGH
SENSOR RESOLUTION SENSOR
SIGNAL SIGNAL GROUND
B C D
C3 B8 C1
C29-A4 C29-A2 C27-D9
5 VOLT CAM HIGH
REFERENCE SENSOR RES
SIGNAL SIGNAL
E
OPTICAL/ FUEL
TEMP SENSOR
F D
C12 C2 A3
C27-C8 A7
C29
SENSOR FUEL TEMP ITS
GROUND SIGNAL HI
Conditions for Clearing the MIL/DTC
• The PCM will turn the MIL off after three consecutive
trips without a fault condition.
• A History DTC will clear when forty consecutive
warm-up cycles that the diagnostic does not fail (coolant
temperature has risen 5°C (40°F) from start up coolant
temperature and engine coolant temperature exceeds
71°C (160°F) that same ignition cycle).
• Use of a Scan Tool (ignition must be cycled before DTC
is cleared).
Diagnostic Aids
When PCM is in backup fuel, fast idle and poor performance
problems will exist. If P0251 is also stored, there is a possible
problem with signal circuit. P0251 and P0370 will set if vehi-
cle has run out of fuel.
Test Description
Number(s) refer to the number(s) on the diagnostic table.
2. This step will determine if this is a hard or intermittent DTC.
4. This step will determine if there is a 5 volt reference.
6. This step will check to see if the sensor is sending a signal
back to the PCM.
INJECTION TIMING
STEPPEN MOTOR (ITS)
M
A B C
A2 A6 A7
A8 A9 A10
ITS ITS ITS
LO LO HI
POWERTRAIN
CONTROL
MODULE
9-S12-070
®