Engine Pressure Sensor Open or Short Circuit Test
Connect Electronic Service Tool to Cab Data Link
and check for ACTIVE 5 Volt Sensor Supply Code.
Check for ACTIVE Engine Pressure Sensor OPEN or SHORT
Circuit Code. If the code is not ACTIVE and the Engine is not running
properly, refer to Troubleshooting Without A Diagnostic Code.
ACTIVE
Supply Code
Refer to +5Volt
Pressure Sensor
Supply Voltage
Circuit Test
NO ACTIVE
Supply Codes
ACTIVE
Open or Short
Circuit Code
Turn the ignition key ON, engine OFF. Disconnect the
Sensor Connector with the ACTIVE Short Circuit
Code in order to create an Open Circuit condition.
Measure Sensor Supply Voltage
with multimeter. Reading should be
within 4.5 to 5.5 VDC range.
Active SHORT
Circuit Code
Active OPEN
Circuit Code
NOT
OK
Refer to Inspecting
Electrical Connectors
Check Electrical Connectors and Wiring.
If a Short Circuit code was Active before the sensor
was disconnected and an Open Circuit code became
Active after the sensor was disconnected then
thoroughly inspect the connector for signs of moisture.
Inspect the seals and connect the sensor. If the code
reappears, the sensor or pigtail harness is the problem.
Temporarily connect a new sensor but do not install it
into the engine. If the code is no longer Active with the
new sensor connected, replace the old sensor.
Code Changes
To OPEN Circuit
SHORT Circuit
Code Remains
Refer to +5Volt
Engine Pressure
Sensor Supply
Voltage Circuit
Test
NOT
OK
Supply OK
Install a short circuit between terminal-B and
terminal-C of the Sensor Connector and check
for an ACTIVE Short Circuit code.
OPEN Circuit Code
Active with jumper
If a SHORT Circuit Code appears,
temporarily connect the suspect
sensor. If an OPEN Circuit code returns,
replace the sensor and verify the
diagnostic code does not return.
SHORT Circuit Code
Active with jumper
Disconnect the ECM Connector J2/P2 and check for Active
diagnostic codes. The OPEN Circuit code should be Active with
the harness disconnected. Install a jumper (short) between the
ECM Connector J2 signal terminal and terminal-3. A SHORT
Circuit code should be Active with the jumper installed.
If the OPEN Circuit code is NOT Active with the harness
disconnected, or the SHORT Circuit code is NOT Active
with the jumper (short) installed, temporarily install a
test ECM. If the test ECM resolves the problem,
install the old and verify problem returns. If the test ECM
works and the original ECM does not, replace the ECM.
NOT OK
Remove the sensor signal wire from ECM connector
P2 and the sensor connector. Bypass the signal wiring by
connecting a jumper wire directly from ECM connector P2
to the sensor connector signal terminal. If the Diagnostic
Code disappears with the bypass wire installed, repair or
replace the harness. Otherwise, restart the test procedure.
OK
Engine Temperature Sensor Open or Short Circuit Test
Connect an Electronic Service Tool and check for an ACTIVE
Engine Temperature Sensor OPEN or SHORT Circuit Code.
If the code is not ACTIVE and the Engine is not running
properly, refer to Troubleshooting Without A Diagnostic Code.
ACTIVE
Open or Short
Circuit Code
With the ignition key ON and engine OFF, Disconnect the
Sensor Connector with the ACTIVE Short Circuit Code
in order to create an Open Circuit condition.
Active SHORT
Circuit Code
Active OPEN
Circuit Code
NOT
OK
Refer to Inspecting
Electrical Connectors
Check Electrical Connectors and Wiring.
If a Short Circuit code was Active before the sensor
was disconnected and an Open Circuit code became
Active after the sensor was disconnected then
thoroughly inspect the connector for signs of moisture.
Inspect the seals and connect the sensor. If the code
reappears, the sensor is the problem.
Temporarily connect a new sensor but do not install it
into the engine. If the code is no longer Active with the
new sensor connected, replace the old sensor.
Code Changes
To OPEN Circuit
SHORT Circuit
Code Remains
With the ignition key On and engine OFF, Install a short
circuit between terminal-1 and terminal-2 of the Sensor
Connector and check for an ACTIVE Short Circuit code.
OPEN Circuit Code
Active with jumper
If a SHORT Circuit Code appears,
temporarily connect the suspect
sensor. If an OPEN Circuit code returns,
replace the sensor and verify the
diagnostic code does not return.
SHORT Circuit Code
Active with jumper
Disconnect the ECM Connector J2/P2 and check for Active
diagnostic codes. The OPEN Circuit code should be Active with
the harness disconnected. Install a jumper (short) between the
ECM Connector J2 signal terminal and terminal-18. A SHORT
Circuit code should be Active with the jumper installed.
If the OPEN Circuit code is NOT Active with the harness
disconnected, or the SHORT Circuit code is NOT Active
with the jumper (short) installed, temporarily install a
test ECM. If the test ECM resolves the problem,
install the old and verify problem returns. If the test ECM
works and the original ECM does not, replace the ECM.
NOT OK
Remove the sensor signal wire from ECM connector
P2 and the sensor connector. Bypass the signal wiring by
connecting a wire directly from ECM connector P2 to
the sensor connector signal terminal. If the Diagnostic
Code disappears with the bypass wire installed, repair or
replace the harness. Otherwise, restart the test procedure.
OK
Electronic Service Tool Will Not
Communicate with the ECM
Try to start the Engine.
Connect another Service Tool to the
ECM, but use the same Service Tool
Cables as in the previous step.
Connect Electronic Service Tool to the Cab data link,
turn ignition key switch ON, engine OFF. Attempt
to establish communications with the ECM.
Ensure after-market engine protection
devices (not installed by Caterpillar) are
not active, preventing the ECM or Ignition
Key Switch from receiving battery power.
Connect the Service Tool using a
completely different set of Service
Tool cables.
Either the vehicle wiring or another
device on the vehicle is causing the
communication problem.
Connect a test ECM. Turn the Ignition Key ON,
engine OFF. If the new ECM communicates and
the old ECM does not, reconnect the old ECM
and verify the problem returns. If the new ECM
communicates and the old ECM does not, replace
the ECM. If the new ECM does not communicate,
there is a problem with the cables or the battery.
A new ECM has an unprogrammed
Personality Module. The engine will not start
or communicate until the Personality Module
in the ECM is Flash Programmed.
Disconnect ECM Connector P1 from
the ECM. Use the Service Tool Bypass
Harness as indicated in the Ignition
Key Switch Circuit Test.
Does
not start
Engine starts, but
with the Service Tool
ECM does not
communicate
No Communications
ECM Communicates
ECM does not communicate
No Communications
No Communications
Engine Cranks, But Will Not Start
Does not crank,
refer to engine
will not crank.
Repair
Service
Tool
Refer to the
Ignition
Keyswitch
Circuit Test
Refer
to the
ECM
Memory
Test
Repair
Cables
ECM and Service
Tool Communicate
OK
OK
OK
Not a New ECM
NOT
OK
NOT
OK
NOT
OK
OK
Check ECM power, ground, and keyswitch
connections with the ignition key switch ON.
Battery voltage at ECM Connector J1/P1
terminal-52, 53 and 70, to terminal-65 and 67
should be 11.0 -13.5 VDC for a 12 Volt
system and 22.0 - 27.0 for a 24 Volt system.
Check the Ignition Key Switch status on the Service
Tool. With the Ignition Key Switch ON, the status
should display ON. If the status displays OFF, check
for voltage from the battery. Battery voltage at
ECM Connector J1/P1 terminal-70 to terminal-65
and 67 should be 11.0 -13.5 VDC for a 12 Volt system
and 22.0 - 27.0 for a 24 Volt system.
If the
has been
, connect and check for an
active 252-11 Incorrect Engine Software.
Personality Module
recently
replaced/updated
Observe from Electronic Service
Tool
the engine. An rpm value
should be displayed on the Service Tool.
engine rpm
while cranking
If the Truck is equipped with a , ensure
is not . Turn the
Ignition Key Switch ON, engine OFF, the
CAT ID will indicate Theft Deterrent Active.
CAT ID
Theft Deterrent Active
Monitor the while cranking - if
is present there may be a fuel
supply or quality problem. Check fuel
pressure, priming, and filters.
exhaust stack
no smoke
Check for combustion problems,
mechanical problems or extreme temperatures.
Refer
to the
Engine
Speed/Timing
Circuit Test
OK
OK
OK
OK
OK
NOT
OK
NOT
OK
Refer
to the
Ignition
Keyswitch
Circuit Test
Refer
to the
ECM
Memory
Test
NOTE: Refer to the Electronic Troubleshooting Guide for detailed information.
NOTE: Refer to the Electronic Troubleshooting Guide for detailed information.
NOTE: Refer to the Electronic Troubleshooting Guide for detailed information.
NOTE: Refer to the Electronic Troubleshooting Guide for detailed information.
SENR9525
November 1999
Schematic
3126B Truck Engine
Electrical System
6PZ1-UP
7JZ1-UP
© 1999 Caterpillar
All Rights Reserved
Printed in U.S.A.
A
B
INJECTION ACTUATION
PRESSURE CONTROL VALVE
J500 P500
J400 P400
B
A
TOP CAMSHAFT SPEED/TIMING-
TOP CAMSHAFT SPEED/TIMING+
BOTTOM CAMSHAFT SPEED/TIMING-
BOTTOM CAMSHAFT SPEED/TIMING+
E963-BK
E964-WH
E965-BU
E966-YL
49
48
13
19
20
21
28
29
30
59
58
31
43
51
22
23
56
57
65
66
69
8
G856-WH
G857-YL
A
B
TOP CAMSHAFT ENGINE
SPEED/TIMING SENSOR
TOP CAMSHAFT SPEED/TIMING-
TOP CAMSHAFT SPEED/TIMING+
J401 P401
J402 P402
B
A
BOTTOM CAMSHAFT SPEED/TIMING-
BOTTOM CAMSHAFT SPEED/TIMING+
BOTTOM CAMSHAFT ENGINE
SPEED/TIMING SENSOR
1
2
ENGINE COOLANT
TEMPERATURE
995-BU
G833-PK
J100 P100
ENGINE COOLANT TEMPERATURE
ENGINE TEMPERATURE SENSOR COMMON
ENGINE INTAKE MANIFOLD AIR TEMPERATURE
ENGINE TEMPERATURE SENSOR COMMON
J103 P103
C967-BU
G833-PK
ENGINE INTAKE
MANIFOLD AIR
TEMPERATURE
1
2
995-BU
C967-BU
G833-PK
C
B
A
G849-BR
G829-GN
G828-WH
A746-PK
G849-BR
A747-GY
G829-GN
G828-WH
P204 J204
ENGINE OIL PRESSURE SENSOR
ENGINE OIL PRESSURE SENSOR COMMON
ENGINE OIL PRESSURE SENSOR +5V
ENGINE ATMOSPHERIC
PRESSURE
NOTE A
GN
YL
OR
ENGINE INJECTION
ACTUATION
PRESSURE
ENGINE INJECTION ACTUATION PRESS
ENGINE PRESSURE SENSOR COMMON
ENGINE PRESSURE SENSOR +5V
GN
YL
OR
P201 J201
994-GY
G827-BU
G826-BR
A
B
C
J200 P200
A746-PK
G829-GN
G828-WH
A
B
C
SENSOR
ENGINE BOOST
PRESSURE
ENGINE BOOST PRESSURE
ENGINE PRESSURE SENSOR COMMON
ENGINE PRESSURE SENSOR +5V
GN
YL
0R
COMPONENTS ARE SHOWN WITH THE KEY OFF
UNLESS OTHERWISE SPECIFIED:
ALL WIRES TO BE 18 AWG OR LARGER SAE J1560 TYPE TXL
OR EQUIVALENT. FOR TWISTED PAIR SPECIFICATION SEE
SAE J1708
INTAKE AIR HEATER RELAY
ENGINE COOLANT TEMPERATURE
ENGINE INTAKE MANIFOLD AIR TEMPERATURE
ENGINE OIL TEMPERATURE
ENGINE TEMPERATURE SENSOR COMMON
ENGINE ATMOSPHERIC PRESSURE
ENGINE PRESSURE SENSOR COMMON
ENGINE PRESSURE SENSOR +5V
IAP CONTROL VALVE
IAP CONTROL VALVE COMMON
ENGINE BOOST PRESSURE
ENGINE INJECTION ACTUATION PRESSURE
P1 J1
ECM GROUND STRAP
G854-PK
G855-PU
TO ECM
MOUNTING
BOLT
ENGINE HARNESS CONNECTOR
VEHICLE HARNESS CONNECTOR
C987-RD
G850-BU
P648
TO CONNECTOR J648
A
B
K995-OR
G850-BU
C987-RD
1
2
ENGINE TIMING CALIBRATION PROBE+
ENGINE TIMING CALIBRATION PROBE-
TO INTAKE AIR HEATER
CIRCUIT FUSIBLE PROTECTION
TO ENGINE CYLINDER HEAD
(NOT CYLINDER HEAD GROUND STUD)
INTAKE AIR
HEATER
G850-BU
P2 J2
ENGINE TIMING CALIBRATION PROBE+
ENGINE TIMING CALIBRATION PROBE-
J501 P501
N/O
RELAY
32
35
33
34
26
18
10
11
12
15
16
17
24
25
41
42
50
52
53
60
9
40
27
6
7
14
3
2
B
A
INJECTOR
CYLINDER
NO.5
1
63
64
4
5
61
62
70
INJECTOR
CYLINDER
NO.6
B
A
J306 P306J305 P305
J304 P304J303 P303
J302 P302
L984-OR
B
A
INJECTOR
CYLINDER
NO.4
INTAKE MANIFOLD GROMMET
J301 P301
INJECTOR
CYLINDER
NO.1
A704-GN
B
A
A705-BU
A703-BR
A702-PU
A701-GY
L985-YL
L984-OR
A706-GY
L985-YL
L983-WH
L983-WH
INJECTOR
CYLINDER
NO.3
B
A
B
A
INJECTOR
CYLINDER
NO.2
P300 J300
L983-WH
L984-OR
L985-YL
A706-GY
A705-BU
A704-GN
A703-BR
A702-PU
A701-GY
INJECTOR COMMON CYLINDERS 1 & 2
INJECTOR COMMON CYLINDERS 3 & 4
INJECTOR COMMON CYLINDERS 5 & 6
INJECTOR CYLINDER 6
INJECTOR CYLINDER 5
INJECTOR CYLINDER 4
INJECTOR CYLINDER 3
INJECTOR CYLINDER 2
INJECTOR CYLINDER 1
44
45
46
55
54
39
38
37
36
47
67
68
12
11
10
9
8
7
6
5
4
3
2
1
3126B ENGINE HARNESS WIRING DIAGRAM
NOTE A
EXHAUST BRAKE
ON/OFF SWITCH
FLYBACK
DIODE
EXHAUST BRAKE
SOLENOID
TO ALLISON AT/MT SHIFT
MODULATOR FOR MECHANICAL
TRANSMISSION
COOLING FAN SOLENOID
NOTE A
INTAKE AIR
HEATER LAMP
TO CONNECTOR P648
J648
100A WITH 130A PEAK AT 11V
C987-RD
TO INTAKE AIR
HEATER RELAY
(CONTACT SIDE)
SPEEDOMETER
NOTE A
NOTE A
SENSOR ACCELERATOR
PEDAL POSITION
BK
RD
WH
J403 P403
ACCELERATOR PEDAL SENSOR/SWITCH SENSOR COMMON
+8 V
ACCELERATOR PEDAL POSITION
B
A
C
L901-GY
C977-BU
G879-OR
H795-PK
H795-PK
A/C HIGH PRESS. SWITCH
N/C
E971-GN
450-YL
451-BR
SIGNAL+
SIGNAL-
NOTE A
TACHOMETER
NOTE A
INPUT SENSOR COMMON #1
CLUTCH PEDAL POSITION SWITCH
+5V
E793-BU
E794-YL
INPUT #12
INPUT #11
INPUT #7
ENGINE COOLANT LEVEL NORMAL
ENGINE COOLANT LEVEL LOW
VEHICLE SPEED IN POSITIVE
VEHICLE SPEED IN NEGATIVE
INPUT #8
20A
14 AWG GXL
14 AWG GXL
14 AWG GXL
14 AWG GXL
AP SENSOR/SWITCH SENSOR COMMON
+8V
ACCELERATOR PEDAL POSITION
OUTPUT #5
18
22
23
2
46
49
54
PASSIVE MAGNETIC
VEHICLE SPEED
SENSOR
G808-BU
G809-GN
H795-PK
H795-PK
SET/RESUME
ENGINE
BLOCK
62
24
409-OR
E971-GN
E793-BU
E794-YL
A249-BK
K990-GN
K900-YL
J906-BR
COMPONENTS WITHIN THIS AREA ARE CATERPILLAR PROVIDED
CLUTCH PEDAL POSITION N/C
SERVICE BRAKE PEDAL POSITION N/C
(PEDAL RELEASED POSITION)
NOTE A
CRUISE CONT ON/OFF
CHECK ENGINE LAMP
WARNING LAMP
+BAT.
-BAT.
J1587 DATA LINK NEGATIVE
J1587 DATA LINK POSITIVE
TO J1939 DATA LINK CONNECTOR
TO J1939 DATA LINK CONNECTOR
TO J1939 DATA LINK CONNECTOR
1
2
D
H795-PK
E
C
F
CAB DIAGNOSTIC CONNECTOR
B
A
5
4
66
11
41
69
32
33
68
VEHICLE HARNESS CONNECTOR
P2 J2
H795-PK
C985-BU
C986-BR
G837-YL
B
A
659-PK
L994-YL
K998-BU
G880-PK
CIRCUITS WITHIN THIS AREA ARE RECOMMENDATIONS
SERVICE BRAKE PEDAL POSITION SWITCH
INPUT #5
INPUT #13
INPUT #6
SIGNAL-
SIGNAL+
+BAT.
NOTE A
(TRANSMISSION IN GEAR)
START
ON
OFF
NOTE A
45
55
47
64
6
C992-PU
G842-GY
G882-WH
G843-OR
K995-OR
C987-RD
C974-PU
C973-GN
993-BR
F713-OR
STARTER
MOTOR
(12V)
FLYBACK
DIODE
NOTE A
CRUISE CONTROL ON/OFF SWITCH
INPUT #3
INPUT #4
59
60
7
BATTERY 2
C975-WH
K982-YL
G841-GN
N/O
RELAY
H795-PK
C985-BU
C986-BR
SET
RESUME
INPUT #1
INPUT #2
INPUT #18
OUTPUT #8
C978-BR
C979-OR
C977-BU
C992-PU
C975-WH
35
44
56
58
40
21
C978-BR
C979-OR
K999-GN
K980-PK
L902-GN
G840-PU
C973-GN
C974-PU
KEY SWITCH
659-PK
GROUND
L994-YL
ALTERNATOR
-
12V
+
-
12V
+ +
P1 J1
12V
-
BATTERY 3
BATTERY 1
G808-BU
G809-GN
G845-PU
ENGINE HARNESS CONNECTOR
J1587 DATA LINK NEGATIVE
J1587 DATA LINK POSITIVE
J1939 DATA LINK SHIELD
J1939 DATA LINK NEGATIVE
J1939 DATA LINK POSITIVE
IGNITION KEY SWITCH
UNSWITCHED +BATTERY
UNSWITCHED +BATTERY
-BATTERY
-BATTERY
OUTPUT #6
OUTPUT #7
52
53
65
67
19
20
51
101-RD
101-RD
229-BK
229-BK
G838-BR
G839-BU
9
8
42
34
50
1
14
70
25
TACHOMETER POSITIVE
TACHOMETER NEGATIVE
38
39
57
43
G844-PK
C983-WH
C984-YL
450-YL
451-BR
H795-PK
FAST IDLE ENABLE SWITCH
L902-GN
N/O MOMENTARY SWITCH
NOTE A
NOTE A
NOTE A
G880-PK
FAST IDLE
ENABLED LAMP
N/O MOMENTARY SWITCH
G844-PK
SPEEDOMETER POSITIVE
SPEEDOMETER NEGATIVE
INPUT SENSOR COMMON #2
INPUT #14
36
37
27
26
3
17
DIAGNOSTIC ENABLE SWITCH
H795-PK
(PEDAL RELEASED POSITION)
+BAT.
29
28
30
31
48
61
10
12
13
15
16
63
NOTE A
G882-WH
SERVICE BRAKE PEDAL POSITION #2 N/O
(PEDAL RELEASED POSITION)
WARNING LAMP
CHECK ENGINE LAMP
OUTPUT #1
OUTPUT #9
TRANSMISSION NEUTRAL N/O
409-OR
OUTPUT #2
OUTPUT #3
OUTPUT #4
E718-PK
E991-GY
G836-WH
3126B VEHICLE HARNESS WIRING DIAGRAM
G846-YL
ENGINE OIL TEMPERATURE
ENGINE TEMPERATURE SENSOR COMMON
J101 P101
G846-YL
G833-PK
1
2
VOLTAGE SUPPLY WIRES
COMMON WIRES
INPUT SIGNAL WIRES
OUTPUT SIGNALS WIRES
SENSOR
SENSOR
SENSOR
ENGINE OIL
TEMPERATURE
SENSOR
SENSOR
Engine Harness Connector P2
ECM AMP Connectors
ECM Side
B
B
A
A
C
C
1
1
2
2
1 2 3 4 5 6
6 5 4 3 2 1
12 11 10 9 8 7
7 8 9 10 11 12
Harness Side
23
31
47
57
14
24
40
48
58 59 60 61 62 63 65 66 67 68 69 70
1 2 3 4 5 6 8 9 10 11 12 13
36
>PEI<
1
14
24
32
40
48
58
13
23
31
39
47
57
70
Harness Side
23
31
47
57
14
24
40
48
58 59 60 61 62 63 65 66 67 68 69 70
1 2 3 4 5 6 8 9 10 11 12 13
36
>PEI<
Vehicle Harness Connector P1
ECM Side
1
14
24
32
40
48
58
13
23
31
39
47
57
70
Deutsch DT Connector Receptacle
Deutsch DT Connector Plug
Pressure
Sensor
Connectors
Temperature
Sensor
Connectors
Injector
Harness
Connectors
NOTE A: A DASHED LINE SPECIFIES AN OPTIONAL FEATURE, NOT AVAILABLE ON ALL SYSTEMS.
NOTE A: A DASHED LINE SPECIFIES AN OPTIONAL FEATURE, NOT AVAIL ABLE ON ALL SYSTEMS.
NOTE A
NOTE A
ENGINE ATMOSPHERIC PRESSURE
ENGINE PRESSURE SENSOR COMMON
ENGINE PRESSURE SENSOR +5V
GN
YL
OR
P203 J203
A747-GY
G829-GN
G828-WH
A
B
C
994-GY
ENGINE OIL PRESSURE SENSOR
ENGINE OIL
PRESSURE
NOTE A
SENSOR
G827-BU
G826-BR
ENGINE OIL PRESSURE SENSOR COMMON
ENGINE OIL PRESSURE SENSOR +5V
