12
Test Procedures
TROUBLESHOOTING AND ELECTRICAL
SYSTEM: DIESEL VEHICLES
TEST PROCEDURE 12 – Wire Harness Diodes
See General Warnings, Section 1, Page 1-2.
A diode is designed to conduct current in one direction only. Depending on the application, diodes are used in the
vehicle to control electrical system logic, or to help protect relay and switch contacts from excessive arcing.
See
following NOTE.
NOTE:
If a diode conducts current in both directions, the diode has failed closed. If a diode will not conduct current in
either direction, the diode has failed open
2m
20
m
200
m
2k
200
200
200
200
20
2
200
m
500
20k
200k
2000
k
1000
OFF
WAVETEK
5XL
V
V
V
A
!
!
COM
200nA
MAX
1000 ---
750V
FUSED
2m
20
m
200
m
2k
200
200
200
200
20
2
200
m
500
20k
200k
2000
k
1000
OFF
WAVETEK
5XL
V
V
V
A
!
!
COM
200nA
MAX
1000 ---
750V
FUSED
2m
20
m
200
m
2k
200
200
200
200
20
2
200
m
500
20k
200k
2000
k
1000
OFF
WAVETEK
5XL
V
V
V
A
!
!
COM
200nA
MAX
1000 ---
750V
FUSED
1
4
5
A
B
6
2
1
2
2
1
2
3
(
)
1
1. VOM Black Lead 2. VOM Red Lead 3. Wiring Diagram Symbol
with Diode Stripe Orientation 4. Good Diode (Probe Scenario A) -
Conducts Current and Shows Continuity 5. Good Diode (Probe
Scenario B) - No Continuity 6. Bad Diode - Meter Beeps In Either
Direction (Probe Scenarios A and B)
1127
Figure 12-21
Basic Diode Information
The wire harness is equipped with several in-line diodes. The following table describes each diode’s function in the
electrical system, the location in the wire harness, and the symptom(s) of a diode failure.
DIODE TROUBLESHOOTING GUIDE
DIODE
IN-LINE WIRE
LOCATION
(WIRES)
FUNCTION
FAILURE
CONDITION
SYMPTOM/COMMENT
Open
Will contribute to the premature failure
of the differential limit switch contacts.
Diode 1
w32 and w35
Differential solenoid coil flyback diode
Closed
With key switch ON, the 30-amp fuse
(on w125) will blow repeatedly until the
diode has been replaced.
TABLE CONTINUED ON NEXT PAGE
Page 12-24
2013 Carryall 295/XRT 1550 Maintenance and Service Manual
Summary of Contents for Carryall XRT 1550
Page 2: ......
Page 28: ......
Page 40: ......
Page 62: ......
Page 106: ......
Page 120: ......
Page 180: ......
Page 224: ......
Page 272: ......
Page 310: ......
Page 344: ......
Page 362: ......
Page 370: ......
Page 418: ...NOTES...
Page 428: ...NOTES...
Page 430: ...NOTES...
Page 431: ......
Page 432: ......