Table 4-2. Unit Troubleshooting (cont'd)
TEST OR INSPECTION
Step 4. Check to see that current is flowing from towing vehicle, using a multimeter.
If proper voltage is obtained, proceed to step 5.
Step 5. Check wiring harness for short circuit.
Check cable for bare spots. Repair as necessary. Make a continuity test of all circuits,
using a multimeter. (Refer to wiring diagrams, starting on page 4-58).
If wiring harness is good, proceed to step 6.
Step 6. Checklightswitchontowingvehicle.
If switch is defective, replace switch.
If switch is good, proceed to step 7 (XM1063) or step 9 (M129A4).
Step 7. Remove resistor box cover and check resistor contact points (1).
Clean contact points.
If points are clean, proceed to step 8.
Step 8. Check resistors for rated ohms marked on resistors, using multimeter (refer to resistor box
wiring diagram, page 4-59).
Replace cracked, chipped or defective resistor (para. 4-23).
M129A4 Registration Numbers NX0RKB and Subsequent LED lighting system voltage
converter boxes no longer contain fuses/circuit breakers.
Step 9. Remove voltage converter box cover and check circuit breakers.
Replace defective circuit breakers.