TM 9-2320-364-20-2 2-1556 44.  TRAILER BLACKOUT TAILLIGHTS DO NOT OPERATE (CONT). Turn signals operate, indicating   wire 1435 at connector   MC15 OK. 22 to 28 vdc supplied to   connector MC15. Wire 1676 OK. Wire 1680 OK. Wire 1435 OK. Blackout taillight relay   R17 faulty. Wire 1680C faulty. KNOWN INFO POSSIBLE PROBLEMS TEST OPTIONS REASON FOR QUESTION Voltage test. STE/ICE-R #89. If 22 to 28 vdc are not present, trailer blackout taillight relay R17 is faulty. YES NO Replace trailer black- out taillight relay (Para 7-95). Verify repair, go to Step 6 of this Fault. 5. Are 22 to 28 vdc measured from wire 1680C at trailer blackout taillight relay R17, terminal 87? Repair wire 1680C (see schematic Fig 2-35) or notify DS Maintenance. Verify repair, go to Step 6 of this Fault. 4. Repair wire 1435 (see schematic Fig 2-35) or notify DS Maintenance. Verify repair, go to Step 6 of this Fault. YES NO Turn signals operate, indicating   wire 1435 at connector   MC15 OK. 22 to 28 vdc supplied to   connector MC15. Wire 1676 OK. Wire 1680 OK. Wire 1435 faulty. Blackout taillight relay   R17 faulty. Wire 1680C faulty. KNOWN INFO POSSIBLE PROBLEMS TEST OPTIONS REASON FOR QUESTION Continuity test. STE/ICE-R #91. If blackout taillights relay R17 is not grounded through wire 1435, relay will not operate. WARNING WARNING Is continuity measured on wire 1435 between trailer blackout taillights relay R17, terminal 85 and a known good ground? Read   WARNING on Page 2-1557 Read   WARNING on Page 2-1557