Speed Sensitive Volume System Does Not Work

WARNING: This page is about a different car, the 2005 Infiniti G35. However, it is still accessible from the selected car via links, so may be relevant.

VEHICLE SPEED OPERATION CHECK
Does speedometer is operated normally?

YES or NO
1. YES: GO TO 2.
2. NO: Check combination meter trouble diagnosis. Refer to "VEHICLE SPEED SIGNAL INSPECTION " in "COMBINATION METERS".
HARNESS CHECK
1. Turn ignition switch OFF.
2. Disconnect audio unit connector and combination meter connector.
3. Check continuity between audio unit harness connector M39 terminal 18 (W/G) and combination meter harness connector M19 terminal 14 (W/G).
  Continuity should exist.
4. Check continuity between audio unit harness connector M39 terminal 18 (W/G) and ground.
  Continuity should not exist.
  
  Fig 1: Checking Continuity Between Audio Unit And Combination Meter
  Courtesy of NISSAN MOTOR CO., U.S.A.
OK or NG
1. OK: GO TO 3.
2. NG:
  - Check connector housings for disconnected or loose terminals.
  - Repair harness or connector.
VEHICLE SPEED SIGNAL CHECK
1. Connect audio unit connector and combination meter connector.
2. Start engine and drive vehicle at more than 40 km/h (25MPH).
3. Check the signal between audio unit harness connector M39 terminal 18 (W/G) and ground with CONSULT-II or oscilloscope.
  18 (W/G) - Ground: Refer to " Terminals and Reference Value for Audio Unit for BOSE System ".
  
  Fig 2: Checking Signal Between Audio Unit Harness Connector And Ground
  Courtesy of NISSAN MOTOR CO., U.S.A.
OK or NG
1. OK: Replace audio unit.
2. NG: Check combination meter system. Refer to "PRELIMINARY CHECK " in "COMBINATION METERS".