Ford Mustang (1999-2004) Service Manual: Pinpoint Test B: LFC 21/DTC B1921 - RCM Bracket Ground Resistance High

Normal Operation

WARNING: The tightening torque of the restraints control module (RCM) retaining bolts is critical for proper air bag supplemental restraint system (SRS) operation. Refer to Restraints Control Module (RCM) in this section for correct torque values.

The restraints control module (RCM) monitors the resistance between the ground connections at its housing and the reference ground at pin 21. If the RCM detects a resistance greater than 100 ohms, it will store a diagnostic trouble code (DTC) B1921 in memory and flash a lamp fault code (LFC) 21 (or higher priority code if one exists) on the air bag indicator.

Possible Causes

High resistance between the RCM housing ground and pin ground can be caused by:

  • incorrect seating of the RCM retaining bolts.
  • incorrect tightening torque of the RCM retaining bolts.
  • high resistance on RCM logic ground circuit 397 (BK/WH).

PINPOINT TEST B: LFC 21/DTC B1921 - RCM BRACKET GROUND RESISTANCE HIGH

Test Step Result / Action to Take
B1 CHECK FOR A HARD OR INTERMITTENT DTC Yes

This is a hard fault. The fault condition is still present. This fault cannot be cleared until it is corrected and the DTC is no longer retrieved during the on-demand self test. GO to B2 .

No

This is an intermittent fault.

The fault condition is not present at this time. GO to B6 .
  • Key in OFF position.
  • Connect the diagnostic tool. Scan Tool
  • Key in ON position.
  • Enter the following diagnostic mode on the diagnostic tool: Retrieve/Clear Continuous DTCs.
  • Retrieve and record any continuous DTCs for use later in this pinpoint test.
  • Enter the following diagnostic mode on the diagnostic tool: On-Demand Self Test.
  • Was DTC B1921 retrieved during the on-demand self
B2 INSPECT THE RCM MOUNTING, MOUNTING BRACKET AND MOUNTING SURFACE Yes

Make sure the RCM, mounting bracket and mounting surface are free of damage, corrosion or dirt and the three retaining bolts are fully seated and correctly tightened. Reattach the RCM and mounting bracket to the mounting surface. GO to B7 .

No

GO to B3 .
  • Remove the RCM. Refer to Restraints Control Module (RCM) in the Removal and Installation portion of this section.
  • Visually inspect the RCM, mounting bracket and mounting surface for damage, corrosion or dirt.
  • Inspect the RCM mounting and make sure that the retaining bolts are fully seated and tightened correctly.

    Refer to Restraints Control Module (RCM) in the Removal and Installation portion of this section for correct tightening torque.

  • Was a significant amount of corrosion or dirt found, the RCM mounting bracket attached to the mounting surface incorrectly or were the three RCM retaining bolts not fully seated and tightened correctly?
B3 CHECK THE RCM HARNESS CONNECTION Yes

GO to B4 .

No

ATTACH the RCM harness connector correctly. GO to B7 .
  • Check the RCM harness connection.
  • Is the RCM harness connector connected to the RCM correctly?
B4 CHECK THE VEHICLE CHASSIS GROUND Yes

REPAIR the chassis grounding system. GO to B7 .

No

GO to B5 .
  • Measure the resistance between a known good chassis ground and the mounting surface of the RCM.
  • Is the resistance greater than 100 ohms?
B5 CHECK THE GROUND CIRCUIT 397 (BK/WH) FOR AN OPEN Yes

REPAIR the circuit. GO to B7 .

No

INSTALL a new RCM. GO to B7 .
  • Key in OFF position.
  • Disconnect: RCM C2041.
  • Measure the resistance between RCM C2041 pin 21, circuit 397 (BK/WH), harness side and a sheet metal ground near the RCM

  • Is the resistance greater than 100 ohms?
B6 CHECK FOR AN INTERMITTENT FAULT Yes

CHECK for causes or intermittent high resistance on circuit 397 (BK/WH). Attempt to recreate the hard fault by flexing the wire harness and cycling the ignition key frequently. REPAIR any intermittent concerns found.

GO to B7 .

No

GO to B7 .
  • Refer to the continuous DTCs recorded during Step B1.
  • Was the continuous DTC retrieved during Step B1 an intermittent fault?
B7 CHECK FOR ADDITIONAL DTCs Yes

Do not clear any DTCs until all DTCs have been resolved.

GO to the Restraints Control Module (RCM) Diagnostic Trouble Code (DTC) Priority Table in this section for pinpoint test direction.

No

RECONNECT the system.

REACTIVATE the system.

PROVE OUT the system.

REFER to Air Bag Supplemental Restraint System (SRS) in this section.

CLEAR all DTCs.
  • Refer to the continuous DTCs recorded during Step B1.
  • Were any continuous DTCs retrieved during Step B1?
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