Airbag System - Code 51 Diagnostics/Repair
Article No.95-4-8
02/27/95
^ RESTRAINT SYSTEM - AIR BAG MODULE - DIAGNOSTIC CODE 51 - IMPROVED DIAGNOSTICS AND REPAIR
^ AIR BAG - DIAGNOSTIC CODE 51 - IMPROVED DIAGNOSTICS AND REPAIR
LIGHT TRUCK:
1994-95 BRONCO, F-150, F-250
ISSUE:
The air bag diagnostic light may flash on some vehicles. The air bag warning indicator may flash as follows:
^ Five (5) flashes, followed by
^ a 1 second pause, followed by
^ a single flash, then
^ the sequence repeats itself.
This may be caused by a previously-repaired short-to-ground or intermittent short-to-ground in the air bag wiring. Air bag diagnostic monitors displaying Diagnostic Code 51 are repeatedly being replaced without having first corrected a possible short-to-ground or intermittent short-to-ground in the system.
ACTION:
Deactivate the air bag system and check for and repair any possible shorts to ground in the air bag wiring. Replace Diagnostic Air Bag Module, reactivate the system and perform an air bag system prove-out. Refer to the following for detailed service procedures.
GENERAL INFORMATION
The diagnostic monitor contains an internal thermal fuse that is not serviceable. The thermal fuse is controlled by a computer inside the diagnostic monitor. The computer will open the thermal fuse whenever a short on the deployment circuits occur. The thermal fuse does not open because of excessive current flowing through it. Do not attempt to jump the thermal fuse with a circuit breaker or any other type of fuse.
WARNING:
DO NOT INSTALL A NEW DIAGNOSTIC MONITOR UNTIL THE CAUSE OF CODE 51 HAS BEEN LOCATED AND REPAIRED. IF A SHORT-TO-GROUND HAS NOT BEEN LOCATED AND REPAIRED, THE SHORT IS INTERMITTENT AND IS NOT PRESENT AT THIS TIME. INSTALLING A NEW DIAGNOSTIC MONITOR WITH AN INTERMITTENT SHORT IN THE SYSTEM WILL RESULT IN REPEAT BLOWN THERMAL FUSES IN DIAGNOSTIC MONITORS AND REPEAT SERVICE.
The diagnostic monitor measures the voltages at the diagnostic monitor connector pins. When certain air bag deployment circuits are shorted to ground, the system may become susceptible to unwanted deployment of the air bags. The diagnostic monitor will sense a short-to-ground on any air bag deployment circuit and if a short is detected, will open the diagnostic monitor's internal thermal fuse. Blowing this fuse removes all power from the air bag deployment circuits. While the short-to-ground exists, the monitor will flash Fault Code 13 or 14 depending where the short is. If the short-to-ground is intermittent and temporarily corrects itself, the diagnostic monitor will flash Fault Code 51. Refer to Figures 5 and 6.
If the short-to-ground returns, the higher priority Codes 13 and 14 will be flashed instead of 51. The diagnostic monitor senses a short-to-ground on any of these circuits and helps prevent unwanted air bag deployment by opening the diagnostic monitor's internal thermal fuse.
If the air bag indicator is flashing Code 51 and a short-to-ground has not been serviced, this means an intermittent short-to-ground exists in the air bag system. The diagnostic monitor should only be replaced after service of the short-to-ground has been completed.
CODE 51 SEQUENCE OF EVENTS
1. A short-to-ground occurs on one (1) or more of the air bag deployment circuits.
2. The diagnostic monitor recognizes the shorted wiring and, depending on which circuit the short occurred. flashes either a Code 13 (air bag circuit shorted) or Code 14 (primary crash circuit shorted). Refer to Figures 1 and 2 for Code 13 and Figures 3 and 4 for Code 14.
NOTE:
FAULT CODES 13 AND 14 WILL ONLY FLASH WHILE THE SHORT-TO-GROUND IS PRESENT.
3. The diagnostic monitor sends a signal to its internal thermal fuse, causing the fuse to open. This disables the air bag deployment circuits.
4. The diagnostic monitor will continue to flash Codes 13 and 14 while the short is present. If the short goes away, Code 51 will appear.
5. The diagnostic monitor only flashes a Code 51 when the internal thermal fuse is open and a short-to-ground is not present.
NOTE:
IT MAY BE POSSIBLE TO REPEAT CODES 13 OR 14 BY MOVING ("SHAKE-WIGGLE" TEST) THE WIRING TO RECREATE THE ORIGINAL SHORT. HOWEVER, THIS WILL REQUIRE THE REACTIVATION OF THE AIR BAG SYSTEM, WHICH WILL AGAIN REQUIRE DEACTIVATION OF THE AIR BAG SYSTEM AFTER THE TEST IS PERFORMED.
CAUTION:
ALL AIR BAG SYSTEM COMPONENTS ARE SERVICED BY REPLACEMENT ONLY. IF ANY OF THESE COMPONENTS OR COMPONENT WIRING IS DAMAGED IN ANY WAY, THEY CANNOT BE REPAIRED AND MUST BE REPLACED.
These components include the following:
^ Driver's Air Bag Module Assembly
^ Sliding Contact (Clockspring)
^ Diagnostic Monitor
^ Safing Sensor
^ Primary Crash Sensors
DETAILED SERVICE PROCEDURE
WARNING:
PRIOR TO PERFORMING ANY AIR BAG DIAGNOSTICS OR REPAIRS, BE SURE TO DISCONNECT THE BATTERY FOR ONE (1) MINUTE, THEN DISCONNECT THE AIR BAG FROM THE CLOCKSPRING CONNECTOR. CONNECT THE CORRECT AIR BAG SIMULATOR TO THE CLOCKSPRING CONNECTOR TO SIMULATE THE AIR BAG PRIOR TO RECONNECTION OF THE BATTERY.
1. Thoroughly check circuitry for any intermittent wiring shorts before attempting to replace the diagnostic monitor. Do not attempt to jump the thermal fuse with a circuit breaker or any other type fuse.
WARNING:
DO NOT INSTALL A NEW DIAGNOSTIC MONITOR UNTIL THE CAUSE OF CODE 51 HAS BEEN LOCATED AND REPAIRED. IF A SHORT-TO-GROUND HAS NOT BEEN LOCATED AND REPAIRED, THE SHORT IS INTERMITTENT AND IS NOT PRESENT AT THIS TIME. INSTALLING A NEW DIAGNOSTIC MONITOR WITH AN INTERMITTENT SHORT WILL RESULT IN REPEAT BLOWN INTERNAL THERMAL FUSES IN DIAGNOSTIC MONITORS AND REPEAT SERVICE.
NOTE:
THE GRAY DIAGNOSTIC MONITOR HARNESS CONNECTOR CONTAINS A SHORTING BAR INSIDE THE CONNECTOR THAT WILL SHORT PINS 2 AND 3 TOGETHER WHENEVER THE DIAGNOSTIC MONITOR IS DISCONNECTED. SINCE PIN 3 IS GROUND, SHORTING PINS 2 AND 3 TOGETHER WILL CAUSE A SHORT-TO-GROUND IN PINS 2 AND 17. ENSURE PLASTIC LOCKING WEDGE IS REMOVED FROM THE GRAY HARNESS CONNECTOR BEFORE CHECKING RESISTANCE OF THESE CIRCUITS TO GROUND.
NOTE:
THE FOLLOWING ARE THE MOST COMMON LOCATIONS WHICH HAVE BEEN IDENTIFIED FOR WIRING SHORTS. IN THE EVENT WIRING SHORTS CANNOT BE FOUND AT THESE LOCATIONS, YOU MUST CHECK ALL OF THE AFFECTED WIRING AS REQUIRED.
^ Water/Liquid may have entered the diagnostic monitor (particularly 1994 models; 1995 models have a water-resistant shield on the monitor. However. water/moisture may have entered the monitor at the electrical connector).
NOTE:
CODES 13, 14 AND 51 MAY BE CAUSED BY THE FOLLOWING CONDITIONS ALSO.
NOTE:
FAULT CODE 51 RESULTING FROM WATER ENTRY INTO THE DIAGNOSTIC MONITOR MAY CAUSE A BACK-UP BEEP SOUND TO BE HEARD AS WELL AS A FLASHING WARNING LAMP (THIS IS A NON-LEGITIMATE CODE).
^ A. Cleaning of vehicle's interior (spraying water on rubber mat or cleaning carpet). Look for water/mud spots on the diagnostic monitor case and surrounding area.
^ B. Application of Armor-All, or similar product, to the instrument panel or rubber floormat or spraying air-freshener under the instrument panel. Check for an oily residue on the diagnostic monitor case.
^ C. Condensation from air conditioner operation may have collected on the duct work under the instrument panel, dripped onto the buzzer/chime module mounting bracket, then into the diagnostic monitor. Look for rust, white residue (corrosion) or water spots on the buzzer/chime module mounting bracket. Look for water spots/water trails on the duct work above the buzzer/chime warning module.
^ D. Condensation may have formed in the diagnostic monitor due to humidity in the air during A/C operation.
Before proceeding, look for water spots on the inside of the diagnostic monitor case and look for white milky-colored spots (corrosion - may be small) on the printed circuit board and/or the components in the monitor.
If it has been determined that water, liquid or condensation has contaminated the diagnostic monitor, but not from any of the above sources, check the following sources of contamination.
^ E. Water leak at cowl weld seam.
^ F. Water leak at windshield.
^ G. Water/mud may have splashed into the monitor from the floormat or carpet.
^ H. Liquid may have been spilled on the instrument panel (above the radio). The radio/keyless entry may also be affected.
^ I. Water leak at the radio antenna grommet. The water will travel along the antenna lead, then drip onto the monitor.
If water/moisture has contaminated the diagnostic monitor, the source of the contamination must be identified. If the contamination is the result of a water leak, water test the vehicle and repair the leak as required.
NOTE:
WHEN WATER TESTING THE VEHICLE FOR A WINDSHIELD OR COWL SEAM LEAK, THE WATER RUNNING OFF THE WINDSHIELD MAY NOT CONTACT THE COWL SEAM. IF NO LEAK IS DETECTED AT THE WINDSHIELD, REMOVE THE COWL TOP PANEL AND DIRECT THE WATER INTO THE COWL AND AT THE SEAM.
Check the following locations for shorts to ground:
^ A. Check for damaged crash sensor wiring.
^ B. Ohm out crash sensors (check for a short-to-ground).
NOTE:
ONE (1) SIDE OF THE SENSOR IS GROUNDED BY DESIGN. THE GRAY DIAGNOSTIC MONITOR HARNESS CONNECTOR CONTAINS A SHORTING BAR INSIDE THE CONNECTOR THAT WILL SHORT PINS 2 AND 3 TOGETHER WHENEVER THE DIAGNOSTIC MONITOR IS DISCONNECTED. SINCE PIN 3 IS GROUND, SHORTING PIN 2 TO 3 WILL CAUSE A SHORT-TO-GROUND IN ON BOTH PINS 2 AND 17. ENSURE RED PLASTIC LOCKING WEDGE IS REMOVED FROM THE GRAY HARNESS CONNECTOR BEFORE CHECKING RESISTANCE OF THESE CIRCUITS TO GROUND.
^ C. Check for damaged shorting bars inside the gray diagnostic monitor connector.
^ D. Check for damaged wiring (small nicks or cut) on the back side of the drive's air bag.
^ E. Check the clockspring wiring down the steering column past the connector into the wire harness for damage.
^ F. Check for damaged wire at fuse block mounting bracket.
^ G. Check for chafed wiring at the clutch pedal return spring (if so equipped).
^ H. Check for damaged wiring between the diagnostic monitor and the right cowl side. The wiring may have been damaged by the attaching screws under the glove compartment.
^ I. Check safing sensor wiring behind the RH kick panel for damage or signs that water may have entered the connector. A water leak at the radio antenna grommet may result in water in the connector.
^ J. Check the safing sensor wire harness under the RH scuff plate. Remove the harness from the trough and check for foreign objects such as sheet metal screws, small pieces of metal or other sharp objects that may damage the wiring. The installation of running boards or similar equipment along with the passenger door opening may result in damage to the wiring. Wiring may be damaged at the passenger's shoulder safety belt floor anchor located to the rear of the scuff plate.
^ K. Check the safing sensor wiring to the safing sensor at the sensor attaching bracket.
^ L. Check for water entry into the safing sensor connector. Water leak at the rear window may result in water in the connector.
^ M. Check the bulkhead connector for water or signs that water may have entered the connector and for loose or damaged pins or terminals.
^ N. Wiring may be damaged at or behind the large aluminum reinforcement located under the instrument panel at the left cowl. There are three (3) known locations the short may occur. They are: 1. At the base of the reinforcement - (inspect this area for damaged wire(s) which may be visible without removing the reinforcement). 2. The wiring may be pinched behind the reinforcement (remove reinforcement). 3. The wiring may have been pinched between the reinforcement and a bracket that is welded to the bulkhead.
REPAIR PROCEDURE
1. Any wiring on air bag system circuits may be repaired as follows:
a. If the insulation on any of the wires is nicked or cut, exposing bare wire, wrap affected area(s) with vinyl electrical tape.
NOTE:
USE FLAME-RETARDANT VINYL ELECTRICAL TAPE FOR ANY REPAIRS IN THE ENGINE COMPARTMENT.
b. If wire(s) are only nicked, wrap affected area(s) with vinyl electrical tape.
c. If wire(s) are cut, proceed to Step 2.
2. Cut a piece of suitable length, same gage replacement wire.
3. Slip Heat Shrink Tubing (WT-5627-A) over cut wire as shown in Figure 7.
4. Join replacement wire to existing cut wire and twist the ends together as shown in Figure 7.
NOTE:
ENSURE HEAT SHRINK TUBING IS ON CUT WIRE BEFORE SPLICING WIRES TOGETHER.
5. Using only 63/37 rosin core solder (do not use acid core), apply solder and heat to spliced wires. (If repair is to be made in the engine compartment, 60/40 rosin core solder may be used.)
WARNING:
USE ONLY A HEAT GUN. DO NOT USE A HAIR DRYER OR ANY FLAME TO HEAT THE SHRINK TUBING.
6. Position spliced wire so that shrink tubing can be positioned over soldered splice as shown in Figure 7.
7. Using Rotunda Heat Gun (107-R0300 or equivalent), apply heat to shrink tubing.
NOTE:
DO NOT OVERHEAT SHRINK TUBING. HEAT ONLY UNTIL ADHESIVE (HOT SEALANT) FLOWS FROM EACH END OF THE SHRINK TUBING AS SHOWN IN FIGURE 7. WHEN THE "HOT SEALANT" FLOWS FROM BOTH ENDS OF THE SHRINK TUBING, THE TUBING HAS BEEN HEATED ENOUGH AND AN ADHESIVE SEAL HAS BEEN FORMED BETWEEN THE HEAT SHRINK TUBING AND THE WIRE INSULATION.
NOTE:
USE FLAME-RETARDANT VINYL ELECTRICAL TAPE FOR ANY REPAIRS IN THE ENGINE COMPARTMENT. DO NOT ALLOW THE REPAIRED WIRING TO COME IN CONTACT WITH ANY METAL SURFACES IN THE AFFECTED AREAS.
8. Wrap the repaired area with vinyl electrical tape.
NOTE:
DO NOT ALLOW REPAIRED AREA TO COME IN CONTACT WITH ANY METAL SURFACES IN THE AFFECTED AREAS.
9. To avoid future contact with metal surfaces, perform the following:
a. Remove any existing burrs or sharp edges from metal surfaces that may come in contact with the repaired wiring.
b. Apply ordinary cloth tape to these areas.
c. Wrap nicked wiring with vinyl electrical tape.
d. Ensure wiring retainers are in the proper retaining holes.
e. Use an appropriate wiring retainer (chicken band) to pull wiring away from metal surfaces.
10. Replace the air bag diagnostic monitor. Refer to Section 01-02B of the appropriate Service Manual for removal and installation procedures.
WARNING:
CONNECT THE APPROPRIATE AIR BAG SIMULATOR TO AIR BAG WIRING BEFORE SYSTEM PROVE-OUT IS PERFORMED.
11. Reactivate the air bag system.
12. Perform the system prove-out as follows:
a. Turn ignition switch from OFF to RUN and visually monitor the air bag warning indicator. The air bag warning indicator will light continuously for approximately six (6) seconds and then turn off.
If an air bag system fault is present, the air bag warning indicator will either fail to light, remain lit continuously or flash after the prove-out. The flashing may not occur until approximately 30 seconds after the ignition switch has been turned from OFF to RUN. This is the time required for the air bag diagnostic monitor to complete testing of the system.
If the air bag warning indicator is inoperative and an air bag system fault exists, a tone will sound in a pattern of five (5) sets of five (5) beeps. If this occurs, the air bag warning indicator will need to be serviced before further diagnosis can be done.
NOTE:
THE ABOVE INFORMATION IS ALSO GIVEN IN SECTION O1-20B OF THE APPROPRIATE SERVICE MANUAL.
WARNING:
REMOVE THE AIR BAG SIMULATOR AND RECONNECT THE AIR BAG(S) WHEN SYSTEM IS REACTIVATED.
Parts Block
OTHER APPLICABLE ARTICLES: NONE
WARRANTY STATUS: Eligible Under The Provisions Of Bumper To Bumper Warranty Coverage And Restraint Systems Coverage
OPERATION DESCRIPTION TIME
950408A Perform Air Bag 0.9 Hr.
Diagnostics And Replace
Monitor
DEALER CODING
BASIC PART NO. CONDITION CODE
14B056 X1
OASIS CODES: 104000, 203000, 203100, 203200