Ford Mustang (1999-2004) Service Manual: Fog Lamps

Refer to Wiring Diagrams Cell 86 , Fog Lamps for schematic and connector information.

Special Tool(s)

73 III Automotive Meter or equivalent 105-R0057

Principles of Operation

The fog lamps will only illuminate when the low beam or parking lamps are illuminated. Voltage from the parking lamp circuit is used to activate the fog lamp relay when the fog lamp switch is ON. Voltage from the high beam headlamp circuit will activate the fog lamp interrupt relay. When the fog lamp interrupt relay is active, the ground path is removed from the fog lamp relay, turning off the fog lamps.

Inspection and Verification

1. Verify the customer concern by operating the fog lamps.

2. Visually inspect for the following obvious signs of mechanical and electrical damage.

Visual Inspection Chart

Mechanical

Electrical

  • Damaged headlamp switch (11654)

  • Damaged fog lamp switch

  • Damaged multifunction switch (13K359)
  • Blown battery junction box (BJB) Fuse 27 (20A)
  • Blown central junction box (CJB) Fuse 38 (20A)
  • Damaged wiring harness
  • Loose or corroded connections
  • Damaged fog lamp relay
  • Damaged fog lamp interrupt relay
  • Damaged fog lamp

3. If the concern is not visually evident, determine the symptom and proceed to Symptom Chart.

Symptom Chart

Condition Possible Sources Action
  • The fog lamps are inoperative
  • BJB Fuse 27 (20A).
  • CJB Fuse 38 (20A).
  • Circuitry.
  • Fog lamp relay.
  • Fog lamp interrupt relay.
  • Headlamp switch.
  • Fog lamp switch.
  • Multifunction switch.
  • GO to Pinpoint Test O .
  • The individual fog lamp is inoperative
  • Fog lamp bulb.
  • Circuitry.
  • GO to Pinpoint Test P .
  • The fog lamps are on continuously
  • Fog lamp relay.
  • Circuitry.
  • Fog lamp switch.
  • GO to Pinpoint Test Q .

Pinpoint Tests

PINPOINT TEST O: THE FOG LAMPS ARE INOPERATIVE

Test Step Result / Action to Take
O1 CHECK THE PARK LAMPS Yes

Turn off the headlamp switch.

GO to O2 .

No

GO to Pinpoint Test L .
  • Place the headlamp switch in the PARK position.
  • Place the multifunction switch in low beam position.
  • Do the park lamps illuminate?
O2 CHECK THE FOG LAMP RELAYS Yes

GO to O3 .

No

INSTALL new relay(s). TEST the system for normal operation
  • Disconnect: Fog Lamp Relay.
  • Disconnect: Fog Lamp Interrupt Relay.
  • Check the fog lamp relay and fog lamp interrupt relay; refer to Wiring Diagrams Cell 149.
  • Are the relays OK?
O3 CHECK THE CIRCUIT 12 (LG/BK) Yes

GO to O4 .

No

REPAIR the circuit. TEST the system for normal operation.
  • Measure the voltage between fog lamp interrupt relay connector Pin 1, Circuit 12 (LG/BK), harness side and ground.

  • Is the voltage greater than 0 volts?
O4 CHECK CIRCUIT 275 (YE) Yes

GO to O5 .

No

REPAIR the circuit. TEST the system for normal operation
  • Measure the resistance between fog lamp interrupt relay connector Pin 4, Circuit 275 (YE), harness side and fog lamp relay connector Pin 85, Circuit 275 (YE), harness side.

  • Is the resistance less than 5 ohms?
O5 CHECK CIRCUIT 477 (LB/BK) FOR OPEN Yes

GO to O6 .

No

REPAIR the circuit. TEST the system for normal operation.
  • Measure the voltage between fog lamp relay connector Pin 30, Circuit 477 (LB/BK), harness side and ground.

  • Is the voltage greater than 10 volts?
O6 CHECK CIRCUIT 188 (WH/BK) Yes

GO to O7 .

No

REPAIR the circuit. TEST the system for normal operation.
  • Disconnect: Fog Lamp Switch C240.
  • Measure the resistance between fog lamp switch C240 Pin 1, Circuit 188 (WH/BK), harness side and fog lamp relay connector Pin 86, Circuit 188 (WH/BK), harness side; and between fog lamp switch C240 Pin 1, Circuit 188 (WH/BK), harness side and ground.

  • Is the resistance less than 5 ohms?
O7 CHECK CIRCUIT 1205 (BK) Yes

GO to O8 .

No

REPAIR the circuit. TEST the system for normal operation.
  • Measure the resistance between fog lamp interrupt relay connector Pin 2, Circuit 1205 (BK), and ground.

  • Is the resistance less than 5 ohms?
O8 CHECK THE FOG LAMP SWITCH Yes

REPAIR Circuit 478 (TN/OG).

TEST the system for normal operation.

No

INSTALL a new fog lamp switch.

TEST the system for normal operation.
  • Measure the resistance between fog lamp switch C240 Pin 1, Circuit 188 (WH/BK), connector side and fog lamp switch C240 Pin 2, Circuit 14 (BN), connector side with the fog lamp switch on.

  • Is the resistance less then 5 ohms?

PINPOINT TEST P: THE INDIVIDUAL FOG LAMP IS INOPERATIVE

Test Step Result / Action to Take
P1 CHECK THE GROUND TO THE FOG LAMP Yes

REPAIR Circuit 478 (TN/OG) for open.

TEST the system for normal operation.

No

REPAIR Circuit 1205 (BK). TEST the system for normal operation.
  • Key in OFF position.
  • Disconnect: Inoperative Fog Lamp.
  • Measure the resistance between inoperative fog lamp C162 Pin 1 (RH) or C152 Pin 1 (LH), Circuit 1205 (BK), harness side and ground.

  • Is the resistance less than 5 ohms?

PINPOINT TEST Q: THE FOG LAMPS ARE ON CONTINUOUSLY

Test Step Result / Action to Take
Q1 CHECK THE FOG LAMP RELAY Yes

GO to Q2 .

No

REPAIR Circuit 478 (TN/OG). TEST the system for normal operation.
  • Disconnect: Fog Lamp Relay.
  • Are the fog lamps off?
Q2 CHECK FOG LAMP RELAY Yes

RECONNECT the fog lamp relay. GO to Q3 .

No

INSTALL a new fog lamp relay. TEST the system for normal operation.
  • Check the fog lamp relay; refer to Wiring Diagrams Cell 149.
  • Is the fog lamp relay OK?
Q3 CHECK FOG LAMP SWITCH Yes

INSTALL a new fog lamp switch. TEST the system for normal operation.

No

REPAIR Circuit 188 (WH/BK) for short. TEST the system for normal operation.
  • Disconnect: Fog Lamp Switch C240.
  • Are the fog lamps off?
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