Pinpoint Test I: The STOPLAMPS Are Inoperative

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

I1 CHECK THE VOLTAGE TO THE STOPLAMP SWITCH
- Key in OFF position.
- Disconnect: Stoplamp Switch C278.
- Measure the voltage between stoplamp switch C278-1, circuit 10 (LG/RD), harness side and ground.
  
  Fig 1: Measuring Voltage Between Stoplamp Switch C278-1, Circuit 10 (LG/RD), Harness Side And Ground
  Courtesy of FORD MOTOR CO.
- Is the voltage greater than 10 volts?
- Yes : GO to I3.
- No : GO to I2.
I2 CHECK CIRCUIT 10 (LG/RD) FOR AN OPEN
- Disconnect: Central Junction Box (CJB) C270f.
- Measure the resistance between CJB C270f-3, circuit 10 (LG/RD), harness side and stoplamp switch C278-1, circuit 10 (LG/RD), harness side.
  
  Fig 2: Checking Circuit 10 (LG/RD) For An Open
  Courtesy of FORD MOTOR CO.
- Is the resistance less than 5 ohms?
- Yes : REPAIR or INSTALL a new CJB. TEST the system for normal operation.
- No : REPAIR the circuit. TEST the system for normal operation.
I3 CHECK THE STOPLAMP SWITCH OPERATION
- Connect a fused (20A) jumper wire between stoplamp switch C278-1, circuit 10 (LG/RD), harness side and stoplamp switch C278-2, circuit 511 (LG), harness side.
  
  Fig 3: Checking Stoplamp Switch Operation
  Courtesy of FORD MOTOR CO.
- Do the stoplamps illuminate?
- Yes : INSTALL a new stoplamp switch. REFER to STOPLAMP SWITCH . TEST the system for normal operation.
- No : REPAIR circuit 511 (LG). TEST the system for normal operation.