Tech Tips (by ALLDATA): ‘Weird’ Ford truck lighting issues

Mileage: 199,556

Problem: When applying the brakes with doors closed and engine running, the dash and speedometer lamps dim. When the door is open and the truck is running, the warning chime sounds when brakes are applied. The problem does not occur when the brake pedal is released.

NOTE: This truck has a cab guard with aftermarket brake lamps and lights installed on the guard.

Details: When the tech checked brake lamps, they all operated normally. Next, he checked some grounds for the brake lamp switch and they were good at ground #G300. There were two fuses that fed power to the brake switch: #34 and #18. The problem stopped when fuse #18 was removed. That fuse fed the rear brake lamps. NOTE: If a stop lamp bulb element is broken and shorting against the park lamp element inside the same bulb, it can cause power to feedback and the illuminate the dash lights. He disconnected each side of the aftermarket cab guard lamps individually. He found that one of the cab guard side lamps was faulty.

Confirmed Repair: The tech replaced that lamp assembly and the truck’s weird electrical issue was repaired.