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Stop Light Lamp Circuit pdf
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This pdf shows the brake light circuit for the NAS LR3.
Note that power for the brake light bulbs can come either thru the brake light switch power feed /contacts plus also from the Hill Decent Control, (HDC), system relay located within the Central Junction Box, (CJB) - yes two separate sources of power for the brake light bulbs. This would tend to explain how a brake light circuit concern shows up as a hill decent fault message; also why lesser vehicles without hill decent braking mechanisms, (HDC), never display this sort problem when a brake light bulb or switch acts up. You never have a problem related to a system that does not exist.
Additional evidence of the relationship of the HDC and the tail light circuit is demonstrated by when the HDC actually acts to slow the 3 down, the brake lights automatically illuminate even if the slowing action was merely retarding the engine speed or shifting to a lower gear, (in other words, no foot on the brake pedal).
There is also a tie in to the Cruise Control and hence a failing brake light switch can affect cruise operation or just create false Cruise Control messages. To further complicate the issue, while there are no messages generated, operation of the radiator electro viscous fan can also be negatively affected by the fouled set of secondary contacts within the brake light switch. This is all because the NC contacts within the brake switch send misleading messages to the ECU.
While the ground wire design is similar to other vehicles, note the grounds for the brake light filament and the reverse light filament are common on each side. That means that if one has a non functioning brake light on one side, the reverse light may not also work or that some problem with the reverse lights could back feed into the brake lights. This situation while unusual, can result when grounds are corroded probably due to swimming in rivers and creeks or just routine driving in a marine environment.
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