Symptom: a 2007 model Shanghai GM Buick LaCrosse 2.4 L sedan, braking occurs when two rear brake lights are not bright, but brake lights normal phenomenon.
Check Analysis: validation failure phenomenon found step on the brake pedal when the meal, the normal brake lights lit, but the two rear brake lights are not lit, the other tail mini elm327 lights are working properly. This thought is very simple, after removing the taillights, brake light bulb replacement can be solved, but found the rear brake replacement brake light bulb, the two rear brake lights still do not light up. After using a multimeter to test two brake lamp, no power supply. Remove the front passenger side of the fuse box cover inspection, brake lights the fuse is good, but on which there is a voltage of 12 V. Check car circuit diagram, which has been found to LaCrosse sedan brake light control circuit and other Buick car is different from the original brake light switch turns the brake pedal position sensor, the sensor signals to the body control module (BCM after), BCM receives the signal output by the terminal J2-30 12V voltage signal, which is divided into two-way, all the way to direct light up brake lights, another way to control parking lights relay, the relay is controlled by two rear brake lights. According to the circuit diagram analysis, braking brake lights can be lit, but the two rear brake lights are not lit, so that ruled the brake pedal position sensor and the BCM failure may fault the range of BCM's output multidiag pro terminals J2-30 after the circuit, including the parking lights to the rear brake lights relay lines. Open the cover of the instrument panel on the left side, under the two relays, the relay is in blue lights relay, the relay measurements, found 12V voltage input, but no voltage output, determined that the relay is damaged.
Troubleshooting: Replace the parking lights relay.
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