Diagnose and fix Dodge headlights don’t work condition
If you encounter a Dodge headlights don’t work condition, you’d better be prepared to learn new technology. In the old days the headlight switch actually switched power to the headlights. No anymore. These days, the Dodge headlight switch just acts as a signal input to the body control module (BCM), telling the BCM your headlight request. From there, the BCM sends a DIGITAL signal to the front control module (FCM) located in the Totally integrated Power Module under the hood. It’s the FCM that actually switches power.
How Dodge headlights work
The headlight switch is built with a set of resistors wired in series. The power comes into the headlight switch from the BCM and, depending on what you’re requesting, the power flows through as many as six resistors wired in series before flowing back to the BCM. The BCM uses the incoming voltage to decipher your lighting request.
Refer to the headlight wiring diagram below
Set the headlight switch to AUTO
When the headlight switch is set in the 0 or AUTO position, the BCM sees a 3.3 volt return voltage. It then consults the ambient light sensor, the IGN switch and the gear you’ve selected. If you’re driving in daylight and the vehicle is equipped with the daytime running lights (DRL) feature, the ambient light sensor will tell the BCM that it’s daylight. When you put the vehicle into a drive gear, the BCM tells the FCM to enter DRL mode. In DRL mode the FCM pulses power to the HIGH BEAMS. By pulsing power, the vehicle can meet the lumen requirements of DRL yet still save power by turning off the high beams for about 10% of the time.
Set the headlight switch to position 2 Park lamps
When the headlight switch is set to the PARK position, the return voltage to the BCM is 2.4-volts. The BCM interprets that voltage as your request to turn on only the parking lamps.
Set the headlight switch to 3 Headlight position
When the headlight switch is set to the HEADLIGHT position, the return voltage to the BCM is 1.4-volts. The BCM interprets that voltage as your request to turn on only the low beams.
Set the headlight stalk to High beams
When the stalk is moved to flash to pass or the high beam position, a separate set of resistors are used (not shown in illustration). The BCM interprets this different voltage as a request for high beams.
Test the Dodge headlight switch
In order to accurately test the system, you
should use a scan tool with live date. Then operate the headlight switch and note the voltages the BCM see at each setting. If you don’t have a scan tool, you can backprobe the white/brown and white/yellow wires and note the voltage readings on a DVOM
See the connector view below
If you can’t test and must take a chances with part replacement, replace the headlight switch first. It has the highest failure rate.
Posted on by Rick Muscoplat
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