Fix headlights speedometer turn signals that don’t work on Chrysler trucks
Remember the days when you could turn on your headlights with the flick of a switch? Back then the switch actually “switched” the power to the headlights. But in late model vehicles, the switch just gives a signal to a body control module (BCM), telling it that you want the headlights.
Dodge has been using this system on its full sized “DR” trucks since 2002. If you’ve got a lighting problem and want to fix it yourself, you’ll have to read this to understand how the system works.
Dodge built their BCM into the instrument cluster, but they call it the cabin compartment node (CCN). They also refer to is as the electro-mechanical cluster (EMIC). The CCN is basically a small computer. It receives data from other modules and processes it. For example, it receives digital information about the vehicle speed sensor (VSS) from the powertrain control module (PCM). The CNN processes the speed information and displays it for the driver. The CNN is programmed at the factory with the vehicle’s vehicle identification number (VIN) and starting mileage. So yes, the CNN becomes your odometer as well.
The headlight switch is wired into the CNN and gets a 5-volt reference signal. When you move the switch from off to parking lights, headlights, high beams, or hazards, the switch sends a multiplexed voltage signal back to the CNN for interpretation. The CNN performs constant diagnostics on the signals from all incoming switches and reports a fault code if it thinks something is off. Once the CNN determines what you’re asking from it, it sends a digital command to the front control module (FCM).
The FCM is basically the power center for all these accessories. It contains solid state switching “drivers” (switching transistors), as well as mechanical relays. Solid state high-side drivers (HSD) switch power on and off in a circuit. Relay drivers switch the ground side of a relay coil on and off, thus energizing or de-energizing the solenoid coil. And solid state push-pull drivers also switch either power or ground to a circuit. The FCM is smart enough to conduct its own diagnostics and report fault codes, just like the CNN and PCM. For example, if the FCM senses excessive current draw, it acts like a fuse and disconnects power to that circuit and sets a trouble code. Similarly, if it detects a short it also disconnects the circuit. Later, it checks the wiring again for a short and if the condition is gone, it resets that circuit.
The FCM uses HSDs to control the headlights, turn signals, backup lamps, brake lamps, and the windshield washer pump.
The FCM uses relay drivers to control the fog lamp relay, horn relay, parking lamp relay, trailer stop/turn signal relays (left and right), windshield wiper high/low relay, and windshield wiper on relay.
The trailer stop/turn relays cannot be changed out, you must replace the entire power distribution center (PDC) or the integrated power module (IPM) [smart Chrysler, really smart—everybody thanks you for that brilliant engineering decision].
Let’s get back to headlights. You switch the headlight switch to ON. The CNN senses the signal and sends a digital message to the FCM. The FCM operates HSDs to operate the headlights. So, if you’ve got a problem with headlights or any of the systems listed here, you have to check the reference voltage at the switch, the wiring harness between the switch and the CNN, the integrity of the data buss between the CNN and the FCM, the operation of the FCM itself, and the wiring and operation of the PDC and IPM.
© 2012 Rick Muscoplat
Posted on by Rick Muscoplat