Headlights and Daytime Running Light Problems in Dodge Caravan
Never had problems with your headlights in your old vehicle? Well, now you know why Chrysler is circling the drain. They’ve made the system so complicated that in some cases you need a scan tool to find out what’s wrong. Don’t own a $3,000 scan tool? Well, you can still do some of your own diagnostics. But first you have to understand the method behind Chrysler’s madness.
It’s like Old Mcdonald’s Farm
Chrysler wants every switch to go through a computer. Then one computer talks to another. The first computer is called the Body Control Module. It is the first stop for controlling all these functions:
Overhead console buttons, Remote keyless entry, Left Rear Lamp Assembly, Left stop/turn signal relay, Right combination relay, Right Rear Lamp Assembly, Left Front Door Lock Motor/Ajar Switch, Left Sliding Door Lock Motor, Left Sliding Door Lock Motor/Ajar Switch, Right Front Door Lock Motor/Ajar Switch, Front Reading Lamps/Switch, License Lamp, Power Liftgate Module, Rear Wiper Motor, Right Sliding Door Control Module, Right Sliding Door Lock Motor, Right Sliding Door Lock Motor/Ajar Switch, Center Dome Lamp, Left B-Pillar Switch, Left Rear Reading Lamp,
Liftgate Cylinder Lock Switch, Right B-Pillar Switch, Right Mid Reading Lamp, Right Rear Reading Lamp, Left Sliding Door Control Module, Left Cylinder Lock Switch,
Rear Automatic Temperature Control Switch, Rear Blower Rear Control Switch,
Right Cylinder Lock Switch, Right Sliding Door Control Module, Driver Heated Seat Module, Headlamp Switch, Overhead Console, Passenger Heated Seat Module Diagnostic Junction Port, Driver Door Lock Switch, Driver Heated Seat Module,
Hood Ajar Switch, Left Liftgate Flood Lamp, Liftgate Ajar Switch, Liftgate Cinch/Release Motor, Right Liftgate Flood Lamp, Center Dome Lamp, Controller Anti-Lock Brake, Floor Console Lamp, Front Reading Lamps/Switch, Left Visor/Vanity Lamp, Right Visor/Vanity Lamp, Left Mid Reading Lamp, Left Rear Reading Lamp,
Right Mid Reading Lamp, Right Rear Reading Lamp, Driver Door Courtesy Lamp,
Driver Door Lock Switch, Fuel Pump Module, Passenger Door Courtesy Lamp,
Passenger Door Lock Switch, Power Mirror Switch, Glove Box Lamp, Rear Automatic Temperature Control Switch
After getting inputs from all those switches and sensors, the BCM sends a digital signal to ANOTHER computer to do the actual switching. In the case of headlights, the BCM sends its digital signals to the front control module (FCM) located in the intelligent power module (IPM). Got that?
How the headlights work
The headlight switch doesn’t actually do any switching. It’s built with a series of resistors. The switch has 12volts coming into it. As you move the switch, the resistors reduce the voltage, depending on where you set the switch—high beams, low beams, parking lights, flashers, etc. The BCM sees the reduced voltages and signals the FCM to switch the power on and off to the lights. Here are the most common headlight problems and how to diagnose them
One headlight works on low beam and another on high beam. Or the lights work erratically.
Start your diagnostics by removing the FCM located in the IPM. You’re going to bypass that module and try to “hotwire” the headlights yourself to eliminate the possibility of a bad FCM. After you pull the FCM, look at the connector on the front of the IPM. You’ll be looking for terminals with numbers 1-9 on the bottom row. The numbers start at the center and count upwards towards the driver’s fender.
Attach a wire to the battery + terminal and touch the other end to terminals 3, 4, 5, and 6 one at a time. Check for headlight operation. Terminal 3 is the left low beam, 4 is the right high beam, 5 is the left high beam, and 6 is the right low beam. If the lights work, that indicates that the IPM is working properly and all the wiring is intact. The problem lies in the FCM, BCM, or the multifunction switch. However, the FCM is the most likely culprit.
If one of the lights does NOT work, then you have to check the green wiring connector located at the bottom of the IPM. You want to make sure you’re getting power through the IPM and into the wiring harness. So, as you jumper each terminal, check for the same voltage on the white/dark blue wire for left low beam, white/light green for left high beam, white/grey wire for right high beam, and white/tan wire for right low beam.
If you get power on all those wires but the headlights STILL don’t light up, then move on to this next step. The IPM provides both power and ground. The ground wire for the left headlight is black/green, and the ground for the right headlight is black/tan. The IPM ground wires are black/white and black/yellow. So, provide an alternate ground to the IPM. If it starts working, that indicates a bad ground on one of the IPM’s ground wires. If that still doesn’t fix the problem, provide an alternate ground to the headlight ground wires. If they work, then the problem is either an open ground wire between the IPM and the headlight, or a bad IPM.
Headlights flashing
This indicates a screwed up FCM. Just like your computer can get locked in a loop, so can the FCM. To “reboot” the FCM, disconnect the battery terminals and touch them together. This procedure shuts off the power and drains all the capacitors in the system. If the problem recurs, you must take the vehicle in to have the software updated. This is covered by TSB # 08-005-04. It is a software screwup where the computer thinks the vehicle has a factory alarm, when it really doesn’t. You’ll have to pay for the reprogramming (of course).
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© 2007 Rick Muscoplat
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