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How to Test Dakota Park LampsPosted 10/1/2008
There was a day when turning on your lights just involved a switch, some wires and bulbs. That seems straightforward, right? But not anymore. Computers and multiplexed switches are the sign of the times. It seems strange that now I need a scan tool just to repair basic exterior lighting problems. Case in point: A 2005 Dodge Dakota pickup truck came in with the parking lamps not functioning. The problems started when the technician found no power at the fuses with the lights switched on. This system uses two modules and a relay to turn on the parking lights. The instrument cluster (IC) receives the switch inputs from the headlamp switch and then sends a bus message to the front control module (FCM) to turn on the lights. The FCM grounds the park lamp relay to turn the lights on.
The question is: Where do you start testing this system? You could start at either end, but I like to start with the switch inputs to the IC. Because the headlamp switch is a multiplexing switch, it varies the voltage supplied by the IC. If your scan tool will access the data in the IC, check the switch input to the IC as the switch is moved. This will indicate if the IC is receiving switch inputs. If your scan tool does not provide this information, use a digital volt/ohm meter (DVOM) to back probe the signal wire at the headlamp switch to monitor the voltage change there. The IC puts 5 volts out on the circuit and the switch varies it to ground; each position of the switch has a different voltage. If the headlamp switch is not working properly, remember to also check the sensor ground circuit - there should be less than 50 mV on this circuit. The sensor ground will not tell you if the IC is seeing the input, but it will tell you if the switch is working. Once you are satisfied that the IC is responding properly to the switch inputs, move to the next area of testing. The IC sends a command for the FCM to actuate the park lamp relay. Using your scan tool, check the FCM for communication codes or problems that might affect the communication between the FCM and the IC. If communication is OK, check the park lamp request from the IC to the FCM. The FCM will ground the control wire to turn on the park lamp relay. Some compatible scan tools will do an actuator test of the circuits, and in this case, it would be for the park lamp relay. During the test, the FCM will cycle the park lamp relay "on/off" every second. If your scan tool will not run the actuator test, have someone cycle the headlamp switch while you hold the park lamp relay to feel if it clicks. Clicking will indicate that the control side of the park lamp relay is working. Next, pull out the park lamp relay and jump pins 30 to 87, bypassing the relay, to see if the park lamps illuminate. This is an easy way to determine if the park lamp relay is bad. To test the operation of the FCM and the integrated power module (IPM), remove the FCM from the IPM (Note: this is the underhood fuse box). Next, ground Pin 48 at the IPM to actuate the park lamp relay. If the parking lamps do not come on, check the resistance between Pin 48 of the FCM connection point and Pin 85 of the park lamp relay. If an open circuit is found, the IPM is bad and will need to be replaced. With all the rapid changes in automotive technology, every few years will bring completely new systems to learn, and also the need for new equipment to help solve even those simple lighting problems. If you need any assistance, just give us a call.
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