Typical fog lights illuminated.

Well-designed vehicle lighting systems enhance vehicle safety by increasing the driver’s visibility while operating the vehicle and by clearly signaling the driver’s intent to those around the vehicle Figure 39-1. Lighting systems are also used inside the vehicle to indicate messages to the driver and provide convenience to any occupants. Manufacturers continue to improve and develop their lighting systems as new technology becomes available. Modern lighting system features include the increased use of light-emitting diode (LED) and xenon lighting, electronic body control units to manage lighting, and high-intensity discharge (HID) lamps, which are much brighter than the traditional sealed-beam or halogen units. Many vehicles also have systems that warn you if lamps are not working properly.

You are the Automotive Technician
A customer brings his 2010 Dodge Avenger into the auto dealership for a 60,000 mile service checkup. You take the vehicle back to the service bay. Part of the 60,000 mile service is to check the lighting and peripheral systems. Your coworker operates each of the lights while you check the front lights and then the rear lights. You notice that the center high mount stop light (CHMSL) on the trunk lid is not functioning, but the brake lights are working normally. The customer agrees to allow you to diagnose it. After referring to the manufacturer’s information and wiring diagrams, you see that the CHMSL is on the same fuse as the brake lights, so that does not need to be investigated further. You see also that the brake switch feeds both the CHMSL and brake lights, so that does not need to be investigated further. Based on your reading of the wiring diagram, you determine that the problem is between the brake switch and CHMSL, between the CHMSL and the ground, or the CHMSL bulbs or unit are faulty. You test the power and ground at the CHMSL terminals with the brake pedal applied. You find 12.2 volts available. Since the CHMSL is a sealed LED-style assembly, you know it will need to be replaced. The customer approves the repair, and you install the new CHMSL and test the vehicle again. Your coworker verifies that the light is working properly. You review the work and invoice with the customer, thank him for his business, and set him up in the automated email system to send a reminder to schedule his next maintenance visit.
  1. Why didn’t you have to check the fuse or brake switch?
  2. What did the 12.2 volt reading at the CHMSL terminals tell you?
  3. How do wiring diagrams save time?