Interior Light Problems and Solutions

Drivers don’t realize how important interior lights are until they stop working. Entering a vehicle on a dark night, locating the correct key and the location of the ignition barrel is difficult with no dome lights. In the 70s, the courtesy light circuit was a gloriously simple system containing just a fuse, bulb and pin switch to turn it on. In the late 80s through current day manufacturers have added timers, modules and advanced electronics to enhance user experience. This article will cover operation and problems with interior lights as well as under hood and trunk area lighting.

Interior Light Operation

The days of the simple single dome light are long gone as interior light packages grow increasingly complex. In addition to a center mounted headliner bulb it’s common to have courtesy lights mounted in the door panels and under the dash to illuminate the foot well areas of the driver and passenger compartment. Although much has changed, for the most part the manual switch that turns on the lamps has stayed the same.

Most automobiles still utilize a pin switch that closes the contacts allowing current to flow to the bulbs when the doors open. There are some manufacturers that prefer to locate this switch inside the door latch. For those wishing to enter the vehicle discreetly canceling mechanisms allow for the opening of the door without turning on the lights. Although manufacturers will use different methods General Motors has a canceling switch mounted near the headlight controls allowing drivers to override the automatic function.

In addition to the interior lights will be additional components that usually run on the same circuit. As an example it is common to find a glove box light that operates off of its own pin switch illuminating a lamp when the glove box is open. In the same respects a hood and trunk light are also included in the courtesy light circuit and protected by the same fuse. Often these two lights are turned on and off automatically by a mercury switch instead of a pin switch. When the hood or trunk swings open a conductive ball of liquid mercury completes the circuit turning on the lamp.

Fancy Illuminated Entry Systems

Illuminated entry systems are often described as lights that come on as you approach the vehicle. Older systems before remote key fobs were widely available often illuminated a ring around the door lock cylinder when the handle was raised. Now that remote controls come with most vehicles, when the unlock button is selected, the illuminated entry system is activated automatically.

Since the lights are turned on before any doors are opened, it requires additional parts to complete this task. Probably the most important is a control module. This component allows for multiple inputs such as lifting the driver side door handle, pushing the remote button and on newer cars receiving a radio signal emitted from the key fob as the driver approaches the vehicle. All of these inputs are capable of turning on the interior light package allowing for easier entry.

Interior Light Problems

Although the systems offered on today’s automobiles continue to get more complicated, courtesy light problems are few and far between. When problems to develop ease of diagnosis is directly related to the complexity of the system being diagnosed. In the case of an interior lighting system that has modules, individual inputs need to be tested for proper operation.

This is best accomplished using a wiring diagram for the specific automobile. This way a meter can be used to check for input signals at the module. If input signals are functioning and no output signals are measured moving away from the module, it is possible for the brain itself to have experienced failure.

Interior light problems are rare but can still develop in basic systems such as blown bulbs, open fuses, corroded sockets and poor wiring connections. Interesting enough, when professional mechanics find an automobile with interior light problems, they often find a vehicle with continuous interior water leaks from T tops or sunroof problems. Constant moisture inside the vehicle can cause malfunctions with connections, corrosion in the sockets and moisture in the housing can cause repeat bulb failures.