Jim Grant's Tech Tips
Grand Marquis, Phantom Dome Lights
I have a '99 Grand Marquis that the dome lights keep cycling on and off
with the ignition off. Sometimes they will cycle on and slowly dim just
like the ignition was turned off but will almost immediately pop back
on. This can go on for hours. Any solutions?....Wayne
The interior lighting in your vehicle is controlled by a Lighting Control
Module (LCM). The LCM has control over the 2 lighting functions provided
in your vehicle, courtesy lighting and demand lighting, in addition it
has a battery saver feature that will turn off these lights and the headlights
if left on with the key off. Erratic light operation would indicate the
possibility of switch problems, wiring or the LCM itself is defective.
Diagnosis of LCM problems is performed the easiest with a computer scan
tool that connects to the communication port for the LCM. The LCM can
perform self-diagnosis and report any problems found to the scan tool
in the form of a diagnostic trouble code that can guide a technician in
diagnosing and repairing the system. If you think that maybe youll
try a used LCM from a salvage yard, there something you should know. LCMs
are programmable, they may look the same but the program in the LCM may
not match. The only way to program a LCM is once again with a scan tool.
Your best bet, get your vehicle plugged in. Itll take the guess
work out of the needed repairs.