Question about 2003 Mercury Grand Marquis

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Headlights won't come on, now car won't start.

For months now the headlights come on when they want so I just pull the brights on....and I have done lots of research on the pc. All say it's a FAULTY "Lighting Control Module" or LCM. But now....I went to start it today and got nothing, not even a lightbulb, and the LCM was clicking vigorously. Now I'm trying to find out if this LCM could be causing my MGM not to start. Someone also said it could be the factory installed alarm. I went looking for the alarm to disconnect it but could not find it. Any comments?

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  • dlaw5025 Apr 20, 2009

    This post is actually for a 2004 Mercury Grand Marquis. Not 2003.

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  • Mercury Master
  • 4,369 Answers

It is definitely the Lighting Control Module. Just fixed one for a friend. It has internal printed circuit board failure.
It is easy repair but it is expensive. New are 500. Controls much more than lights. Alarm, air bag, door key pad, etc.

You need to have correct part for 2003. tascafordparts,com has the for about $250.

It's just above brake pedal. remove the sond panel. The module will come out without removing other components.

Lighting Control Module (LCM)
The LCM eliminates the electronic interference and heat associated with the thermoelectric devices used previously. The LCM is a microprocessor- based module that controls several vehicle subsystems. It responds to electrical input signals from various switches, sensors, and external modules.
LCM exterior lighting subsystem functions include:

  • headlamps with autolamps (if equipped)
  • turn signal lamps/hazard flasher lamps
  • cornering lamps
  • parking lamps
  • battery saver
Principles of Operation
The lighting control module (LCM) is a microprocessor-based module that controls several vehicle subsystems. The LCM responds to electrical input signals from various switches, sensors, and external modules. The LCM is located under the LH side of the instrument panel. Subsystem functions of the LCM include headlamps with autolamps, turn signal lamps/hazard flasher lamps, cornering lamps, parking lamps, courtesy lamps, demand lamps, battery saver and control backlighting.
Module —Lighting Control (LCM) Removal and Installation

  1. Remove the LH instrument panel lower insulator.
    1. Remove the pin-type retainers.
    1. Disconnect the courtesy lamp.
Headlights won't come on, now car won't start. - 2baa00d.gif
  1. Position the LCM aside.
    1. Remove the screws.
    1. Release the locking tab.
b8e3948.gif
  1. Disconnect the electrical connectHeadlights won't come on, now car won't start. - ecda3be.gifove the LCM.
ecda3be.gif

Posted on Apr 20, 2009

  • gerry bissi Apr 20, 2009

    The Part Number you need is 4W7Z-13C788-BC (New updated part).



    The part in your car now is:



    Engineering Part Number - 3W7T-13C788-AH. This is the part number on your existing part. It is not the service part number. The service part number is 3W7Z-13C788-AH. So, if you see this one on Ebay, it will work. The 4W7Z above replaced this one for service.



    You will find some on Ebay, but you have to have one for a Crown Vic/Grand Marquis with this engineering part number.



    If you are a techie, what actually fails is the 2" flat plastic tape that connects the 2 circuit boards. Inside it are about 6 flat foil like "wires" that have cracks in them. I am going to try soldering jumper wires in the one I have and see if I can fix it.

    Good luck, and please rate this as a fixya if this helps you.

  • gerry bissi Apr 21, 2009

    I believe the 4W7Z applies to 2004 as well.

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Posted on Jan 02, 2017

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SOURCE: Please help me i need the Lighting Control Module diagram

http://i28.photobucket.com/albums/c216/thomaspedemon/99lcm1of3.gif

http://i28.photobucket.com/albums/c216/thomaspedemon/99lcm2of3.gif

http://i28.photobucket.com/albums/c216/thomaspedemon/99lcm3of3.gif

follow the three links for complete wiring diagram of lcm

Posted on Jun 30, 2009

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