Question about 2003 Mercury Grand Marquis

1 Answer

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?

Posted by on

  • dlaw5025 Apr 20, 2009

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



1 Answer

  • Level 3:

    An expert who has achieved level 3 by getting 1000 points


    An expert that got 20 achievements.


    An expert that got 10 achievements.


    An expert that got 5 achievements.

  • 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.
  1. Disconnect the electrical connectHeadlights won't come on, now car won't start. - ecda3be.gifove the LCM.

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.



2 Suggested Answers

  • 2 Answers

SOURCE: I have freestanding Series 8 dishwasher. Lately during the filling cycle water hammer is occurring. How can this be resolved

A 6ya expert can help you resolve that issue over the phone in a minute or two.
Best thing about this new service is that you are never placed on hold and get to talk to real repairmen in the US.
The service is completely free and covers almost anything you can think of (from cars to computers, handyman, and even drones).
click here to download the app (for users in the US for now) and get all the help you need.
Good luck!

Posted on Jan 02, 2017

  • 94 Answers

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

follow the three links for complete wiring diagram of lcm

Posted on Jun 30, 2009

Add Your Answer

Uploading: 0%


Complete. Click "Add" to insert your video. Add



Related Questions:

2 Answers

What can cause the headlights to flicker steady an constant from dim to bright from time u start the car annturn it back off?

I am thinking a faulty alternator. Take it to an auto electrician to test the voltage output.

Before hand, you could try removing the battery leads, cleaning the contacts and coating with Vaseline and reconnecting.

How old is the battery? 5 to 6 years is the average life.

Nov 15, 2015 | Cars & Trucks

1 Answer

Headlights seem to drain the battery quickly... whats up?

check alternator - should have 13.5 to 14.8 volts when running

Jan 18, 2013 | 2000 Jeep Grand Cherokee

1 Answer

The dimmer switch on my 1999 firebird won't change the headlights from dim to bright, or vice versa. It has a good click to it and the blue light in the dash comes on when they're supposed to

That is a common problem, the switch needs changing if the high and low beams do work, the only problem is that it's an expensive part to replace, try a local wrecking yard they should have a bunch of suitable replacements at a reduced price.

Sep 19, 2012 | Pontiac Cars & Trucks

1 Answer

Need wiring diagram of headlights on 1999 dodge dakoa

This is a common problem and is caused by a burnt headlight switch and headlight switch connector, replace both of them,

Apr 22, 2012 | 2002 Daewoo Nubira

1 Answer


For all types of car headlights repair and service issues.Click the link below and follow the troubleshooting:----All headlights not working?
--------headlights flashes while start?
---------Headlights flickering and car hesitates to start?
----------Both headlights and turn signals not working?
--------Headlight wiring diagram ?
---------This will help.Thanks.

Feb 19, 2012 | 1999 Kia Sephia

1 Answer

Won't turn over

Turn on your headlights and go check to see how bright they are or if they are on. If they are not on when the headlight switch is pulled then check to see if you have a dead battery or your cables are loose on the battery.

Dec 12, 2011 | 2004 Toyota RAV4

2 Answers

When I pull back to put headlights on bright it doesn't stay. I do not feel the click. Headlights will go bright as long as I hold back on it. Everything else is working fine. I am being told it will cost...

/// new column needs a multi function switch 500 dollars but i would wire around it and add old time floor switch done it lots and it works well get a diagram from the library to find the wires to cut ground switched i believe

Feb 27, 2010 | 2000 Chevrolet Impala

1 Answer

Headlights wont dim but work when clicked on bright

Replace both low beam headlights. It's common to have one headlight burned out and you may not realize it for months. Then the other one goes out and "puff" you have NO low beams.

Feb 20, 2010 | 1990 Ford LTD Crown Victoria

Not finding what you are looking for?
2003 Mercury Grand Marquis Logo

1,646 people viewed this question

Ask a Question

Usually answered in minutes!

Top Mercury Experts


Level 3 Expert

78267 Answers


Level 3 Expert

5655 Answers

Colin Stickland
Colin Stickland

Level 3 Expert

22326 Answers

Are you a Mercury Expert? Answer questions, earn points and help others

Answer questions

Manuals & User Guides