A good number of modern production passenger vehicles use some sort of
computer to control the headlights, parking lights, turn signals, dome
lights and other illumination. 1995 and later ford crown victorias are
no exception, the only exterior lighting that is not computer
is the brake lights and spotlight (if so equipped). The factory service
manual calls the computer in question a "Lighting Control Module" (LCM)
but many mechanics refer to it as a GEM (Generic Electronic Module).
The module has
some nice features, like if you accidentally leave the headlights on
you shut off the ignition, the car will shut them off automatically
10 minutes. The same is true if you leave the door ajar, the computer
automatically turn the domelight off after a predetermined interval so
as not to drain the battery.
Opening a door is only an input to the computer, the computer then
decides how to act on the output. In the case of police interceptors
with "Dark Car" mode
enabled, the response is to ignore the open door signal and not turn on
the domelight. Turning on the switch to your headlights or parking
lamps is likewise just a input to the computer.
Note that 1995 and later crown vics & grand marquis do not
use a conventional standalone turn signal flasher module. But this does
not deter numerous aftermarket parts stores and backyard mechanics from
insisting that they do. I find it humorous everytime I hear of an
backyard mechanic purchasing a can flasher for $5.99 and then tearing
half their vehicle's dashboard looking for the old one, but the vehicle
owner is typically very frustrated and mutters many curse words in the
of looking for the nonexistent part. Some have also observed that the
on 95'-00' crownvics has a properly shaped place where a can flasher
fit, but there are no electrical contacts in the socket. This is
the ATC fuseblock is a common part shared among numerous vehicles and
saw no need to redesign it until the crown vic went with mini-fuses in
2001 model year.
Below are a couple pictures of the circuit boards inside the
control module case. The smaller relay is for the turn signals, the
larger ones are for the low beam headlights, high beam headlights,
lamps and demand lighting output. The round device labelled "TMX-06
is the seatbelt/headlight/airbag audible warning buzzer. Besides the
controlling the lighting, the LCM also controls rear window defroster,
but the relay for defroster is located in the engine bay and not on the
LCM circuit boards like the rest of the relays.
Physically Locating the LCM:
It should be noted that the pictures above show the lighting control
module with it's black metal case removed. The lcm may appear smaller
than the pictures above portray because the the white ribbon cable
the two circuit boards is flexible and the two circuit boards lay one
top of each other inside the case, not side by side like pictured
Also note that the lcm circuit boards have a "conformal coating"
applied to them to inhibit corrosion of the metal parts underneath.
This makes the circuit boards feel somewhat "gooey" if you touch them
with your fingers and also creates a "white out" effect when taking a
picture of them with a camera.
The lcm is mounted in the same location in all 1995 and later crown
vics. It's under the dash, above the gas pedal in between the climate
controls and steering column. If you're having trouble locating it,
engage the hazard flashers and listen to where the source of the
clicking sound is coming
from. You can also place your keys in the ignition and open the drivers
door and track down the module by listening to where the warning buzzer
tone is coming from.
If you need a visual reference, pictures of a mercury grand marquis
dashboard removed from the car with the lcm still attached are
avaliable by clicking here.
Year to Year compatibility:
Some have asked about LCM compatibility between model years, below
is a chart listing the lighting control processors used in crown
victorias and grand marquis.
|Service Part Number
|Legacy Service Part Numbers
|Extra electrical connector for highbeam
headlight bulbs that 1998-2002 LCM's do not have
|New body style with one dual filament bulb per
ID# XW7T-13C788-AA, XW7T-13C788-BA
|New Airbag Restraints Control Module
|Electrical Connectors Redesigned
Also used in the 2003-2004 mercury marauder
|New pcm with canbus support.
New steering column. Brake shift interlock solenoid now controlled
directly by lcm. New key in ignition lock sensor setup.
overspeed function moved inside lcm instead of using external module
|Electrical Connectors Redesigned
New can bus instrument cluster
Lincoln towncar now uses the same lcm as the ford crownvic.
- 1992-1994 crownvics do not use an lcm. The headlight switch
carries the full electrical load of the headlight bulbs and parking
- 1995-1997 lcm's have one additional electrical connector on them
the highbeam circuit that the 1998 and later models do not.
Additionally, 1997 and prior vics have a 4 headlight bulb system, 1998
and later ones have a 2 headlight bulb system.
- 2000 was the first year for the "belt minder" system that sounds
warning buzzer every few minutes until the driver buckles their
seatbelt, it was also the first year for the all red police taillights
with two bulbs fewer than its civilian counterpart. The 2000 electrical
connector pinouts are identical to the 1998 and 1999 connectors.
- 2001 and 2002 crownvics use the same electrical connectors as
years but the function of a few of the pins has changed a little due to
the addition of airbag restraints control module.
- 2003 and later vehicle use connectors that have physically
dimensions than earlier vehicles did.
- Police interceptors use the same lcm's as the civilian crownvics
- Do note that the 1995-2005 crownvic/grandmarq lcm's will NOT fit
in the lincoln towncar. However starting in the 2006 model year,
lincoln towncars started using the same lcm as the ford crown victoria.
- An interesting microsoft excel document listing the engineering
part numbers for all the 2003-2005 lcms and what the differences are
between them is avaliable by
- A document showing the configurable options inside a 2008 lcm is
avaliable by clicking here.
- TSB 02-9-7 is avaliable here.
- The part numbers listed in the "Legacy Part Numbers" column have
been discontinued. When you order a new lcm, you're going to recieve
the one listed in the "Part Number" column unless your particular ford
dealer has some old stock sitting on their parts shelves.
LCM related problems:
The buzzer inside the lcm will periodically chime if the airbag bulb in
the instrument cluster has burnt out or has been removed. If you hear
random chimes from the lcm, check to make sure that the airbag light
turns on for a couple seconds after you start the car. Having all of
the lights in the instrument cluster turn on for a couple seconds and
then go off is is a built-in diagnostic procedure called bulb prove out.
If you're trying to diagnose a lighting related issue and are
considering installing a used salvage yard processor, be warned that
lcm's from wrecked vehicles often get damaged in the collision.
According to Bluesmobile (firstname.lastname@example.org), "there is about a 90%
chance of the thing being bad if it comes from a wrecked car, they get
fried if any of the lights were shorted out during the wreck or if the
car sustained a heavy jolt it will also fry it."
And here's another warning from the ford factory service manual for
police equipment installers: "Any Auxiliary Warning Lights, controlled
by the Lighting Control Module (LCM), need diode protection to prevent
current spike damage to the LCM."
Erratic Turn Signals:
A few crown vic owners have reported the turn signals flashing very
fast at random points in time when the climate control unit fan speed
is set on "high". There is typically no set pattern to the behaviour,
appears to occur at the greatest frequency when the vehicle is driven
in stop and go traffic. I had seen this behaviour on my 98' police
interceptor a couple times, but after replacing the wornout blower
motor the problem started occuring almost every time I drove the
vehicle and had the blower motor
set on high. Further investigation revealed that I was getting close to
a 2V voltage drop between the alternator output stud and some of the
inside the car after the ignition switch when the blower motor was set
Suspecting that the erratic turn signals were caused by too low of
a voltage getting the multifunction switch and other ignition switched
components, I decided to rewire my vehicle to test the theory. My
interceptor has two police power leads, one always hot, the other hot
the key is in the run or acc positions. First I removed the climate
head from the dash, then cut the BR/O wire in two on the back. Took one
of the coil wires of a generic 30amp SPST relay and hooked it to
hooked the other coil wire to the BR/O wire coming out of the dash
Connected one terminal of the relay contacts to the constant hot police
lead and the other to the BR/O wire that ran into the climate control
Had now reduced the load for the blower motor that flows through
ignition switch from over 20Amps to around 0.2Amps. Turn signals behave
now. The sporadic problem of only the left turn signal indicator, not
right flashing when I engaged the hazard flashers had also corrected
Further details about this topic are avaliable by clicking here.
Alternatively, some people have reported successfully restoring a
normal blinking rate to their turn signals by wiring another turn
signal bulb and socket in parallel with one of the existing bulbs.
LED (Light Emitting Diode) Turn Signal Bulbs:
Installing LED turn signal "bulbs" will cause the lcm to think that
a bulb has burned out because they consume very little power compared
to the incandesant bulbs that the car came with from the factory. The
practical way to get the 98+ cars to work with led turn signal bulbs
be to reinstall the original factory turn signal bulbs or rewire the
and install an aftermarket flasher module designed to work with the LED
turn signal bulbs. Be warned the rewiring project mentioned is not for
someone that doesn't know how to read electrical schematics and use a
2003-2005 Headlight Problems
In the 2003 model year, ford went with a smaller low profile relay to
control the headlights. With this change came reports of the headlights
flickering or not working at all. The National Highway Traffic Safety
Administration investigated lcm failures
in the 2003-2005 cars that caused the headlights to work
intermittently or not at all. But this investigation has been closed,
and is unlikely to be reopened now since all the affected vehicles are
several years old and the nhtsa realizes that cars do not last forever
without requiring repairs.
The official ford solution to headlight problems caused by the lcm is
to replace the whole lighting control module module with a new one.
This is an easy task, but the part alone costs around $500 at your
local ford dealer. A few owners of affected cars with electronics
knowledge have found that they can often repair
lcms by soldering an external headlight relay in place of
the onboard broken one.
Do note that the headlight switch in the 2003-2005 cars is almost never
the cause of malfunctioning headlights. But this does not deter some
shadetree mechanics from replacing the headlight switch a couple times
only to find that their headlights still do not work properly.
The LCM sometimes gets blamed for problems it does not cause. For
instance, police departments usually have a headlight flasher installed
under the hood on their patrol vehicles. Installation and removal
from department to department, but a good number use crimp on
which break the conductors inside the wire jacket and will also corrode
time. The multifunction switch on the steering column is also wired in
with the headlight output of the lcm, if the switch or the connectors
the switch are defective, the headlights may flicker or turn on and off
random times. The basic check here is to hook up a voltmeter or small
to the headlight output terminals of the lcm and observe the readings
the lights start malfunctioing. If you've got +12V showing at the lcm
the headlights shut off, the problem is probably not with the LCM.
if you've got a spare known good lighting control processor around, you
install it in your vehicle and see if the problem corrects itself.