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Chris (aka- Moose)
Chris (aka- Moose), Mercury Technician
Category: Mercury
Satisfied Customers: 47990
Experience:  16 years experience with Mercury
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98 crown vic. Headlight lcm went bad. Made my own wire

Customer Question

98 crown vic. Headlight lcm went bad. Made my own wire harness ues i put a inline fuse in. Lights work until i shut them off, fuse blows, when they are hot. When cool i can turn them off and on again no problems.
Submitted: 11 months ago.
Category: Mercury
Expert:  Chris (aka- Moose) replied 11 months ago.

Welcome, I'm Chris (aka Moose) I prefer to work when others don't, I am here around lunch and after dinner till after midnight.

I will need to know more about this wire harness to help diagnose this since I have no diagrams to match a bypassed LCM.

Customer: replied 11 months ago.
what do you need to know?
Customer: replied 11 months ago.
It is a stand alone wire harness. One switch controls the low and another controls the high beam. Power is directly from battery with a fuse @battery. And its ground back to the battery. The lights work so it must be wired correctly. The problem i have is when i turn them off the fuse blows. Only when they have been on for a while. Within the first 10-15 minutes i can turn them on and off with no problems.
Expert:  Chris (aka- Moose) replied 11 months ago.

Is there a fuse in this circuit for the low beams and a fuse for the high beams?

What amp fuse is used and what gauge wiring?

Did you cut out the old wiring to the headlights or just splice in your new wiring?

Customer: replied 11 months ago.
Only one fuse because both switches are on the same hot.
30 amp fuse. 12 gauge wireAll new wiring. New bulb connectors also. Nothing old was used.
Expert:  Chris (aka- Moose) replied 11 months ago.

So you have it wired something like this?

Customer: replied 11 months ago.
No. The ground goes from the switches through the light circuit back to battery
Expert:  Chris (aka- Moose) replied 11 months ago.

Okay so your constant powering the light and controlling the lights with a ground switch.

You shut the lights off, then the fuse immediately blows?

You said you can turn them off and on when cool. Cool meaning outside air temp or when the bulbs have not been on long and are cool?

Customer: replied 11 months ago.
Switches have a led hence the ground @ switches.
Not controlling with a ground switch.
The fuse only blows when lights have been on for some time and are hot.
Customer: replied 11 months ago.
I will send you a diagram of my wiring in am. Unable to do that now.
Expert:  Chris (aka- Moose) replied 11 months ago.


Expert:  Chris (aka- Moose) replied 11 months ago.

Hello again Chris here,

  • I am following up with you to see if you got everything you needed from my answer.
  • I have not heard back from you, so I assume you were not pleased. We're you pleased with my help?
  • If you were not pleased with my help, please let me know by replying so I can help more.
  • This post is coming up on the 7 day mark where it times out. When a post times out, that means it's no longer on my active list (you reply and I may never know it.)

Thanks Chris

Customer: replied 11 months ago.
Not really pleased at all. I could not explain it any better. Its a wiring problem. You dont seem to have any ideas on how to help me. All you did was ask question after question. With no things to try on my end. So no im not happy.
Expert:  Chris (aka- Moose) replied 11 months ago.

So are you wanting me to have the post closed?

You said on the 12th you were going to send me a diagram.

Diagnosing a vehicle that has been rewired different than OEM diagrams I have to ask questions since yours is not like other vehicles. I have to know what you did to help you.

Customer: replied 11 months ago.
Red- constant hot
Black- ground
Yellow- low hot
Pink- hi hot
Expert:  Chris (aka- Moose) replied 11 months ago.


The wire gauge is fine, the fuse amp rating is fine. Are the bulb wattage higher than normal?

Is the wiring at the back of the headlight getting hot?

If you remove the ground from the switches does this stop the fuse failing.

I believe the contacts in the switches are warping under load and grounding out or the bulbs are drawing to many amps from high watt bulbs.

Customer: replied 11 months ago.
Okay. That gives me a idea that i couldn't think of. Ill try that soon. Hopefully tomorrow.
Expert:  Chris (aka- Moose) replied 11 months ago.