2000 Mustang GT, Taillights stay on (dim) when headlight switch is Off

[jayrw]

I recently noticed when my motor is running the tail lights are always on but in a dim state even if the headlight switch is completely off, This isnt normal right?

They're lit at around half brightness compared to if I turn the parking lights or headlights on and I verified they work proper with brakes applied as well as turn signals.

Is this some sort of feedback perhaps? bad ground? any ideas appreciated.

EDIT: My tailights are LED assemblies (model info here) , I'm fairly certain these led lights didnt stay on when first installed like they do now, if anything LED draws much less current so I don't think anything would be damaged in the long run? Ive had them for a little over a year now.

 

[Barnaby]

Sounds like a bad ground somewhere.

 

[fast97gt]

nope head light switch, better get it fixed before it grounds out and fills your interior with smoke

 

[Rusty67]

+1
First thing I would do is disconnect the headlight switch.

You can test out the leads from there to find if there is a short or not.

You might also be able to test the headlight switch with a multimeter.

 

[wmburns]

If it were my car, the first thing I would check is the grounds in the trunk. Take out one of the bulbs and measure the resistance from the ground terminal back to battery negative (car off).

Next, it would be handy to know if the stray voltage is coming through the stop lights or the parking lights.

Pull the following fuses one at a time and report the results. F2.41, F2.18, F1.10, F2.19. Note, F1 is the Battery junction box, F2 is the central junction box.

This problem could be in the electronic flasher module or the multifunction switch as well as the head light switch. Where to look depends upon where the voltage is coming from.

 

[warment]

Sounds like the light switch is bad. I had one go bad on a 1990 Ford Escort GT. The lights would cut off at night while I was driving and then cut back on a few seconds later. Dangerous!

 

[jayrw]

Hm well I replaced the headlight switch today and the problem still persists, I will have to try what wmburns suggested next.

It's worth noting the taillights are aftermarket LED units but I don't recall them doing this when I first installed them. (heres pic I took shortly after installing them Link).

 

[wmburns]

Might have been handy for us to know these are aftermarket LED assembles.

Some cars have problems with LED lights because they draw so little current. This sometimes causes the lamp monitor to "think" bulbs are blown.

The ususal fix is to install a ballast resistor to add addtional load in order to fool the monitoring circuits. Note, the problem usually presents as a fast blink. Some cars will give a tone when the brakes are applied.

 

[jayrw]

Might have been handy for us to know these are aftermarket LED assembles.

Some cars have problems with LED lights because they draw so little current. This sometimes causes the lamp monitor to "think" bulbs are blown.

The ususal fix is to install a ballast resistor to add addtional load in order to fool the monitoring circuits. Note, the problem usually presents as a fast blink. Some cars will give a tone when the brakes are applied.

Sorry I should've metioned the LED part in my first post.

The blinkers are currently working at normal speed and I dont hear any tone with brakes applied. Ive had a bulb go out before I switched over to LED so I know what the fast blink is like.

 

[BurnerJ79]

I've about had it

I have the exact same setup on a 02 GT and I discovered this problem last night after installing a 03 cobra bumper and fogs. tail lights and 3rd brake light glow very very dim any time the engine is running. This is just the latest problem in tracking this electrical gremlin I've had. The headlight switch was replaced 6 months ago to see if it helped my voltage issues which it didn't. I'm now leaning towards a bad multifunction switch or GEM (generic electronic module?!?) After this light problem started last night my car died when I'd pull the headlight switch to turn it on.

Did you find what the problem was?


--------------different but maybe related?!?---------
I also sometimes open my car door and the car voltage drops down to 9volts. I shut the door and it goes back to 12.4. If this happens I can hit the door lock button on my keyless entry and the problem goes away.

 

[Rusty67]

Also check your relays. One of them may be going bad....

 

[jayrw]

To be honest I haven't tried tracing the problem since I replaced the headlight switch, The tailights still have the same symptoms. I have no other symptoms other than the dim tailights so I guess it hasn't been a huge priority.

But if you happen to figure it out please share your findings.