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View Full Version : Brake Pedal and Trunk Light: "The Haunting Part I"


ProfChaos
03-18-2006, 08:31 PM
I have a '96 Cobra that now has a haunted trunk light. :)

What I mean by that is that when I apply the brake pedal, the trunk light comes on. The charge-indicating ammeter also jumps around a bit when the car sits at idle. (I noticed this after dark, so the headlights were also on at the time.) The meter does not show severe movement, but the occasional, slight-and-jerky needle movement toward the left (-) seems a little erratic to me.


Any ideas on what might be causing this ground loop that makes the trunk light come on when I apply the brake pedal?

Thanks for your help.

--Professor Chaos

Levi
03-20-2006, 09:25 AM
I am not a mechanic, so this is just a guess. Check the grounds to the battery to chassis and/or due to this the alternator may not be charging the battery and the drain from the accessory is pulling more then the system can handle.
If the third brake light is in the trunk lid (center brake light), then it may just be glowing in the truck when you apply the brakes.

dewone
03-20-2006, 12:30 PM
Levi is on to something there with the light, the ammeter is showing that the system is charging. But not at idle so it is showing the load as it is at that time once your above 1000 rpms it should begin charging again and jump or move towards +. It should always be above 13 volts while running if not go to auto zone and have them hook up there diagnostics tools. Could be the altenator, voltage regulator, battery or a loose ground as Levi stated.

SNAKEYE
03-22-2006, 09:23 PM
Sounds like either a bad high brake light or regular brake light ground feeding through one or the other. Trace wire harnesses from high brake light and regular brake lights over around into the left rear quarter panel area of the trunk where you will find a couple of multiple-wire connectors. Unplug them and then plug them back together to clean up connections.
Give it a go!

ProfChaos
03-28-2006, 07:00 PM
Sounds like either a bad high brake light or regular brake light ground feeding through one or the other. Trace wire harnesses from high brake light and regular brake lights over around into the left rear quarter panel area of the trunk where you will find a couple of multiple-wire connectors. Unplug them and then plug them back together to clean up connections.
Give it a go!


It turns out that it is the "high brake light." The lamp assemblies for the high brake light are located on either side of and just above the trunk light assembly. Since the lamp assemblies for the high brake light reside above the trunk lamp itself by only three inches or so, when you apply the brake, it can create the illusion that the trunk light is coming on. This occurs because there is no bezel (dark plastic cover) on the rear of the high brake light assembly, to block the light from shining back into the trunk.

The issue with ocassional erratic movement on the ammeter turns out to be a worn-out negative terminal mount. The copper on the cable terminal mount is so worn and stretched and the cable terminal mount's bolt assembly is so corroded that the negative terminal will no longer tighten securely. (This is obviously a separate issue from what I thought was a "haunted" trunk light, as there is actually nothing wrong with the trunk light.)

The bad news is that Ford will sell only the entire battery cable assembly (+ and -) for $220--Ford will not sell merely the negative cable assembly . (Those rascals!) :mad:

My solution to the ridiculous price that Ford wants for the entire assembly is that, since Lorena the Cobra goes to the garage this week anyway for a differential replacement, I'm going to ask my SVT tech if he can merely replace the cable end and leave the wiring harness in-place. (I'd hate to rig-up something myself from cheesey generic auto parts and then find out later that the rigged-up cable assembly has become intermittent and a real pain in the neck. If my mechanic has the tools to merely replace the cable end, I'm set.)

Thanks for your helpful advice.

cheers,
--Professor Chaos