WEIRD ELECTRICAL PROBLEM

PonyGTrider · Apr 21, 2022

Hi all,
I hope someone light me up on this electrical issue on my 90 GT.
This has been an ongoing issue and have no clue why it is happening, it is more noticeable now that I have LED light bulbs on the three stop lights. The problem is that as soon as I start the car or if just turn the ignition switch to the accessories position my brake lights come on dim. I removed the third light from the rear wing, checked the voltage without the key in the ignition and shows 00.0
As soon as I turn the ignition switch ON or swithc to accesories ir shows 5.30 Volts DC on my volt meter and the LED bulbs come on.
My battery has been draining quite often if I just sitting listening the radio. Please guide me where to start.

I really hope someone help me with this pesty problem.

Thank you all

Mustang5L5 · Apr 21, 2022

The cruise control monitors the brake light circuit to turn it off of you hit the brakes. Since the LEDs have much less resistance that the halogen bulbs, they will shine simply as there is a path to ground through the CC modules

If you have someone press the clutch pedal switch in, I bet they go out.

With the LEDs the cruise may not function at all (or it might depending on the specific LEDs) but to eliminate the problem you have a few solutions.

Add resistors, or if you don’t care about cruise, disconnect the plug on the right side of the pedal cluster that goes to the cruise control cut off switch

PonyGTrider · Apr 21, 2022

Mustang5L5 said:
The cruise control monitors the brake light circuit to turn it off of you hit the brakes. Since the LEDs have much less resistance that the halogen bulbs, they will shine simply as there is a path to ground through the CC modules

If you have someone press the clutch pedal switch in, I bet they go out.

With the LEDs the cruise may not function at all (or it might depending on the specific LEDs) but to eliminate the problem you have a few solutions.

Add resistors, or if you don’t care about cruise, disconnect the plug on the right side of the pedal cluster that goes to the cruise control cut off switch

Thanks Mustang5L5 for your response.
I have afew things bothering me, one of them is that when I disconnect the stop pedal switch I have 5.3 Volts and then if I pull the three brake bulbs out of their sockets I read 12.3 Voltios al the pedal switch connector.... So now I have two wires with 12.3 volts, one constant and one coming thru the ignition switch. That doesn't make any sense to me, I guess I'm not following the logic...
Any how, my Cruise Control has been dissabled for a while, so you're saying that if I unplug that CC connector will solve that dim light issue? And how far that connector is? or is it connected to some sort of box?

Man I really appreciate help! Thanks again

Mustang5L5 · Apr 21, 2022

A lot of these circuits work by providing constant voltage and controlling/switching grounds. Al you may find 12v where you don’t expect it. Provide a path to ground and weird stuff happens.

Anyway, go under the drivers side dash and fish around on the right hand side of the pedal cluster. You’ll find a plug on that sode with a green and red wire that connect to the box just above the steering column. This is the cruise control clutch shut off switch. Disconnect it. That should clear the problem by disconnecting the alternate path to ground

PonyGTrider · Apr 21, 2022

Mustang5L5 said:
A lot of these circuits work by providing constant voltage and controlling/switching grounds. Al you may find 12v where you don’t expect it. Provide a path to ground and weird stuff happens.

Anyway, go under the drivers side dash and fish around on the right hand side of the pedal cluster. You’ll find a plug on that sode with a green and red wire that connect to the box just above the steering column. This is the cruise control clutch shut off switch. Disconnect it. That should clear the problem by disconnecting the alternate path to ground

You were absolutely correct, I just unplugged that CC module and no more dim light. Actually first I did what you suggested, turn ignition switch to the accessories position end there were the dim lights agai then I pressed the clutch pedal and the lights came off, proving what you said
Thanks a bunch!

PonyGTrider · Apr 22, 2022

PonyGTrider said:
You were absolutely correct, I just unplugged that CC module and no more dim light. Actually first I did what you suggested, turn ignition switch to the accessories position end there were the dim lights agai then I pressed the clutch pedal and the lights came off, proving what you said
Thanks a bunch!

Well I this it was too good to be truth. Last night I unplugged the CC module and it turned the dim brake lights so I thought problem solved... NOT! after I pluged everything up I turned the ignition switch to accessories and there are the dim brake lights again and reads the same 5.30 Volts. If I press the clutch pedal they come off, if brakes pedal is pressed they glow normally but when release it the lights stay dim.... Is there any other way other than placing the resistor? I'm getting frustrated

Mustang5L5 · Apr 22, 2022

So they still go out when you press the clutch pedal?

Which switch did you disconnect previously? It’s possibly you disconnected the wrong one.

You can also reach up on the drivers side firewall and just disconnect the two cables to the cruise control amplifier.

7991LXnSHO · Apr 22, 2022

A battery draining quickly when listening to the radio has always indicated a low reserve power of a soon to be replaced battery for me. If it is charged and load tested, do not be surprised if it fails.

@Mustang5L5 would putting incandescent bulbs back in be a good idea?

PonyGTrider · Apr 22, 2022

Mustang5L5 said:
So they still go out when you press the clutch pedal?

Which switch did you disconnect previously? It’s possibly you disconnected the wrong one.

You can also reach up on the drivers side firewall and just disconnect the two cables to the cruise control amplifier.

Guess what? I just slapped myself really hard!!! Yesterday I disconnected a little box at the right side of the fuses center and for some reason the lights went off, maybe I pressed the clutch inadvertently and I got excited. Today I disconnected the CC switch that is located and activated by the clutch quadrant and this time the damn dim lights went off for good.

Thanks man for the patience!

PonyGTrider · Apr 22, 2022

7991LXnSHO said:
A battery draining quickly when listening to the radio has always indicated a low reserve power of a soon to be replaced battery for me. If it is charged and load tested, do not be surprised if it fails.

@Mustang5L5 would putting incandescent bulbs back in be a good idea?

That is truth but the dim brake lights with the engine running or with the ignition switch on the accessories position just bothered the hell out of me but thanks to Mustang 5L5 that issue has been resolved.
But you are right about the failing battery reserve. Right now I have a brand new battery with a 110 minutes reserve, with this everything shoul be happy

Thanks man

7991LXnSHO · Apr 22, 2022

PonyGTrider said:
That is truth but the dim brake lights with the engine running or with the ignition switch on the accessories position just bothered the hell out of me but thanks to Mustang 5L5 that issue has been resolved.
But you are right about the failing battery reserve. Right now I have a brand new battery with a 110 minutes reserve, with this everything shoul be happy

Thanks man

If you still have battery life issues, you could have charging system issues, and the FLAPS can test the car for free.
I hope the gremlins are gone.

PonyGTrider · Apr 23, 2022

7991LXnSHO said:
If you still have battery life issues, you could have charging system issues, and the FLAPS can test the car for free.
I hope the gremlins are gone.

My charging system is fine. I have a 200 Amp alternator and just relocated my battery to the trunk with 1/0 gauge cable that should carry enough juice. I feel better with no damn dim brake lights.

Thanks man

WEIRD ELECTRICAL PROBLEM

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