FYI FORD MustangSteve's Ford Mustang Forum / brake light voltage

**Posted by Alan** 5/04/2024 3:27 PM

On the way to Lincoln, Bob noticed my brake lights not working. Didn''t have Dan handy to crawl under the dash, so i got under there and wiggle the switch and they came on, but the left is dimmer than the right. Got home and measured the voltage. The right measures 12.7 at the bulb, but the left only measures 10.5. Any suggestions as to what is causing this? Turn signal switch?

**Posted by Nos681** 5/04/2024 6:05 PM

Take a look at your side marker lights and tail lights.
You can get weird readings with burned out filaments.

**Posted by Toploader** 5/05/2024 5:29 AM

This isn't really helpful to your current issue, but when I switched to LED brake lights and then measured the total current, it measured at .3A on my car. I was amazed.
Surely this has to take a load off the electrical system?
Maybe LED is something that you can consider in the future? In my case, the result was far brighter lights at a fraction of the current draw.

Last edited by Toploader (5/05/2024 5:33 AM)

**Posted by John Ha** 5/05/2024 6:14 AM

Alan wrote:
The right measures 12.7 at the bulb, but the left only measures 10.5. Any suggestions as to what is causing this? Turn signal switch?

Could be. Is there a good ground to both of the lamp sockets in the buckets? Have you tried swapping the bulbs from one side to the other (it's unlikely that the issue is a bulb but it could be)?

**Posted by Alan** 5/05/2024 6:26 AM

I was actually adding led's to the bucket so make them brighter. After i had added them to the left side, i tried the brake again and noticed it was still not very bright. That was when i started checking the voltages. Made no difference whether they were hooked up or not on the voltage.

**Posted by John Ha** 5/05/2024 7:47 AM

I would check the resistance between the less-bright bucket and ground. It should be zero ohms.

There are two separate wires that come from the turn signal connector to the rear turn/brake lamps. You could disconnect and reconnect that connector a few times to clean the contacts and confirm that none of the contacts are pushed out of the connector. My wiring diagram says that there is a connector between the rear lamps and the turn signal connector. You could try disconnecting and reseating that too. It should have an orange/blue stripe wire and a green/orange stripe wire.

**Posted by BobE** 5/05/2024 8:04 AM

I agree with checking the ground connections, these can degrade over the years and may not be making good contact causing a low voltage condition.

**Posted by Alan** 5/05/2024 8:57 AM

I am measuring the voltage on the wire coming back to the rear and using a good ground on the meter so i don't think the ground at the bulb is the problem. Will do some more looking this pm.

**Posted by John Ha** 5/05/2024 9:14 AM

This should have been the first question I asked ... is the turn signal is also dimmer on that side than on the other side?

Sometimes gently jiggling the turn signal lever up and down will change things - if you can do that with someone watching or maybe with a mirror or white board behind the car it would be a useful experiment.

Last edited by John Ha (5/05/2024 9:15 AM)

**Posted by Mach1Driver** 5/05/2024 1:49 PM

Power comes from the light switch (there is a circuit breaker in there and the lights do no have to be on). Then it goes to the brake switch (you are good up to this point), and from there to the turn signal switch where it goes to each side. This is done to interrupt the power from the brake switch when the turn signals are on, or you wouldn't see it blink with the brakes on. The turn switch could be partially fried (high resistance) or a bad connection/wire somewhere. If you have a wire diagram, see if there are any plugs on that wire. I believe the brake/turn signal circuit is the most often asked about circuit on our cars.

**Posted by BobE** 5/06/2024 7:40 AM

Alan wrote:
I am measuring the voltage on the wire coming back to the rear and using a good ground on the meter so i don't think the ground at the bulb is the problem. Will do some more looking this pm.

The possible poor ground is between the housing and the body. The fact that you had full voltage and were connected to a good ground indicates that there isn't an issue with +12v power going to the bulb.
The bulb 'grounding' is accomplished by way of the housing, if the housing to body connection is 'poor', the ground will be poor. Or if the bulb housing isn't making good contact with the housing of the bulb, you'll have the same condition.

**Posted by wsinsle** 5/06/2024 8:33 AM

There is a green/orange wire on the plug coming out of the steering column. This goes to the left turn/brake. If the voltage is good there, may be a bad plug connection. If it is bad there, may be in the turn switch. Orange/blue is the other turn/brake.

**Posted by Alan** 5/07/2024 8:17 PM

got 10.59 volts out of the steering column plug, so i guess next thing is to pull the steering wheel when i have time