Electrical When the headlights are on..... 91LX

The first is not an issue. It is not an amp meter, it is a voltage meter. Under high load at idle, adding the brake lights should likely lower the car’s voltage a bit, especially with a stock alternator. No biggie.
Before you go to LED light bulbs that might be a hassle for no real benefit, check the condition of your charging wires (positive and ground) and all their connections. If a ground connection is corroded, it will add resistance and lower the efficiency of the system.

The other problem with the brake light switch is a separate issue. Check for connections, that it’s getting actuated, that one side has a constant 12v, then replace the switch.

Mustang5L5 said:

What sort of numbers were you seeing, and what pullies/alternator do you run?

A typical 12V car battery should have a resting voltage of 12.4-12.6 volts when fully charged. When hooked up to a running vehicle, anything less than that is actually discharging and pulling more energy from the battery than the alt is putting in.

Typical alternator charging voltage is around 13.6-14.8 volts. This is where you want to try to be most of the time typically.

So, at idle with everything on, what was your voltage?


LED bulbs will help with the voltage, but many of the cheap ones on the market are actually dimmer than the stock bulbs. I went through half-dozen or so different brands to find a set of taillight LED's that were equal to stock bulb brightness.

Click to expand...

And then with many led bulbs, there is the change to the load and which requires a different blinker or added resistors so the load does not change.
Do you remember what bulbs worked best and brightest?

The guys have given you solid advice above. You need to let them know if the electrical system on the car is stock or not and if any aftermarket gear has been added to it.

1) If this problem all of a sudden started then I would lean towards a resistance issue as 7991LXnSHO stated. You should check all major battery cables for connection corrosion or just being loose. For example, I literally had a 4 ga ground cable from the battery down to the K-Member internally oxidize to the point of causing the car hot start issues. Also, check the headlight harness grounds, should be one at each corner of the core support on the top near the fenders. Should have a green 7mm or 9/32" hex head on the bolt that secures the ground wire to the core support.

2) Have you added any electrical load to the car? For example, electric fan(s), MSD box, stereo, etc. I believe the stock alternator is a 75 amp unit and cannot support much more than what the car came with. If you are adding electrical load you really need to start thinking about a 3G conversion if it has not been done already. If you want to do this go here and scroll down to the "Electrical" section and there are two writeups on how to do it.


3) LED headlights - not much experience here but all I can say is do the research and your focus should be on beam pattern and having a clear cutoff.

4) If the headlight bulbs are not the stock units i.e. they were replaced with something hotter like the Sylvania Silverstars (this is what I run) you should look at putting relays in so the headlight switch stays cool. Do a search and you will see what I am talking about. This also made my headlights brighter as it increased the voltage at the bulb. You can wire this yourself (what I did) or if you do not want to put the parts together you can get kits:



Again, I would start with making sure the battery is in good shape (have it load tested), making sure the alternator is is top shape, checking the battery cables (positive and ground both) and then looking at the headlight harness grounds in that order.

I remember when I first got my mustang in 2013, the battery gauge would drop, the lights would dim and the thing would just die in the middle of traffic. It was the alternator, mine was all stock except for a pioneer radio. I guess that was too much for the electrical system because as soon as I upgraded to a 3G alt all that vanished. Ford put the bare minimum needed to run the car.