94 Regal Custom electrical lights dim

[BKA]

Great forum I have been looking around for the past few days.

1. At night my headlights will dim when I step on the brakes.

2. On the dash the lights dim down some and go back to norm. This is going on just driving around, no brakes applied.

3. I see other checking there codes, how do I do this?


If any of this has been posted I could not find it.

[82Stang]

Might be time for an alternator.

If I recall correctly, old timers used to unhook the negative battery cable while the vehicle was running. If it stalled, the alternator was bad. Not sure, but think that's how it was.

[BKA]

I will give that a try.

Thanks

[Mr_D]

Have someone with some electronics knowledge do a few tests.

Rather than disconnecting the battery cable (which will probably spark since it would be under load or charging) It would be safer and more accurate to simply have someone with a DC multimeter take a few readings.

A fully charged 12V battery should have about 12.6V on it when the vehicle is off. If it's down to 12 Volts that is too low.

Most alternators charge the battery at 14 - 14.5V while the engine is running and slightly less if the battery is fully charged. So you'll see it highest right after you start the car since it has to recharge a bit. Then it drops off in a few minutes after the battery is charged. At that point it would be down to 13 - 13.6V with the engine running.

If you suspect the alternator is not charging then start the car and put a small load on the battery while observing if the alternator keeps up.
To do this you measure the voltage at the battery terminals and then turn on the headlights, blower motor, or dome lights and watch if the battery voltage drops off as you increase the load. If the alternator keeps up the voltage will not drop much at all.

Check for a loose or corroded wire around the battery or alternator.

Also check for excessive current draw (a short circuit) in your brake light circuit that is puling the voltage down. That could cause nearby circuits to also have low voltage so you see the lights go dim. If it's a minor short it may be enough to load down the electrical system while not being enough to actually blow a fuse.

That can also be tested pretty easily. I use a DC amp meter with a range that handles 15 -20 amps and test the current draw on the circuit in question at the fuse box. I pull the fuse and insert the meter inline so the current flows through the meter instead of the fuse. Make sure to only use a meter with a built-in fuse just to be safe. I would expect maybe 3 to 6 amps to run the brake lights. I could go measure mine if you really need a spec but it should be something like that.

A 3-6 amp load should not cause any other lights to dim.

While reading the current you can pull the brake lights bulbs one at a time and you should see an corresponding drop in the current as the total load goes down. If there is still a significant load (> 5amps) with all the bulbs out then I'd suspect a short to ground in the brake light system.

Mr_D

[BKA]

Originally Posted by 82Stang

Might be time for an alternator.

If I recall correctly, old timers used to unhook the negative battery cable while the vehicle was running. If it stalled, the alternator was bad. Not sure, but think that's how it was.

alternator, battery and cables are all good.

[BKA]

@ Mr_D I will give that a go, I will post back what I come up with.

Thanks