Battery light keeps coming on

[maillemaker]

1995 Mustang GT convertible.

My battery light has started intermittently coming on. Comes on once the engine heats up. Starts by blinking randomly, then comes on and stays on.

I have plugged a voltmeter USB adapter into my cigarette lighter and I'm getting 13.7 volts. Pretty consistent whether at idle or at speed.

I'll check tomorrow what it registers cold.

Any ideas?

 

[ttocs]

does the volt meter on the dash fluctuate at all?

 

[maillemaker]

does the volt meter on the dash fluctuate at all?

No, it sits pretty much dead center. Maybe a little to the right of dead center.

Just went and fired her up cold. Volts at cigarette lighter read 14.1 and the battery light is off. I'm guessing below 14 volts it starts to kick on.

 

[ttocs]

with electrical problems we need to start simple which means checking the grounds. The heat from the motor could be causing expansion of the connections and start to make them loose and gain resistance. To check the ground remove it completely, sand down any corrosion/paint or crap in the way and then tighten it back down till it does not wiggle at all.

 

[lwarrior1016]

I had one do that to me for a long time. Turns out the brushes had a groove worn in them that would sometimes break contact, always charged fine. A new alternator fixed it.

 

[maillemaker]

Drove in this morning (cooler weather), started at 14.2V, but within 5 minutes was down to 13.8. But the light never came on. We'll see what it does on the way home.

 

[maillemaker]

Went out today at lunch. Was rather warm - engine temp on dash was past mid-way. Voltage got as low as 13.2V No light came on.

Last night I jiggled the wires on the back of the alternator, unplugged the stator wire - it seemed to come off very easily. Plugged it back in. I don't think the stator wire is part of the circuit for the alternator lamp though. I think that runs through the ignition wire at the alternator.

 

[lwarrior1016]

Going down to 13.2 with heat would kind of make me think the alternator is on its way out.

 

[ttocs]

does that harness come off easy when the motor is cold?

 

[maillemaker]

Yeah the stator wire plug came off easy. I didn't fool with the AIS harness.

 

[ttocs]

it should take a pretty good tug to get it off hot or cold honestly and might be worth trying to replace before you throw a new alt at it.

 

[ctandc]

The '03 I bought a while back has a battery light that comes and goes randomly. Previous owner had the car 4 years - he said it did it the entire time. I haven't gotten around to it yet, but I plan on replacing the pigtail to the alternator, soldering / heat shrinking a new connector in and see what happens. Car has sat for days at a time, fires right up - zero issues.

 

[ttocs]

yes if you do replace it be sure to solder/shrink. I see a lot of people use crimp connectors under the hood and they will fail.

 

[MyLittlePony]

One thing to consider if nothing else fixes it it is to just consider the alternator. Ever since 2016, my 94 would have the same issue. Battery light would come on and off, randomly. As the years went by it would stay on longer. I would have the alternator tested on the car, off the car, always good. Always passing. But again, light became more of a constant even after redoing practically all the electrical.

Then last month I was driving and my radio stopped working. Kept driving, but then my RPMs dropped to zero! I decided to turn around and go back home. Then the entire cluster stopped working altogether! Car ended up dying halfway into the garage.

Took the alternator back to get it tested, and FINALLY it failed. Thankfully I had a spare one, and the car no longer has a battery light! So for SEVEN YEARS I’ve had this obnoxious light! My first thought after swapping out the alternator: “hm. That’s odd. The bulb must have finally burnt out.”

 

[maillemaker]

After having disconnected and reconnected the stator wire on the alternator (and it's just a 1-blade connector, it doesn't firmly lock on at the best of times) I haven't had it come on again. But, I haven't done any 30-minute + runs to get it really warm yet, either.

 

[ttocs]

it could be as simple as a connector that is a little loose. It allows corrosion to develop over time and there is a point with electronics where it will not be happy. At some point the resistance/corrosion will be right at that tipping point and it can go on/off if you hit a bump and it moves the wire. Taking it off and putting it back on will often clean the connection enough to get back below that tipping point/threshold voltage and work for a short time.

When I was 16 once I disconnected the battery ground when I was doing some work on the car and I put it back on to where I could just barely still wiggle it, but being young/dumb I figured metal is touching metal so it is all good! It worked for about a month and then the starter would slowly start slowing down, lights dimmer, ect. The first time it would not start I was at work with no tools and I grabbed the ground and wiggled it back/forth a few times and then it started back up and ran great, for another 2-3 weeks....