Battery Light Is Coming On at Idle, Goes Off When Engine Is Revved Up

Joined
Feb 28, 2003
Messages
11,977
Location
Cajun Country, La.
2007 F150, 4.2 V6, 171K Miles
While going through the CVS drive thru earlier tonight my battery light decided to come on. I revved up the engine and it went off.
Driving home I noticed the lights going dim. As I would slow down the battery light would come on again.
When I got home, I turned the engine off and it restarted just fine, but the battery light came on until I revved the engine.
I'm thinking the alternator is going bad. I checked all the connections, and nothing is loose or corroded. Belt is fine and not slipping.
A friend is coming over in the morning to check my electrical system. Who else thinks it's the alternator?
 
My multimeter is broke. That's why I haven't said that I checked it.
Ok, I fixed my MM. It needed a battery. :rolleyes:
I set it to DCV 20 and tested the battery. 12.85 volts. I started the engine, 11.68 volts. I turned on the headlights, high beam, a/c, and radio, 11.15 volts. Battery light stayed on. I revved up the engine, 11.88 volts, battery light went off until it returned back to idle. Turned everything off, let the engine run, 11.55-11.61 volts, battery light still on.
Shut off the engine, 12.79 volts at the battery.
Needs an alternator.
 
Ok, I fixed my MM. It needed a battery. :rolleyes:
I set it to DCV 20 and tested the battery. 12.85 volts. I started the engine, 11.68 volts. I turned on the headlights, high beam, a/c, and radio, 11.15 volts. Battery light stayed on. I revved up the engine, 11.88 volts, battery light went off until it returned back to idle. Turned everything off, let the engine run, 11.55-11.61 volts, battery light still on.
Shut off the engine, 12.79 volts at the battery.
Needs an alternator
Test it at the parts store first before buying. A small possibility might exist with wiring or connection affecting the regulation of the charging system. I am surprised the static battery voltage is at 12.85v.
 
I had a 2003 F150 with around 150000 miles when the same thing happened to me. Light would come on at idle, go away when driving. Turned out it was the brushes which had worn down. Replaced them and was all set to go with no more battery light coming on.
 
Well, l like buying certain car parts from RA. When it comes to electrical parts (starter, alternator, some sensors, etc.) I like to buy them local. You get a Lifetime warranty in which you can return it very easily and get another one right then and there.
I bought a rebuilt/remanufactured one from O'Reilly's for $199.95 + tax = $217.57 with a $45 discount $172.57 total and a Lifetime warranty.
My son paid for it as a birthday present (my birthday is tomorrow March 5th). My best friend owns a mobile auto repair business and installed it FREE of charge. We do favors for one another and he's my shooting buddy.
So, all in all l had a very good birthday.
My lady friend is making my favorite cake tomorrow, pineapple upside down cake. She gave me my other birthday present this morning.
:censored:
 
Well, l like buying certain car parts from RA. When it comes to electrical parts (starter, alternator, some sensors, etc.) I like to buy them local. You get a Lifetime warranty in which you can return it very easily and get another one right then and there.
I bought a rebuilt/remanufactured one from O'Reilly's for $199.95 + tax = $217.57 with a $45 discount $172.57 total and a Lifetime warranty.
My son paid for it as a birthday present (my birthday is tomorrow March 5th). My best friend owns a mobile auto repair business and installed it FREE of charge. We do favors for one another and he's my shooting buddy.
So, all in all l had a very good birthday.
My lady friend is making my favorite cake tomorrow, pineapple upside down cake. She gave me my other birthday present this morning.
:censored:
hey @BlueOvalFitter happy early birthday!! 🎂 Nothing like getting the car fixed for a birthday present! Hopefully this alternator lasts you a long time!
 
hey @BlueOvalFitter happy early birthday!! 🎂 Nothing like getting the car fixed for a birthday present! Hopefully this alternator lasts you a long time!
Thank You very much Sir.
I made a mistake in my OP. My truck has 172K miles instead of 171K.
Man, the factory one was shot to Hades and back. There was a little bit of play in the shaft, and it would spin A LOT!
I think it's kind of funny. I was out in the parking lot checking my electrical system at 11:30 PM last night. After I replaced the battery in my tester I didn't care if it was 2 AM, I was determined to test it.
 
Thank You very much Sir.
I made a mistake in my OP. My truck has 172K miles instead of 171K.
Man, the factory one was shot to Hades and back. There was a little bit of play in the shaft, and it would spin A LOT!
I think it's kind of funny. I was out in the parking lot checking my electrical system at 11:30 PM last night. After I replaced the battery in my tester I didn't care if it was 2 AM, I was determined to test it.
hey you're most welcome.. and.. wow still.. at 171k or 172k, it definitely had a good run. That's a good BITOG member for ya.. gotta get the answers, who cares what time it is! lol... (y)
 
OK @BlueOvalFitter . What is the output now? Glad I was able to help with the Sunoco race fuel in your beautiful Ford GT40. Don't forget to check the nut behind the wheel.
Happy birthday, young man. Another spin around the sun...
 
Well, l like buying certain car parts from RA. When it comes to electrical parts (starter, alternator, some sensors, etc.) I like to buy them local. You get a Lifetime warranty in which you can return it very easily and get another one right then and there.
I bought a rebuilt/remanufactured one from O'Reilly's for $199.95 + tax = $217.57 with a $45 discount $172.57 total and a Lifetime warranty.
My son paid for it as a birthday present (my birthday is tomorrow March 5th). My best friend owns a mobile auto repair business and installed it FREE of charge. We do favors for one another and he's my shooting buddy.
So, all in all l had a very good birthday.
My lady friend is making my favorite cake tomorrow, pineapple upside down cake. She gave me my other birthday present this morning.
:censored:
All that AND cake ? Man, thats livin right there.
 
Write on your battery, your numbers,every once in a while doing maintenance you can just give it a quick check over.Example,different situation, last spring I bought new lawn tractor.I wrote on battery voltage,and while running.Few years down the road I will have some ideas.Things old farts do
 
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