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Burned Out Battery (Alternator) Light Indictor...Major Drain

Posted: Feb 17, 2021 6:13 PM
by Preacher's 5
Hey guys, I was wondering how many of you know that if the battery/alternator light bulb blows, that it will cause a negative drain on battery. Yesterday I started my car and let it idle to warm up, but did not realize that when I turned the ignition switch on the battery/alternator light was not on. When I turned the car off, it would not start. First I thought the battery was dead. So I waited about 10 minutes and the cars started right up. Then I thought I needed an alternator. Today, when checked the battery and alternator voltage, it was reading 11.2 volts. I know that it should 14. I called my mechanic, Wayne, and he told me to turn the ignition switch on and see if the battery/alternator light was on, I told him..NO! He told me to pull the cluster out and replace the bulb, then check to voltage, which I did...now it is reading 14.02 volts. WOW! Problem solved! Saved a couple hundred bucks! Car is cranking and running great.
I have owned this car for 35 years and had never heard that before. You learn something new every day. I wonder how many people have replaced their alternator, (as I was about to do); or replaced the battery because of a $2 bulb, (which I have a bunch of). Just curious and passing on some info. I truly thank God for my mechanic, Wayne Lins. Great car brain.

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Posted: Feb 17, 2021 9:27 PM
by Shawn D.
This is fairly well-known, but it's not intuitively obvious. This was also an issue on GM cars that used a similar circuit to excite the alternator.

FWIW, later instrument clusters have a resistor parallel to the bulb so that if the bulb blows, the alternator will still get power from that circuit.

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Posted: Feb 17, 2021 10:28 PM
by Preacher's 5
Shawn D. wrote: Feb 17, 2021 9:27 PM This is fairly well-known, but it's not intuitively obvious. This was also an issue on GM cars that used a similar circuit to excite the alternator.

FWIW, later instrument clusters have a resistor parallel to the bulb so that if the bulb blows, the alternator will still get power from that circuit.
Thanks Shawn. 35 years and never had the issue until yesterday. Glad to know some-one else is up on that revelation.

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Posted: Feb 18, 2021 8:57 AM
by BMWCCA2
It was a fairly common issue on 2002s, too.

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Posted: Feb 19, 2021 7:12 PM
by demetk
These cheap voltmeters are useful tools, if and when you remember to look at them :)

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Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Posted: Feb 22, 2021 6:39 PM
by indycar89
demetk wrote: Feb 19, 2021 7:12 PM These cheap voltmeters are useful tools, if and when you remember to look at them :)

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Nifty meter in the check panel - remember what/where you found it?

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Posted: Feb 22, 2021 9:00 PM
by demetk
indycar89 wrote: Feb 22, 2021 6:39 PM Nifty meter in the check panel - remember what/where you found it?
https://www.amazon.com/gp/product/B00YA ... UTF8&psc=1

My check panel came apart easily so installing the meter wasn't difficult.

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Posted: Feb 23, 2021 12:19 PM
by gadget73
Shawn D. wrote: Feb 17, 2021 9:27 PM This is fairly well-known, but it's not intuitively obvious. This was also an issue on GM cars that used a similar circuit to excite the alternator.

FWIW, later instrument clusters have a resistor parallel to the bulb so that if the bulb blows, the alternator will still get power from that circuit.
Fords do this too. Not so much a drain, just it doesn't turn the alternator on. There is a resistor but its on the cluster so it doesn't work if you have that out for whatever reason.

I had a Beetle years ago that wouldn't charge, same thing with the bad bulb.

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Posted: Feb 24, 2021 9:40 PM
by craigb93
indycar89 wrote: ↑22 Feb 2021 18:39
Nifty meter in the check panel - remember what/where you found it?

https://www.amazon.com/gp/product/B00YA ... UTF8&psc=1

My check panel came apart easily so installing the meter wasn't difficult.

Where were you able to get the voltage signal from? Inside the Check Panel or ?