This is an intermittant problem that only happens when driving with the headlights on -- but does happen every time. Any ideas how to troubleshoot it?

For awhile the needle would oscillation between the normal 50-60% position a few degrees after driving a half hour or more with the headlights on. Then I cleaned all the connections in the starting systems from battery terminal to hot connections and grounds, and the problem stopped.

Then last week, I had driven to a store at dusk with the lights on - no problem indicated. I left the lights on for 20 minutes while in the store. Then when I started, the battery needle start oscillating from 50% to 100% regularly, and the battery warning light started blinking off and on.

Car was running fine so I turned off the headlights and just put on the parking lights so I could drive the 2 miles home. After driving about 1/2 mile the oscillating and flashing stopped.

I have tried to duplicate the problem so I could show a mechanic, but all is back to normal for now.

Without knowing anything, I would guest that the computer detected some kind of low voltage and the swinging and flashing was an attention-getter, and not the actual reading.

Any ideas how to troubleshoot this?

On 05/21/2016 1:17 AM, Snuffy "Hub Cap" McKinney wrote:
....

For awhile the needle would oscillation between the normal 50-60%
position a few degrees after driving a half hour or more with the
headlights on. ...
....
Any ideas how to troubleshoot this?

My money would be on the internal regulator in the alternator is acting
up...

--

On Saturday, May 21, 2016 at 8:20:56 AM UTC-4, dpb wrote:
On 05/21/2016 1:17 AM, Snuffy "Hub Cap" McKinney wrote:
...

For awhile the needle would oscillation between the normal 50-60%
position a few degrees after driving a half hour or more with the
headlights on. ...
...
Any ideas how to troubleshoot this?

My money would be on the internal regulator in the alternator is acting
up...

--

That's a likely possibility. I'd start with having the battery load tested,
which you can get done at most auto parts stores for free. Many will
also test the charging system at the same time. If the battery is good,
you're pretty much left with the alternator or a bad connections. The
alternator common failures are either the VR already mentioned, or the
brushes wear out. Those can usually be replaced separately but most
people just swap out the alternator for a rebuilt one.

"dpb" wrote in message ...
On 05/21/2016 1:17 AM, Snuffy "Hub Cap" McKinney wrote:
...

For awhile the needle would oscillation between the normal 50-60%
position a few degrees after driving a half hour or more with the
headlights on. ...
...
Any ideas how to troubleshoot this?

My money would be on the internal regulator in the alternator is acting
up...

Thanks, is that something that can be checked before it acts up again?

On 5/21/2016 2:17 AM, Snuffy "Hub Cap" McKinney wrote:
This is an intermittant problem that only happens when driving with the headlights on -- but does happen every time. Any ideas how to troubleshoot it?

For awhile the needle would oscillation between the normal 50-60% position a few degrees after driving a half hour or more with the headlights on. Then I cleaned all the connections in the starting systems from battery terminal to hot connections and grounds, and the problem stopped.

Then last week, I had driven to a store at dusk with the lights on - no problem indicated. I left the lights on for 20 minutes while in the store. Then when I started, the battery needle start oscillating from 50% to 100% regularly, and the battery warning light started blinking off and on.

Car was running fine so I turned off the headlights and just put on the parking lights so I could drive the 2 miles home. After driving about 1/2 mile the oscillating and flashing stopped.

I have tried to duplicate the problem so I could show a mechanic, but all is back to normal for now.

Without knowing anything, I would guest that the computer detected some kind of low voltage and the swinging and flashing was an attention-getter, and not the actual reading.

Any ideas how to troubleshoot this?




Leaving the lights on 20 minutes would put more load on the alternator
when you did start the car. The first thing that comes to mind is that
for some reason the alternator is unable to work properly under high load.

The second thing that comes to mind is some serious sounding problems
have simple solutions. Higher load makes the alternator work harder.
It can be something as simple as a slipping belt. I'd tighten or
replace it before rebuilding the alternator. Yes, I've seen it happen
more than once. .

On Saturday, May 21, 2016 at 10:59:03 AM UTC-4, Ed Pawlowski wrote:
On 5/21/2016 2:17 AM, Snuffy "Hub Cap" McKinney wrote:
This is an intermittant problem that only happens when driving with the headlights on -- but does happen every time. Any ideas how to troubleshoot it?

For awhile the needle would oscillation between the normal 50-60% position a few degrees after driving a half hour or more with the headlights on. Then I cleaned all the connections in the starting systems from battery terminal to hot connections and grounds, and the problem stopped.

Then last week, I had driven to a store at dusk with the lights on - no problem indicated. I left the lights on for 20 minutes while in the store.. Then when I started, the battery needle start oscillating from 50% to 100% regularly, and the battery warning light started blinking off and on.

Car was running fine so I turned off the headlights and just put on the parking lights so I could drive the 2 miles home. After driving about 1/2 mile the oscillating and flashing stopped.

I have tried to duplicate the problem so I could show a mechanic, but all is back to normal for now.

Without knowing anything, I would guest that the computer detected some kind of low voltage and the swinging and flashing was an attention-getter, and not the actual reading.

Any ideas how to troubleshoot this?




Leaving the lights on 20 minutes would put more load on the alternator
when you did start the car. The first thing that comes to mind is that
for some reason the alternator is unable to work properly under high load..

The second thing that comes to mind is some serious sounding problems
have simple solutions. Higher load makes the alternator work harder.
It can be something as simple as a slipping belt. I'd tighten or
replace it before rebuilding the alternator. Yes, I've seen it happen
more than once. .

Good point about checking the belt.

On Saturday, May 21, 2016 at 8:20:56 AM UTC-4, dpb wrote:
On 05/21/2016 1:17 AM, Snuffy "Hub Cap" McKinney wrote:
...

For awhile the needle would oscillation between the normal 50-60%
position a few degrees after driving a half hour or more with the
headlights on. ...
...
Any ideas how to troubleshoot this?

My money would be on the internal regulator in the alternator is acting
up...

--

get the battery load tested, batteries can get internal intermittent shorts. had that happen once, it was terrible and couldnt be found. the battery was pretty new.......

finally decided to repplace battery, problem went totally away..

a year later a buddy cut the batter apart andfound on cells plate was bent, and must of intermittetly shorted

"bob haller" wrote in message ...
On Saturday, May 21, 2016 at 8:20:56 AM UTC-4, dpb wrote:
On 05/21/2016 1:17 AM, Snuffy "Hub Cap" McKinney wrote:
...

For awhile the needle would oscillation between the normal 50-60%
position a few degrees after driving a half hour or more with the
headlights on. ...
...
Any ideas how to troubleshoot this?

My money would be on the internal regulator in the alternator is acting
up...

--

get the battery load tested, batteries can get internal intermittent shorts. had that happen once, it was terrible and couldnt be found. the battery was pretty new.......

finally decided to repplace battery, problem went totally away..

a year later a buddy cut the batter apart andfound on cells plate was bent, and must of intermittetly shorted

Thanks BH. Will check it out. I remember that the meter started slightly cycling from time to time when the headlights were one before the battery was replaced.

On 05/21/2016 10:00 AM, trader_4 wrote:
....

Good point about checking the belt.

Albeit doesn't virtually everything now use just the single multi-rib
serpentine belt with the auto tensioner setup? Not to say can't
eventually stretch where there's insufficient takeup and I have had the
tensioners fail/hang up where not doing its job but overall not nearly
the issue used to be w/ separate, manually adjusted v-belt setup.

The oscillating nature makes me wonder also about the symptom, but could
alternatively (so to speak ) grab/slip a little I suppose...

Agree the symptom being so much more observable after the battery drain
is indicative it's happening under the heavier load...

--

On Sat, 21 May 2016 07:39:00 -0700, "Snuffy \"Hub Cap\" McKinney"
wrote:

"dpb" wrote in message ...
On 05/21/2016 1:17 AM, Snuffy "Hub Cap" McKinney wrote:
...

For awhile the needle would oscillation between the normal 50-60%
position a few degrees after driving a half hour or more with the
headlights on. ...
...
Any ideas how to troubleshoot this?

My money would be on the internal regulator in the alternator is acting
up...

Thanks, is that something that can be checked before it acts up again?

Or the brushes are worn out.

On Fri, 20 May 2016 23:17:42 -0700, "Snuffy \"Hub Cap\" McKinney"
wrote:

This is an intermittant problem that only happens when driving with the headlights on -- but does happen every time. Any ideas how to troubleshoot it?

For awhile the needle would oscillation between the normal 50-60% position a few degrees after driving a half hour or more with the headlights on. Then I cleaned all the connections in the starting systems from battery terminal to hot connections and grounds, and the problem stopped.

Then last week, I had driven to a store at dusk with the lights on - no problem indicated. I left the lights on for 20 minutes while in the store. Then when I started, the battery needle start oscillating from 50% to 100% regularly, and the battery warning light started blinking off and on.

Car was running fine so I turned off the headlights and just put on the parking lights so I could drive the 2 miles home. After driving about 1/2 mile the oscillating and flashing stopped.

I have tried to duplicate the problem so I could show a mechanic, but all is back to normal for now.

Without knowing anything, I would guest that the computer detected some kind of low voltage and the swinging and flashing was an attention-getter, and not the actual reading.

Any ideas how to troubleshoot this?


My 1991 Aerostar did the same. I replaced the regulator and brushes
and it was good for a couple months, then started doing it again. The
local wrecker had a rebuild off a wreck I was able to purchace for
less than another new regulator with a 2 year replacement warranty -
it lasted several years 'till I sold the truck.

On Saturday, May 21, 2016 at 12:44:00 PM UTC-4, dpb wrote:
On 05/21/2016 10:00 AM, trader_4 wrote:
...

Good point about checking the belt.

Albeit doesn't virtually everything now use just the single multi-rib
serpentine belt with the auto tensioner setup? Not to say can't
eventually stretch where there's insufficient takeup and I have had the
tensioners fail/hang up where not doing its job but overall not nearly
the issue used to be w/ separate, manually adjusted v-belt setup.


Agree, the belts today would typically be a lot less susceptible,
particularly with automatic tensioner.



The oscillating nature makes me wonder also about the symptom, but could
alternatively (so to speak ) grab/slip a little I suppose...

Agree the symptom being so much more observable after the battery drain
is indicative it's happening under the heavier load...

--

I'd say an alternator problem, most likely VR or brushes, is the most
likely problem.

On Sat, 21 May 2016 07:39:00 -0700, "Snuffy \"Hub Cap\" McKinney"
wrote:

"dpb" wrote in message ...
On 05/21/2016 1:17 AM, Snuffy "Hub Cap" McKinney wrote:
...

For awhile the needle would oscillation between the normal 50-60%
position a few degrees after driving a half hour or more with the
headlights on. ...
...
Any ideas how to troubleshoot this?

My money would be on the internal regulator in the alternator is acting
up...

Thanks, is that something that can be checked before it acts up again?

Sadly, the answer is basically no. You MIGHT be able to trick it into
misbehaving again by putting a carbon pile load tester on the battery
to draw down the voltage with the vehicle running - but I wouldn't
count on it.
That's why, knowing the problem was in the alternator, and having
already replaced the regulator and brushes once on mine, I simply
replaced rhe alternator when it started intermittently doing it again.
I couldn't be bothered digging deaper into it for the $30 cost of a
used, very recently rebuilt unit from the local "used parts emporium"

On 5/21/2016 1:25 PM, trader_4 wrote:
On Saturday, May 21, 2016 at 12:44:00 PM UTC-4, dpb wrote:
On 05/21/2016 10:00 AM, trader_4 wrote:
...

Good point about checking the belt.

Albeit doesn't virtually everything now use just the single multi-rib
serpentine belt with the auto tensioner setup? Not to say can't
eventually stretch where there's insufficient takeup and I have had the
tensioners fail/hang up where not doing its job but overall not nearly
the issue used to be w/ separate, manually adjusted v-belt setup.


Agree, the belts today would typically be a lot less susceptible,
particularly with automatic tensioner.



The oscillating nature makes me wonder also about the symptom, but could
alternatively (so to speak ) grab/slip a little I suppose...

Agree the symptom being so much more observable after the battery drain
is indicative it's happening under the heavier load...

--

I'd say an alternator problem, most likely VR or brushes, is the most
likely problem.


What is he driving? Could be a '62 Chevy with no auto tensioner.

On Sat, 21 May 2016 16:35:55 -0400, Ed Pawlowski wrote:

On 5/21/2016 1:25 PM, trader_4 wrote:
On Saturday, May 21, 2016 at 12:44:00 PM UTC-4, dpb wrote:
On 05/21/2016 10:00 AM, trader_4 wrote:
...

Good point about checking the belt.

Albeit doesn't virtually everything now use just the single multi-rib
serpentine belt with the auto tensioner setup? Not to say can't
eventually stretch where there's insufficient takeup and I have had the
tensioners fail/hang up where not doing its job but overall not nearly
the issue used to be w/ separate, manually adjusted v-belt setup.


Agree, the belts today would typically be a lot less susceptible,
particularly with automatic tensioner.



The oscillating nature makes me wonder also about the symptom, but could
alternatively (so to speak ) grab/slip a little I suppose...

Agree the symptom being so much more observable after the battery drain
is indicative it's happening under the heavier load...

--

I'd say an alternator problem, most likely VR or brushes, is the most
likely problem.


What is he driving? Could be a '62 Chevy with no auto tensioner.
I believe the OP drives a 1995-ish Aerostar, but I could be mistaken.

wrote in message ...
On Sat, 21 May 2016 07:39:00 -0700, "Snuffy \"Hub Cap\" McKinney"
wrote:

"dpb" wrote in message ...
On 05/21/2016 1:17 AM, Snuffy "Hub Cap" McKinney wrote:
...

For awhile the needle would oscillation between the normal 50-60%
position a few degrees after driving a half hour or more with the
headlights on. ...
...
Any ideas how to troubleshoot this?

My money would be on the internal regulator in the alternator is acting
up...

Thanks, is that something that can be checked before it acts up again?

Sadly, the answer is basically no. You MIGHT be able to trick it into
misbehaving again by putting a carbon pile load tester on the battery
to draw down the voltage with the vehicle running - but I wouldn't
count on it.
That's why, knowing the problem was in the alternator, and having
already replaced the regulator and brushes once on mine, I simply
replaced rhe alternator when it started intermittently doing it again.
I couldn't be bothered digging deaper into it for the $30 cost of a
used, very recently rebuilt unit from the local "used parts emporium"

Thanks. $30 is a bargain. Alternators here are $150 new with lifetime warranty, $100 rebuilt w/ 1 year. If it were easier to swap out, that's what I would do. Still looking for a way to isolate to battery or alternator first. Already checked that connections are all good.

On Sat, 21 May 2016 14:20:33 -0700, "Snuffy \"Hub Cap\" McKinney"
wrote:

wrote in message ...
On Sat, 21 May 2016 07:39:00 -0700, "Snuffy \"Hub Cap\" McKinney"
wrote:

"dpb" wrote in message ...
On 05/21/2016 1:17 AM, Snuffy "Hub Cap" McKinney wrote:
...

For awhile the needle would oscillation between the normal 50-60%
position a few degrees after driving a half hour or more with the
headlights on. ...
...
Any ideas how to troubleshoot this?

My money would be on the internal regulator in the alternator is acting
up...

Thanks, is that something that can be checked before it acts up again?

Sadly, the answer is basically no. You MIGHT be able to trick it into
misbehaving again by putting a carbon pile load tester on the battery
to draw down the voltage with the vehicle running - but I wouldn't
count on it.
That's why, knowing the problem was in the alternator, and having
already replaced the regulator and brushes once on mine, I simply
replaced rhe alternator when it started intermittently doing it again.
I couldn't be bothered digging deaper into it for the $30 cost of a
used, very recently rebuilt unit from the local "used parts emporium"

Thanks. $30 is a bargain. Alternators here are $150 new with lifetime warranty, $100 rebuilt w/ 1 year. If it were easier to swap out, that's what I would do. Still looking for a way to isolate to battery or alternator first. Already checked that connections are all good.



and OEM new alternators are closer to $600.

I checked the wreckers and took the best one they had - had been
installed less than 2 weeks bdfore the car was wrecked and cost the
same as a 10 year old one.

On 5/20/2016 11:17 PM, Snuffy "Hub Cap" McKinney wrote:
This is an intermittant problem that only happens when driving with the headlights on -- but does happen every time. Any ideas how to troubleshoot it?

For awhile the needle would oscillation between the normal 50-60% position a few degrees after driving a half hour or more with the headlights on. Then I cleaned all the connections in the starting systems from battery terminal to hot connections and grounds, and the problem stopped.

Then last week, I had driven to a store at dusk with the lights on - no problem indicated. I left the lights on for 20 minutes while in the store. Then when I started, the battery needle start oscillating from 50% to 100% regularly, and the battery warning light started blinking off and on.

Car was running fine so I turned off the headlights and just put on the parking lights so I could drive the 2 miles home. After driving about 1/2 mile the oscillating and flashing stopped.

I have tried to duplicate the problem so I could show a mechanic, but all is back to normal for now.

Without knowing anything, I would guest that the computer detected some kind of low voltage and the swinging and flashing was an attention-getter, and not the actual reading.

Any ideas how to troubleshoot this?



Cross the bi-pole crystals into sub-panel B, then hope for the best.

On Saturday, May 21, 2016 at 5:21:34 PM UTC-4, Snuffy "Hub Cap" McKinney wrote:
wrote in message ...
On Sat, 21 May 2016 07:39:00 -0700, "Snuffy \"Hub Cap\" McKinney"
wrote:

"dpb" wrote in message ....
On 05/21/2016 1:17 AM, Snuffy "Hub Cap" McKinney wrote:
...

For awhile the needle would oscillation between the normal 50-60%
position a few degrees after driving a half hour or more with the
headlights on. ...
...
Any ideas how to troubleshoot this?

My money would be on the internal regulator in the alternator is acting
up...

Thanks, is that something that can be checked before it acts up again?

Sadly, the answer is basically no. You MIGHT be able to trick it into
misbehaving again by putting a carbon pile load tester on the battery
to draw down the voltage with the vehicle running - but I wouldn't
count on it.
That's why, knowing the problem was in the alternator, and having
already replaced the regulator and brushes once on mine, I simply
replaced rhe alternator when it started intermittently doing it again.
I couldn't be bothered digging deaper into it for the $30 cost of a
used, very recently rebuilt unit from the local "used parts emporium"

Thanks. $30 is a bargain. Alternators here are $150 new with lifetime warranty, $100 rebuilt w/ 1 year. If it were easier to swap out, that's what I would do. Still looking for a way to isolate to battery or alternator first. Already checked that connections are all good.

Driving over to one of the auto parts stores that does free battery and charging system tests is too hard?

"Ed Pawlowski" wrote in message ...
On 5/21/2016 2:17 AM, Snuffy "Hub Cap" McKinney wrote:
This is an intermittant problem that only happens when driving with the headlights on -- but does happen every time. Any ideas how to troubleshoot it?

For awhile the needle would oscillation between the normal 50-60% position a few degrees after driving a half hour or more with the headlights on. Then I cleaned all the connections in the starting systems from battery terminal to hot connections and grounds, and the problem stopped.

Then last week, I had driven to a store at dusk with the lights on - no problem indicated. I left the lights on for 20 minutes while in the store. Then when I started, the battery needle start oscillating from 50% to 100% regularly, and the battery warning light started blinking off and on.

Car was running fine so I turned off the headlights and just put on the parking lights so I could drive the 2 miles home. After driving about 1/2 mile the oscillating and flashing stopped.

I have tried to duplicate the problem so I could show a mechanic, but all is back to normal for now.

Without knowing anything, I would guest that the computer detected some kind of low voltage and the swinging and flashing was an attention-getter, and not the actual reading.

Any ideas how to troubleshoot this?




Leaving the lights on 20 minutes would put more load on the alternator
when you did start the car. The first thing that comes to mind is that
for some reason the alternator is unable to work properly under high load.

The second thing that comes to mind is some serious sounding problems
have simple solutions. Higher load makes the alternator work harder.
It can be something as simple as a slipping belt. I'd tighten or
replace it before rebuilding the alternator. Yes, I've seen it happen
more than once. .

I'll try to take a look at this weekend post what I find out if anything. But if I find a nice BBQ on sale, all bets are off.

"Snuffy "Hub Cap" McKinney" wrote in message ...
"Ed Pawlowski" wrote in message ...
On 5/21/2016 2:17 AM, Snuffy "Hub Cap" McKinney wrote:
This is an intermittant problem that only happens when driving with the headlights on -- but does happen every time. Any ideas how to troubleshoot it?

For awhile the needle would oscillation between the normal 50-60% position a few degrees after driving a half hour or more with the headlights on. Then I cleaned all the connections in the starting systems from battery terminal to hot connections and grounds, and the problem stopped.

Then last week, I had driven to a store at dusk with the lights on - no problem indicated. I left the lights on for 20 minutes while in the store. Then when I started, the battery needle start oscillating from 50% to 100% regularly, and the battery warning light started blinking off and on.

Car was running fine so I turned off the headlights and just put on the parking lights so I could drive the 2 miles home. After driving about 1/2 mile the oscillating and flashing stopped.

I have tried to duplicate the problem so I could show a mechanic, but all is back to normal for now.

Without knowing anything, I would guest that the computer detected some kind of low voltage and the swinging and flashing was an attention-getter, and not the actual reading.

Any ideas how to troubleshoot this?




Leaving the lights on 20 minutes would put more load on the alternator
when you did start the car. The first thing that comes to mind is that
for some reason the alternator is unable to work properly under high load.

The second thing that comes to mind is some serious sounding problems
have simple solutions. Higher load makes the alternator work harder.
It can be something as simple as a slipping belt. I'd tighten or
replace it before rebuilding the alternator. Yes, I've seen it happen
more than once. .

I'll try to take a look at this weekend post what I find out if anything. But if I find a nice BBQ on sale, all bets are off.
---

Still trying to find the time. I was able to catch it cycling just now. No time to check at the alternator tonight, but here's some voltages at the battery terminal.... Turn signals seems to cause the cycling tonight, although I have seen it cycling with just the lights on.

Battery with engine off - 13.6
Turned engine on
Idling with no lights - 15.0 steady
Turned on parking lights - 15.0 steady
Turned on headlights - 15.0 steady
Turned on headlings and turn signal - 15.2-15.8 cycling
No lights, turn signal on - 15.2-15.8 cycling

On Saturday, May 28, 2016 at 11:25:53 PM UTC-4, Snuffy "Hub Cap" McKinney wrote:
"Snuffy "Hub Cap" McKinney" wrote in message ...
"Ed Pawlowski" wrote in message ...
On 5/21/2016 2:17 AM, Snuffy "Hub Cap" McKinney wrote:
This is an intermittant problem that only happens when driving with the headlights on -- but does happen every time. Any ideas how to troubleshoot it?

For awhile the needle would oscillation between the normal 50-60% position a few degrees after driving a half hour or more with the headlights on. Then I cleaned all the connections in the starting systems from battery terminal to hot connections and grounds, and the problem stopped.

Then last week, I had driven to a store at dusk with the lights on - no problem indicated. I left the lights on for 20 minutes while in the store. Then when I started, the battery needle start oscillating from 50% to 100% regularly, and the battery warning light started blinking off and on.

Car was running fine so I turned off the headlights and just put on the parking lights so I could drive the 2 miles home. After driving about 1/2 mile the oscillating and flashing stopped.

I have tried to duplicate the problem so I could show a mechanic, but all is back to normal for now.

Without knowing anything, I would guest that the computer detected some kind of low voltage and the swinging and flashing was an attention-getter, and not the actual reading.

Any ideas how to troubleshoot this?




Leaving the lights on 20 minutes would put more load on the alternator
when you did start the car. The first thing that comes to mind is that
for some reason the alternator is unable to work properly under high load.

The second thing that comes to mind is some serious sounding problems
have simple solutions. Higher load makes the alternator work harder.
It can be something as simple as a slipping belt. I'd tighten or
replace it before rebuilding the alternator. Yes, I've seen it happen
more than once. .

I'll try to take a look at this weekend post what I find out if anything. But if I find a nice BBQ on sale, all bets are off.
---

Still trying to find the time. I was able to catch it cycling just now. No time to check at the alternator tonight, but here's some voltages at the battery terminal.... Turn signals seems to cause the cycling tonight, although I have seen it cycling with just the lights on.

Battery with engine off - 13.6

You sure your meter isn't busted? Fully charged battery is just 12.7V




Turned engine on
Idling with no lights - 15.0 steady
Turned on parking lights - 15.0 steady
Turned on headlights - 15.0 steady
Turned on headlings and turn signal - 15.2-15.8 cycling
No lights, turn signal on - 15.2-15.8 cycling

Those look like they are about 1V high too. I suspect your meter
is off. You could try a fresh 9V battery to check, that should be around 9..5V.

Assuming the meter is off, I don't see anything unusual in your
readings. It's not dropping anywhere that would indicate a
charging problem that would put the dash light on. Exactly what
puts the light on varies from car to car. Some use the old method,
where it's essentially a light bulb tied to 12V on one end, the
other goes to the alternator. If the voltage there drops low,
the bulb lights up. Others now have the computer monitoring the
voltage at the alternator and it decides when to turn the light on.