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Discussion Starter · #1 ·
Any idea what could be causing an intermittent battery light? Alternator and battery tested good today. Had battery cables replaced last year with OEM. Car runs fine otherwise, no issues with lighting, idle, driving dynamics. Warning light will be flickering on and off every approx every 20 seconds and will be on 70% of the time. This just started two weeks ago. I had a similar problem a year ago in the winter and had the battery cables replaced and it went away. Alternator is 4 years old, a aftermarket brand. I wonder if it has something to do with the cold….car is in good shape otherwise, no leaks.

2003 4dr Lx, 4 cyl. 140k.
 

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Gearhead Girl 🤟
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Do you know the specific brand of the alternator? Some brands hold up well, and some don't.
 

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Are all the cable connections, including ground wire connections tight?

With cold weather, things contract and what may have been a good connection could suddenly become loose...
 

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Make sure all battery cables are clean and tight. Be sure to check the body ground to make sure it is clean and tight also. Check the engine to body ground and the transmission to body ground too.

Check the alternator belt as well and make sure it is not slipping.

If you have a multimeter check the voltage while the car is running.

The alternator light is controlled by the voltage regulator inside the alternator.

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Check all grounds & cables for tightness, make sure they are corrosion free. Could be corrosion in the middle of the cable but would seem unlikely since they were replaced last year. Some aftermarket alternators, the pulleys channel where the belt goes on them aren't deep enough causing the belt to slip / wear so check that. Many have their alternator rebuilt or get a used one as some elect. parts bought @ chain stores can become problematic, my Bosch one lasted only 2 yrs.
 

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Discussion Starter · #6 ·
Thank you for the replies, I will check all connections this morning. Will check the belt for wear too.

Checked all cables and grounds and all are tight and clean, we are in Texas and it’s garage kept. Battery terminals are clean and in good shape.

If the car runs fine is there any hurt still driving it? We are trying to buy a minivan but prices are just so high right now have been trying to delay.

Don’t have a record on what brand the alternator is, took a look and didn’t see any identifying badging on it.
 

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I suspect you have a diode failure or is about to fail, possibly a bad voltage regulator or worn brushes. Again, to test this you can get an inexpensive battery/alternator/starter tester or use a multimeter.

What you want to see is the voltage while the car is running. Also, you want to see if there is AC ripple coming from the alternator.

Personally, If all grounds and cables check out as well as the belt, I would find a used OEM alternator with as low miles as possible and replace it.

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Discussion Starter · #8 · (Edited)
Below the results when testing with a multi meter at the battery terminals. Is the lower read when the AC is on a indicator the alternator is bad?

Checked the belt and it didn’t seem to be slipping.

12.35: Engine off, lights off
14.14: Engine on, headlights off, air conditioner off.
12.03: Engine on, air conditioner on, headlights on, cabin lights on
 

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12.35v to start is a bit low, and the 14.14v sounds a little low as well. Definitely 12.03v is low with the AC and headlights pulling power.

If you have a chance flip the multi meter from DC to AC and see how much AC ripple the electrical system is seeing.

My guess is you have an issue with the alternator, maybe the diode bridge or voltage regulator but the alternator is not supplying the current needed.

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Discussion Starter · #10 · (Edited)
Flipped to AC and read 30, is that bad? I will pay the shop to replace it in Monday. They quoted $560 using a Denso unit. Would do it myself but we need both our cars.

Thank you for the help. It’s very nice to see folks acting in a community, especially in these Covid times. This was fun to learn about also. If I had space to keep this car as a hobby I could see myself doing that.
 

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Elvira
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30 what ? mVolts or volts (it's probably not 30 volts but we should try to be as accurate with the reported data as possible. It's good practice)

.030 volts (30 mVolts ) is OK. 30 volts is horrible. Maximum would be 500 mVolts (0.5 volts) anything over that is a problem with the diodes as @greg1c posted.
 

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Discussion Starter · #12 ·
I actually was not sure in the units of voltage. The manual for the tester makes me think it’s volts, but that sounded really high. Looking again below, seems like its volts…could a bad alternator make it that high?
Watch Finger Electrical wiring Gas Cable


Sleeve Font Material property Paper Pattern
 

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Elvira
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Youch ! according to what I see here, the 200 VAC range selection IS indicating 30.1 VAC.

You would want to see 0.5 or less.

The resolution of 100 mV would read 0.1 vac even if it were 30 mVac. That resolution is OK for our purposes here right now though.

That high of a voltage is telling me there IS something wrong with that alternator.

Are you having problems with any of the electronics in the car?
 

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If that is correct, and it appears to be 30v of AC current. The alternator produces AC current and the diodes convert AC to DC. If you are seeing 30volts of AC, one or more Diode pair has failed. A typical alternator has 6 diodes in the rectifier and that is 3 pairs of diodes. An Alternator can generate in excess of 300vac. Here is a typical alternator wiring diagram.


Font Rectangle Schematic Slope Parallel


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Discussion Starter · #15 ·
Purely electrical, I don’t think so. When I sit at a red light the car shakes a little bit which gets worse when the air conditioner kicks on, but I just always figured that was the car getting older and losing power. It never bothered me too much since I’ve been around old tractors - though the kids noticed and thought it was funny.
 

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If that is correct, and it appears to be 30v of AC current. The alternator produces AC current and the diodes convert AC to DC. If you are seeing 30volts of AC, one or more Diode pair has failed. A typical alternator has 6 diodes in the rectifier and that is 3 pairs of diodes. An Alternator can generate in excess of 300vac. Here is a typical alternator wiring diagram.


View attachment 537093

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If that is correct, and it appears to be 30v of AC current. The alternator produces AC current and the diodes convert AC to DC. If you are seeing 30volts of AC, one or more Diode pair has failed. A typical alternator has 6 diodes in the rectifier and that is 3 pairs of diodes. An Alternator can generate in excess of 300vac. Here is a typical alternator wiring diagram.


Font Rectangle Schematic Slope Parallel


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Discussion Starter · #18 ·
If that is correct, and it appears to be 30v of AC current. The alternator produces AC current and the diodes convert AC to DC. If you are seeing 30volts of AC, one or more Diode pair has failed. A typical alternator has 6 diodes in the rectifier and that is 3 pairs of diodes. An Alternator can generate in excess of 300vac. Here is a typical alternator wiring diagram.


View attachment 537094

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Thank you for the diagram. This is very helpful.
 

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Elvira
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You would be experiencing pulsating lights/dash /dome light or more likely dimming and brightening of the bulbs. Could also be the cause for the rough tractor-like feel.

EDIT: I should have figured this earlier when you posted in #1 that the battery light was flickering every 20 seconds.

I would really suggest you don't run that car until that is fixed or you run the possibility of trashing the ECU and other EXPENSIVE electronics.

Who and how was the alternator tested by? If there is/was that high of ripple they would have noticed that (I'd hope). If they ONLY checked the DC component for 12.6-145 V they cheated you in their diagnosis.

I fear that the original battery cables that went bad MAY have caused this problem for the alternator. If the cables were open circuit or very high resistance caused by internal corrosion, the alternator maight have trashed itself.

This would be similar to running the alternator without a battery connected. That ONLY workes/worked with non-electronically controlled cars from eons ago.

That possible 300 volts that the alternator is capable of producing is usually when the load of the battery is removed. @greg1 ?
 
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Discussion Starter · #20 ·
Will drive it to the shop 1 mile tomorrow and that’s it. They must not have tested the AC ripple. My wife dropped off and I think that they were puzzled that the battery light was on when it tested at 14 V DC with no headlights on. At that point they probably gave up and never bothered to test the voltage under higher load or to test the AC voltage. They didn’t charge for the diagnosis so I guess I cannot be too hard on them. Will keep that it in mind for the future though. Will take it to our other mechanic for the repair.
 
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