Car repair expert opinions desired: alternator test

In My Humble Opinion
1

A quick and dirty AutoZone test on my 2000 Civic failed me on the alternator “charging current” item. I haven’t had serious or consistent issues with starting the car, but considering that I’m thinking of driving a couple of hours to a place without cell service, I’m guessing that it’s likely a cause for concern.

How serious is this result, potentially? Is my instinct of needing to address this before my trip correct? How much will it be, approximately? Will I need to wait for any parts for a car of advanced age like mine?

2

The hard issue is always the reliability and specificity of the test. Neither of which facts you know.

If your alternator is stone dead, you’ll have total car failure within a couple of hours of operation: won’t run, won’t start, won’t do nuthin’.

If your alternator is weak, and you don’t run the headlights, air conditioner, or stereo, the car might operate fine for weeks that way. or maybe only a day. And however long it would last not using those things, it’ll die sooner if you do use them. The more short trips with attendant starts, the sooner it quits.

Bottom line:
Your worst case scenario is pretty bad: you stranded in the boonies. But we really don’t have enough info to know whether that worst case is a certainty or only a remote possibility or anywhere in between.


Cost:
The problem may not be the alternator itself. It may be the voltage regulator (“VR”) which is the gizmo that controls alternator output. If a bad VR is telling a perfectly good alternator to put out little or no juice, well, that’s what it’ll do.

For round numbers, autozone says VRs cost $20 to $80 and alternators cost $100 to $250. On some models of car the VR is an integral part of the alternator, and on others it’s a separate box. you/we would need the exact model, engine size and type, etc., to get the exact part(s).

Both of those items are easy enough for a backyard mechanic to change, but not somebody who’s never worked on a car and owns no relevant tools. A reasonable but still half-assed rule of thumb for paid-for car repairs is 1/3rd parts, 2/3rd labor. So $300 of parts would cost $600 to install for a total of $900, etc. That rule of thumb is all sorts of inaccurate / problematic, but without any further data to go on it’s the best you / we can do.

IANA Honda guru, but about now it might well be time to replace the drive belt for the alternator too.

3

I think the voltage regulator is built in the alternator.

Yeah. Mechanic time. Better before the trip than during. IMO

4

Were there any symptoms which prompted you to get it tested?

5

My $0.02 is assume the worst will happen. Then decide what you need to do to avoid it.

It may be an unfortunate cost you didn’t expect but there it is. Best dealt with sooner than later I think (a lesson I have learned the hard way but maybe the OP will get lucky…maybe).

6

Thanks for the advice so far. I’ve gotten advice elsewhere to get a cheap multimeter and measure the battery myself to confirm the test. I think I might do that before contacting a mechanic - might save me money and trouble.

7

An alternator test and a battery test are very different things.

A multimeter won’t tell you shit about a battery or an alternator other than: it’s dead, Jim. What you need for diagnosis is the qualitative details between perfectly healthy and stone irretrievably deceased.

8

So sounds like you think i I should have a professional (ie not myself or a random retail dude) make that determination? The suggestion I got was to take it in if the multimeter registered less than 13.5 v.

9

“It” the battery or “it” the battery w the car running? If you don’t know which or why it matters, you’re already in too deep.

All those shadetree diagnostic techniques work ok on 1980s cars. Not so much modern ones. And your antique Honda is modern enough to matter.

10

I meant “the suggestions I got from multiple parties was to test the battery with the multimeter while the car was running (with AC, radio, etc. on or off depending on who was talking), and if it registered less than 13.5 volts, to take the car in for repairs (and assumably not do so if it were above).”

11

Sorry to sound hostile. Not on ourpose, still, that’s all on me.

Not an unreasonable test, but probably still inconclusive.

As @filmore asked … What, if anything, prompted you to have autozone do an alternator test to begin with?

12

Ah, well, if I can’t just blindly trust that I’m good to go into rural nowhere if the test passes, then yeah, might as well just call a professional.

Not because of any performance issues whatsoever. I knew I was going on that trip, and because I’d recently parked with my headlights on for half an hour, I decided to check so I’d be assured that I wouldn’t be stuck in the wilderness. After testing the battery, the Autozone dude asked if I wanted to check the alternator too, since it used the same device, and I said sure, why not. That’s it.

13

Decided to take it in to my usual repair place for a more detailed diagnostic. Fingers crossed. Thanks for the advice!

14

Good luck!!

15

Just to wrap up the story (hopefully), it was the battery and not the alternator, according to the repair guy. I have no idea how that happened, but at least my odds of electrical failure in the boonies are likely way down.

16

Yaay!!!

17

Eh… kinda.

With the car off, the battery should read close to 12.6 volts. Around 12.5 to 12.6 is good. Down around 12.0 volts is pretty well discharged. If the alternator is working, the battery shouldn’t discharge this low, assuming that the battery is actually good.

After measuring the battery with the car off, now repeat the test with the car running. The voltage measured at the battery should go up by maybe a volt or so. Your 13.5 number is good, but it assumes that the battery voltage with the car off is 12.5 or so.

But basically, the quick and dirty test is to make sure that the voltage goes up by a volt or so, maybe a bit more if you rev the engine.

In other words, to test the alternator, you’re looking for the voltage to go up by at least a volt or so. You’re not necessarily looking at the actual voltage. If the battery voltage is 12.1 with the car off and it goes up to 13.1, it’s probably the battery that is bad and not the alternator.

Note that this test works well on an older car like your 2000 Civic. It’s not necessarily a valid test on a modern car with a newer fancy shmancy smart charging system.

If you don’t measure the battery voltage with the car off first, then you won’t really know if it’s the battery or the alternator that is dragging the voltage down.

They tend to undersize batteries in cars these days, partly to save costs and partly to save weight. Unfortunately, this also means that car batteries often don’t last very long. The battery gets discharged too far during normal starting and starts to chemically self-destruct.

A lead-acid battery is made up of lead and lead oxide plates with sulfuric acid and water in between them to act as the electrolyte. When the battery discharges, both the lead and the lead oxide plates turn into lead sulfate, and the sulfuric acid turns into water. When you charge the battery, this reaction reverses, and the plates go back to lead and lead oxide, and the water goes back to sulfuric acid. If the battery gets discharged too far, the lead sulfate can form
crystals that don’t break up when the battery is charged, leading to a loss in battery capacity. This is called battery “sulfation”.

If you put in a larger battery, the battery won’t get discharged as far during use, and your battery plates won’t end up a sulfated mess. Instead of replacing the battery every 3 or 4 years, you might not ever have to replace the battery over the life of the car.

Of course, this assumes that you can actually fit a larger battery into your car. Many cars these days are made with just a very teeny tiny space for the battery, and you physically can’t fit a larger one in its place.

Temperature extremes are bad for batteries. Cars are more difficult to start in very cold weather, which puts more of a strain on the battery. Hot weather is also bad, since it increases the self-discharge rate for batteries (a battery that is just sitting there will discharge on its own, hence “self-discharge”). If the battery sits too long at a low voltage (below 12.2 volts or so), sulfate crystals are more likely to form.

If the car is going to sit for any length of time, you can get battery tenders to keep the battery fully charged and prevent plate sulfation. I got an el-cheapo one from Harbor Freight for about $10 that works pretty well. You can probably find something similar on Amazon.

18

Just going to confirm what engineer_comp_geek has stated about fancy-shmancy charging systems.

On my 2017 Honda CR-V, the battery charging voltage has a high and a low mode. When the car is loafing along, the charging voltage to the battery is around 13.5 volts. When there is more of a load on the car (or for whatever reason the designers thought was optimal), the voltage drops well below 13 volts, though not usually below 12.5. If you just happen to be measuring the voltage at THIS moment, it will look like the battery is not being charged. If you happen to measure it at THAT moment, it looks like you’re charging system is working perfectly.

I know this from watching my USB cig lighter charger’s digital voltage display. Honda confirms this in their owner’s guides as well.