Alternator Going Bad?

Trixie

Moderator
Staff member
The Kid came home from a road trip with my car and said he thinks my alternator may be going bad. Apparently after they stopped for gas and took a rest, the headlights came on very dim (the new ones Hubby installed while he took my original HIDs... yeah... still annoyed), but then got brighter while driving on the highway. I tend to agree with him but Hubby won't believe us unless you all agree.

So, I forgot, can an alternator be tested, and if so is it reliable? Or could this be some other issue? Thanks!
 
An alternator can be tested at your local autoparts store and on my gen 2 takes maybe 5 minutes to get off. You remember how your son gets to live at home and drive your car? We'll its time for him to spend a few minutes in the cold icy tundra that is the Auto Zone parking lot, removing the alternator and battery for testing.
 
The Kid came home from a road trip with my car and said he thinks my alternator may be going bad. Apparently after they stopped for gas and took a rest, the headlights came on very dim (the new ones Hubby installed while he took my original HIDs... yeah... still annoyed), but then got brighter while driving on the highway. I tend to agree with him but Hubby won't believe us unless you all agree.

So, I forgot, can an alternator be tested, and if so is it reliable? Or could this be some other issue? Thanks!

Yes, alts and batteries can be tested at the parts store.
 
He doesn't have to remove anything. Advance Auto has a rollout unit that will test alternator and battery. If you have a high amp meter you can also test yourself, be sure it can handle high amp output from your alternator or you can fry your test meter.
 
You can also test it with a simple voltmeter.

Measure the voltage of the battery with the car off, then start it, and measure it again while it's running. It should be around 13.5 volts or higher when running.
 
You need a digital voltage meter to test the charging system yourself. Here's what you need to do.

1. Engine off - battery standing voltage.
With the engine off and fully charged battery, hook up the meter directly to the battery terminals(not clamp, bolt, nut, etc...). Read the meter as volts DC. 12.8 volts is 'perfect' for a typical fully charged wet cell battery in new condition at room temperature. Be aware battery voltage will decrease with temperature.

2. Engine running - lo idle - no load charging voltage.
Engine running at idle, all lights, accessories and AC off. Read the meter after voltage stabilizes from starting engine. Usually 0.5 to 1.0 volts higher, typically mid 13 volts.

3. Engine running - hi idle - no load charging voltage.
Engine running at 1500 RPM, all lights, accessories and AC off. Read the meter. Should not be over 2.0 volts above battery standing voltage.

4. Engine running - full load charging voltage.
Engine running at 2000 RPM, lights on high beams, AC on MAX(blower on high). Read the meter. Full load charging voltage should be a minimum of 0.5 volts above standing voltage of battery.

Also note that a hotter alternator will decrease voltage output.

Another good test is to check the voltage with the negative lead on the negative battery terminal and the positive lead on different points starting from the alternator, fuse box distribution point and then the battery positive post.

If the battery is not sealed, inspect the cells making sure each is up to the minimum level. During charging, observe the bubbling in each cell. A bad cell will have little or no bubbles compared to the others. Observe all proper safety precautions at all times when testing and charging any lead-acid battery. You can also test the electrolyte in each cell with a tester available at most parts stores.

Never believe those 'charge indicators' on sealed batteries.

The only 'proper' battery test is a load test on a fully charged battery.

A load test on an alternator is best as well if available.
 
If the lights get brighter with higher RPMs.....95% of the time the Alternator is taking a dump.
 
Just got back from having it tested. Apparently alternator is fine, but the battery could use a charge. Maybe having her sit for about 2 months outside untouched wasn't such a good idea? But I thought the alternator charges the battery when run on long trips?
 
I'd take the car to someone else and have THEM test the system..... An alternator IS supposed to charge the battery on a not so long run too!! Did they check the fluid level in the battery?
 
Yup the Alt charges the battery. Sounds to me like the battery is in poor condition. How old is it and what kind of battey?
 
Thought I replaced the battery a couple years ago. Will make Hubby check connections, etc. as that was a problem last time. I never did get the heavy duty battery cable thingies.

Then again, today I was running a few errands, short trips with stops, so possibly no chance for battery to charge up, although car started right up all day with no probs.
 
About 2 weeks ago, my mark had the ground at the battery fail, went to start it up and nothing. Left the key on and wiggled the pos and neg connections and when I wiggled the neg connection, everything came on. Went to start it, and as soon as I went to the Start position everything shut off again. Replaced the negative battery terminal connection, and everything was good! Def check your connections and see if that battery is at 12.8v =)

EDIT: Check it in the morning before you drive it anywhere. If you check it after you had the car running, it gives false readings because the battery had recently been charging.
 
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