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Alternator, bad? Definitive test?

Yes, I have the feeling I’ll be replacing both the alt and the battery. :frowning: Thanks for the word!

Darned google - see my edited post above.

Very close, but the instrument cluster looks different. Thanks, texases!

That’s a nice looking truck cab! Great photo.

Tester has already mentioned it, but it’s important enough to mention again. Never, ever run the engine w/the battery disconnected, especially on modern electronic fuel injected cars. Very expensive damage to the car’s electronics can result. The alternator produces – what would be the river equivalent — of a raging rapids of current flow. The battery act like a dam on the river, with a reservoir behind it, creating a nice calm pool of current flow to supply the car’s electronics.

A quick way to do a basic check on the alternator & battery, assuming you have a shop volt meter to hook to the battery terminals, is to check these measurements. They should measure

  • 12-12.6 volts with engine turned off, after car has been sitting over night
  • 13.5-16 volts with the engine running, after first starting an overnight-cold engine
  • 13-14 volts after the engine is warmed up & has been running 30 minutes or so.

That’s where to start. Report back what you measure and we’ll go from there.

George,

A battery that reads 12.0 VDC is a dead battery.

The battery should read no less that 12.40 VDC.

Tester

If a battery measures 12.0 volts and won’t robustly crank the engine, no disagreement at all.

http://faq.ninja250.org/wiki/How_to_tell_if_your_battery_is_dead

Tester

Testing the alternator in the vehicle IS the preferred method

That way, you’re also taking the wiring harness, fuses, battery, fusible links, accessory drive belt, tensioner, etc. into consideration

If you bring the alternator to the store to be tested, you’re really not getting the full picture

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The use of a voltmeter is highly recommended.

Failing that, you might turn the key to the RUN position (engine not running) and touch the alternator pulley with the tip of a screwdriver. You should feel the magnetic attraction of the pulley if the alternator is good.

If there is no magnetic attraction this means the alternator fields are not being energized through the dashboard battery warning light. If the bulb is out the alternator will not charge.

If there IS a magnetic attraction then you need to check the circuit between the alt. and the battery positive post. Maybe a fusible link has given up.

In most cases I’d guess this symptom, when it is first observed this time of the year, in the fall as ambient temperatures are falling, is a battery on its last legs. Works fine all summer b/c it’s hot enough that the weak battery doesn’t show itself. But as soon as the temperatures start to drop, voila, the weak battery rears its ugly head. I wouldn’t be messing much with the alternator theory until the battery has been properly tested or replaced.

It was my understanding that isolating the alternator and performing a bench test would assist to determine if it truly was the alternator at fault. These rebuild shops are set up for that and should be able to check your alternator to full specification. Once again I’m no expert, but I’ll have to check with my local rebuild shop for the reasoning. Also, they can determine if you just need the voltage regulator replaced or if a full rebuild is required.

@always_fixing
This is true. But with the alt. still installed you can test the whole circuit. If the voltage is low or nonexistent at the battery you can trace backwards from the battery to find where you lose current. If your first step is to take the alt. out for testing and you find it is good you just wasted probably a couple hours and accomplished nothing. Its better to do all the testing you can before removing things.

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Charged the battery yesterday, drove it to the store and back (six mile trip), then a neighbor borrowed it for a bit last night and drove back with the lights on. He said they didn’t dim. But - per the suggestion of one kind soul here - I bought a hydrometer and checked the cells. Five out of the six tested on the low end of “Fair” (1200) and the sixth tested “Water.”

I also measured 12.4 volts this morning; cold engine. Now I’m going to re-read all your kind posts and spend some more time with the multi-meter and test again. The meter tests a few days ago indicated – as I interpreted the results – that the alt was changing the battery, but weakly.

I’ve got a friend driving the fifty-five miles into town tonight. I’m seriously considering just having her get me a new battery to eliminate that as a possible problem, especially considering the hydrometer tests.

Pardon my in and out with this thread. I have a job on the internet and I can’t always pick my hours.

THANKS TO ALL WHO HAVE TRIED TO HELP.

With a screwdriver on the belt, I didn’t feel an electronic field. If I put the screwdriver on the other side of the bracket and place it on the alternator, I do feel an electronic field.

Thanks for your help, ok4450!

First test you mention, I got 12.4 volts.
Missed your second test (bad planning)
Your third test, after running the engine for 30 minutes, I got 13.17 volts.

Don’t know if you picked up on my earlier comment, so will repeat it here for you.


"Charged the battery yesterday, drove it to the store and back (six mile trip), then a neighbor borrowed it for a bit last night and drove back with the lights on. He said they didn’t dim. But - per the suggestion of one kind soul here - I bought a hydrometer and checked the cells. Five out of the six tested on the low end of “Fair” (1200) and the sixth tested “Water.”

I also measured 12.4 volts this morning; cold engine. Now I’m going to re-read all your kind posts and spend some more time with the multi-meter and test again. The meter tests a few days ago indicated – as I interpreted the results – that the alt was changing the battery, but weakly.

I’ve got a friend driving the fifty-five miles into town tonight. I’m seriously considering just having her get me a new battery to eliminate that as a possible problem, especially considering the hydrometer tests.


THANKS GeorgeSanJose !!!

So I got the charging system reader you suggested, assuming I have a cigarette lighter “outlet” to plug it into, only to find out I don’t. Sheeeeesh. :frowning:

Still a great suggestion, tho. :slight_smile:

If the second part of the test measurement (the part which you weren’t able to do yet) if it corresponds to the numbers I posted above, and given the specific gravity measurements, most likely sounds like you have a battery problem. This is easily confirmed one way or the other by a battery load test. Shop’s have a special test gadget they use which makes this a simple job. But it may require they charge the battery with a battery charger for an hour or two first.

OK, thank you GeorgeSanJose!!

Well you can still use the that device with a couple of light jumper cables like I used on my mower. Red is the center connector to positive on the battery and black is the side contact to negative, then start the car and look at the charging voltage.

Oh, interesting. If my friend doesn’t take the battery into town tonight to get a replacement, I’ll try that! Thanks!