Need Help Diagnosing Bad Alternator

[grober02]

Just need a second opinion before buying a new alternator. For the past month, my VX will not start without jumping it. All electronics work, i.e. headlights, windows, radio. I had a mechanic connect a volt meter, and I believe he said the reading was 12 volts which is below the norm. The battery tested okay. This cannot be a bad starter, right? The starter clicks fast, then fades away. But like I said above, I can jump it. So it’s gotta be the alternator, right? Also, never replaced an alternator before, but it looks fairly easy. Can anyone tell me how many bolts are actually holding it on? I have the repair manual, but based on the diagram, I cannot tell if it’s 3 bolts or 6. THX

[etlsport]

i dunno, if your alternator wasnt working you would only be able to drive the vx a few miles before it no longer had power and died on you.. if you drive your vx for like 20 minutes, then get out for a couple minutes, come back and try to drive it, will it start? it sounds like you might have something drawing power while your vx sits rather than a dead alternator

[grober02]

I can drive it 20 minutes or an hour. As soon as I shut it off, I have to jump it again.

[JHarris1385]

Have you actually driven it for an hour since. Seems to me if your altenator was failing that would be nearly impossible.

[etlsport]

hm.. i guess it could be alternator not producing enough current to charge the battery and run the vx both.. and u said u had the battery tested, but that would have been my next guess

[grober02]

I have driven it more than an hour once, but mostly short 20 minute drives. Yup, the battery tested okay. Just today, I checked my VX--Of course it wouldn't start, but the lights, radio, & windows work after sitting untouched for about 2 weeks. How can I tell if something is draining the power while it's sitting (not in use)?

[Dino]

I have driven it more than an hour once, but mostly short 20 minute drives. Yup, the battery tested okay. Just today, I checked my VX--Of course it wouldn't start, but the lights, radio, & windows work after sitting untouched for about 2 weeks. How can I tell if something is draining the power while it's sitting (not in use)?

Try this:
http://vehicross.info/forums/showthread.php?t=11247

[biju]

ALTERSTART
150amp
50% at idle, 100% over 1400rmp
Plug and play
$299

-biju.

[etlsport]

aww dooooood dont show me that... now i have to get a second job

[grober02]

ALTERSTART
150amp
50% at idle, 100% over 1400rmp
Plug and play
$299

-biju.

That's funny...I was reading one of your posts about the Alterstart. Have you gotten good results with this alternator and does it bolt right on like stock? I think Advanced Auto Parts wanted $230 for a refurb, so I might as well spend a few extra $$$ and get a better product...

[biju]

That's funny...I was reading one of your posts about the Alterstart. Have you gotten good results with this alternator and does it bolt right on like stock? I think Advanced Auto Parts wanted $230 for a refurb, so I might as well spend a few extra $$$ and get a better product...

The alt is EASY to install, and is indeed plug n play. It's quality for the money. Not sure what else to add really... It was a good purchase/decision.

Not only does it run my car/charges (multiple batteries) with ease - it also runs my new audio system without breaking a sweat.

-biju.

[Locomigo]

Easy test for alt/bat.

Start the vehicle. Remove the ground from the battery. If the vehicle dies, it is definitely your alternator.

If the vehicle continues to run, this is where you have issues. It could be a low output level where it is not fully charging the battery.
It could also be that the battery (even though it tested good) gave a false reading and not fully charging requiring a new battery.

Is the battery the OEM one, or aftermarket?
How are you testing the bat?
It can easily have enough juice to run the radio, etc but not to start the vehicle.

Ive had to change both, the alt was due to a mudhole. that I dropped into while out on a trail.


Good luck.

[Dino]

Loco is right. It could be BOTH!

Here's a little insight from years of troubleshooting automotive charging systems. It seems long winded but it's full of some good info.

When the engine is running, put a volt meter on the battery terminals. It should read something ABOVE 12 volts. If it does, turn on the headlights and watch what the voltage does. If it drops a bit then comes back up and stabilizes ABOVE 12 volts, alternater function is good. If it drops and stays dropped, and or, keeps dropping, you have a weak alternator and maybe a bad battery.

Here's the deal with alternators:

They need voltage to energise the electromagnet (field winding) inside them. The part that spins, the rotor, has poles that pass by the poles in the field winding and this produces pulses of AC electricity. The diodes inside the regulator, usually built into the alt., smoth out the AC to DC voltage. This then flows through the battery. The regulator determines how much flow based on the current draw on the battery.

When you do the "pull a battery cable while it's running..." test, there is enough residual magnetism in the field windings to make enough voltage to keep things going. If it stays running, you know that its making voltage, but but it may not be making much current.

If your battery is borderline, it might test OK. Did they do a FULL LOAD TEST???? It might fail that test. THAT'S why it starts when you add another good battery while jump starting it.

A note on batteries is in order here, bear with me...
When you connect two batteries together as in jump starting, they will equalize towards the lower voltage battery. This is why a car with a completely dead battery needs to sit and charge before it will even crank over. In fact, it will start right up if you just bypass the battery and jump it. The dead battery is an extra load on the charging systems peak voltage output. If your battery has a dead or weak cell, it would not have enough amperage to operate your starter by itself. Jumping it does.
First things first. Get the battery tested at full load. I would suspect your battery has a bad cell based on what you've said. A new one will run you less than $80. Well worth the bucks. A good battery is EVERYTHING in any car. A must have. Once you've verified the battery being OK with a full load test AND a hydrometer test on EACH CELL, then you should get the alnernator tested at an auto parts store. Most will come out to the parking lot and test it with a hand held unit or a roll around, cart mounted older unit. ...and they'll do it for free.

Full load test, Full load test, Full load test, Full load test, Full load test....

one more mechanics opinion.