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hi my 97 3.5 isnt charging .... when first started , i t charges a few minutes , then stops and the light comes on . all the wiring connections are clean and tight... (i dont wanna buy an alt for nothing)
thanks in advance... im gonna need .... ps pump/lines, cowl panel, hood hinges, front sway bar links
PAINT!!
 

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How many miles on that bad boy? How old is the battery.
 

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220k. It may be that the alternator is worn out. When the brushes wear out, they are too short to firmly contact the commutator, and this often shows up as intermittent (may make contact when cold, but loose solid contact as things heat up). I would think that that condition would show up in an alternator test as the brush pressure against the commutator is going to be very low even at cold startup when it’s working just enough to keep the battery light from coming on. I agree that you don’t want to replace a good factory alternator (I’d trust a well-used factory alternator more than I would an aftermarket rebuild right out of the box), so have it tested to be sure it’s bad before replacing.
 

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Easiest way to test an alternator is to connect a Voltmeter up to the battery. It should read about 12.5V. Start the vehicle and take another reading, the battery should register about 14V. If it's less than 12.5 the alternator is bad.
 

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Easiest way to test an alternator is to connect a Voltmeter up to the battery. It should read about 12.5V. Start the vehicle and take another reading, the battery should register about 14V. If it's less than 12.5 the alternator is bad.
If I could add to that, the spec. Is 13.5 - 14.5 volts at a little over 2000 rpm engine speed. In warmer weather, it regulates down closer to the 13.5 end of the range, and moves towards the 14.5 volts upper end in colder weather.

My point about having it tested was to see how the output holds up with a specified current load. If the brushes are worn to the point that they are making only light contact with the slip ring, that would drop it’s current capability. He’s getting no dash light at startup, then gets a light. The possibility that I’m proposing is that the brush contact is marginal due to wear - makes enough contact at start up (just above the threshold for lighting the dash light), but contact pressure drops as it warms up so voltage drops just below that threshold. My suggestion as a possibility.

There could be other explanations: Loose connection (resistance and voltage drop rise as the loose connection gets hotter due to current flow), fusible link between alternator and battery eaten halfway through with battery corrosion, etc.
 

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If I could add to that, the spec. Is 13.5 - 14.5 volts at a little over 2000 rpm engine speed. In warmer weather, it regulates down closer to the 13.5 end of the range, and moves towards the 14.5 volts upper end in colder weather.

My point about having it tested was to see how the output holds up with a specified current load. If the brushes are worn to the point that they are making only light contact with the slip ring, that would drop it’s current capability. He’s getting no dash light at startup, then gets a light. The possibility that I’m proposing is that the brush contact is marginal due to wear - makes enough contact at start up (just above the threshold for lighting the dash light), but contact pressure drops as it warms up so voltage drops just below that threshold. My suggestion as a possibility.

There could be other explanations: Loose connection (resistance and voltage drop rise as the loose connection gets hotter due to current flow), fusible link between alternator and battery eaten halfway through with battery corrosion, etc.
I said 12.5, since 13V is still within a margin of error for cheap voltmeters, since most cannot afford a decent one like a Fluke. Corroded or loose connections is also a possibility, but easily remedied with a little elbow grease.
 

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I said 12.5, since 13V is still within a margin of error for cheap voltmeters, since most cannot afford a decent one like a Fluke. Corroded or loose connections is also a possibility, but easily remedied with a little elbow grease.
For $20, you can get an accurate meter on eBay: https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m4084.l1313.TR5.TRC1.A0.H0.Xmultimeter+uni-t.TRS0&_nkw=multimeter+uni-t&_odkw=multimeter

I have one of those because I didn’t want the expense of a Fluke for home use. For dc volts readings, accuracy is 0.5% of reading + 5 counts of least significant digit on the scale you’re reading. You don’t need a Fluke for enough accuracy for this kind of thing. So on 20-volt scale with 4 digits (called 3-1/2 digit meter), reading 13.5 volts, possible error would be 70mV + 50mV, or 0.12 volts (less than 1% total error of that reading). So if you’re reading between 13.38 and 13.62, you could actually be at the very bottom of the acceptable 13.5-14.5 volt range.
 
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There are several Fluke model multi-meters that won't "Break the Bank" nowadays. However a $20 accurate meter as shown above is even better!
 

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Any progress here?
 

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The $5 harborfreight multimeters are within a 1/10 of a volt or so compared to the flukes, and I have a $300 fluke, I use the cheap HF meter more, it's close enough for what most will use it for working on their own cars.

Anyways I'm wondering how you know the car charges for a few mins and stops? Usually the PCM will tell the alt to charge, or not charge. Not just random charge and stop, or an alt will charge or not charge, not just do it sometimes.

He hasn't posted for a while..

Sent from my SM-G965U using Tapatalk
 

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The $5 harborfreight multimeters are within a 1/10 of a volt or so compared to the flukes, and I have a $300 fluke, I use the cheap HF meter more, it's close enough for what most will use it for working on their own cars.

Anyways I'm wondering how you know the car charges for a few mins and stops? Usually the PCM will tell the alt to charge, or not charge. Not just random charge and stop, or an alt will charge or not charge, not just do it sometimes.

He hasn't posted for a while..

Sent from my SM-G965U using Tapatalk
Good question. If he’s just going by the dash light, a marginal condition in which the system is wavering around the threshold of good vs. bad charge out of the alternator could explain no light at startup, and light after a few minutes (the example of the brushes being worn down so they’re not pressing very hard against the slip rings, and temperature change can mean the diff. between output being above or below the threshold of dash light/no dash light).

Could also be a bad diode in the alternator so it’s very limited in its current (amps) capability to maintain the average voltage above the threshold to keep the dash light off. That, again, could be a marginal condition wherein one minute it’s above the threshold and the next minute it’s not due to temperature changes as things heat up after startup.
 

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Discussion Starter #17 (Edited)
it would charge (by the idiot light ).... for a few minutes while idling... less if revved up. it never went back to charging once it quit ...... bought an aftermarket reman. no junkyard alt available. trouble fixed .... ill save the oem alt and get it rebuilt (im sure it is full of ps fluid)
 

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Suggestion to readers: Buy a car cigarette lighter voltmeter to have on hand for situations like this. Provides very useful information that could help troubleshoot situations like the OP’s.

Amazon has many choices - some with USB charging ports - $10-$20:

https://www.amazon.com/s?k=cigarette+lighter+voltmeter&ref=is_s
 

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