Charging Problem

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95SUBURBANLM7

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Hey all, I've got a problem. Recently my volt gauge would drop to 11.5 and jump up to its usual 14.4+ area. It kept doing this until finally the needle spent more time on the left side of the gauge. I finally took my alternator in and the brushes were nearly gone so they were replaced and the alternator tested fine @ 14.5+v @ 120a load. The short of the story is that this kept happening. The alt tested good on the bench but as soon as it was installed, the voltage hovers around 12v. I even took it to AutoZone and they tested it twice and it passed all three times. So for whatever reason, once it's installed in my truck, it doesn't want to charge . I even bought a new battery, but the same thing happens. I've cleaned the mounting points of the alternator and the alternator bracket and I've checked all three major grounds and they are all checking out good. The PCM has been tuned out, so the brown wire is only connected to the indicator light. I even connected the jumper cables from the alternators log directly to the battery and from the case directly to the battery and I still have the low-voltage problem. What am I missing?
 
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evilunclegrimace

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Is the belt slipping? Have you checked the wiring to make sure that there is not excessive resistance?
 

95SUBURBANLM7

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Is the belt slipping? Have you checked the wiring to make sure that there is not excessive resistance?

How can I tell if my belt is slipping? The belt tensioner would have to be broken if that was the case, correct? I'm not running any AC so all I have is the alternator power steering pump and the water pump.

I checked the resistance of the fusible link and it read at .3ohms.
 

redfishsc

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Completely disconnect all three grounds and clean the contact points well. Just showing 0 ohms on a multi meter doesn't confirm that you actually have a good ground.

I've done this on both of my 1999 Suburbans and it made about a 1 to 1.5 consistent increase in voltage as seen by the OBD2 port (ie, digital, not dash gauge)
 

95SUBURBANLM7

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Completely disconnect all three grounds and clean the contact points well. Just showing 0 ohms on a multi meter doesn't confirm that you actually have a good ground.

I've done this on both of my 1999 Suburbans and it made about a 1 to 1.5 consistent increase in voltage as seen by the OBD2 port (ie, digital, not dash gauge)

I have cleaned ALL the connections and the alternator mounting points. There's no paint between the bracket and the block. Plus I connected the alternator directly to the battery, ground and positive.

Again this worked fine all summer, fall, up until three weeks ago.
 

Bob L

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I had a similar problem with my 98 . intermittent charging and it turned out to be a bad plug to the alternator on the small wire.
 

95SUBURBANLM7

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I had a similar problem with my 98 . intermittent charging and it turned out to be a bad plug to the alternator on the small wire.

Was this on an LS swapped truck? As I wrote in the original post, I connect to the alternator directly to the battery with jumper cables. This still did not help my issue.
 

Urambo Tauro

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Test alternator output directly at the alternator first. After verifying good charging voltage, work your way towards the battery one test point at a time until you find voltage drop.
 

95SUBURBANLM7

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Test alternator output directly at the alternator first. After verifying good charging voltage, work your way towards the battery one test point at a time until you find voltage drop.

I have done that, and from the alternator the voltage is low. That's why this is so confusing. Today I went and had it tested at AutoZone, it tested fine, as soon as I install it the voltage starts at 14, the drops to 12 and stays there.
 

Urambo Tauro

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That's peculiar. Could there be a short or something in the vehicle pulling the voltage down? Or is the belt tension perhaps cocking the pulley just enough to cause an issue?

I think the next thing I'd try is disconnecting the main charge (+) cable from the alternator to isolate it while checking output.
 
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