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Discussion Starter #1
Hi,

My battery light comes on and my batteries are slowly dying so truck barely starts. Replaced both batteries with brand new ones and removed the alternator and it tested fine. What else could it be?
Thanks for your help.
 

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Discussion Starter #2
Also, we tested the charge at both batteries with the truck running and the alternator just doesn't charge it, yet the alternator tests fine off of the truck.
 

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I can never trust the run of the mill auto parts stores to test alternators. Quite a few post like yours lately. I would suspect the alternator.

Sent from my phone that somebody didn't help me get.
 

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OP: whats the voltage off the back of the alternator?
 

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Around 13.6v is average, testing that under a load is preferred.

Sent from my phone that somebody didn't help me get.
 

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A few things to check:

-- With the truck running and everything connected, check voltage off the back of the alternator. Should be 13.5v or better after the glow plugs finish cycling

-- Again with the truck running and everything connected, read voltage across the wire going from the alternator to the passenger side battery (should be 0.5v or less -- if it's more, the fusible links in the wire bundle have popped)

-- Unplug the 2 pin connector on the alternator and turn the key on but don't start the truck (key on, engine off). Should see close to 12v off of both pins.

-- Check all your grounds and ensure they're well connected and free of corrosion / dirt / grease / undercoating

-- Check all of your battery connections and ensure they're well connected and free of corrosion

-- Did you have your batteries load tested? If not, take them in. If they pass the load test, fully charge them while they're in the truck. I can't explain why it happened in my case, but I couldn't get more than 13.2v out of an alternator that tested good and 2 brand new batteries that passed the load test. I charged the batteries one day so I could continue troubleshooting, and suddenly everything worked like it was supposed to. I still haven't been able to explain it.

That should get you started, and please keep us updated.
 

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Good info.

Sent from my phone that somebody didn't help me get.
 
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