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Discussion Starter · #1 · (Edited)
Hi All,

Last night I went to leave my sister's house and the car battery was dead. I was at her house for about 5 hours. When I went back to the car I was hearing relays clicking on and off. I disconnected the battery and put it back on and it stopped. This morning I jumped the car and after a few minutes it started. I have a Topdon AB101 and after running the car for about 30 minutes I ran a load test of 640CCA (Interstate battery from Costco) and it indicates to replace it.
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I just don't know how the battery would just die all of a sudden like that. I verified that I didn't have any indoor lights lights on and if I had the outside lights on it would have beeped when I was exiting.

I then put a load on the alternator to see what the current was generated and with the A/C, outdoor and indoor lights radio, rear defrost I was only getting around 12.78V. That seems low correct? I also ran the load test and it was kind of confusing but I was running the RMPs at 4000 after I pressed ok (which from what I read in the manual you run it at 2500 RPMs for 5 seconds then press ok). I will re-run this test again to verify. I assume I need to replace the battery first correct before going for an alternator replacement? Also the alternator is making a moaning sound under load, when I turn off the lights and A/C it goes off. I have verified that the fans are not the culprit as they aren't running once the A/C is off.


I don't remember when I put this Autozone one on but should I chance brining it to them to verify the test before pulling it our and having them test it or does my initial testing seem to indicate the alternator is also failing? Any help would be appreciated.
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Thanks
 

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Sounds to me like you have bad connections and not bad hardware. Do a voltage drop test between the positive battery post and the alternator post (should be below 200mV) and the negative post to the body and block (both should be under 50mV).
 

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Discussion Starter · #3 · (Edited)
Sounds to me like you have bad connections and not bad hardware. Do a voltage drop test between the positive battery post and the alternator post (should be below 200mV) and the negative post to the body and block (both should be under 50mV).
Thanks I will check it out tomorrow and report back. I am looking for a drop when the engine is running correct?
 

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Discussion Starter · #5 ·
Correct. Voltage drop is only meaningful when current is flowing.
I posted yesterday but looks like it didn't commit. I am seeing .03-.04A on the positive side and .06-.08 on the negative side. I went ahead an replaced the battery and it seems to have helped. The load seems to be ok as well. Under all load it is around 13.1 amps. I was reading some other forums that mentioned anything about 13 with everything on is acceptable. Thanks for your help in troubleshooting this issue.
 

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That 13.1V is low, and 60~80mV on your ground wire is very high. The reading between the block and negative battery post should be under 50mV, and 15mV is probably average for a healthy ground cable. Check the connection where the cable bolts to the tranny, and also the cable itself. Ohm says 70mV with a 10A load from the ignition will become a full 1.05V loss under 150A load from the starter. That's more than enough to cause starting issues.
 
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