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#1
10-18-2009, 10:43 PM
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Erratic Voltage - Battery Killer
My son's '98 Firebird.... Has L36 V6 motor.
Old battery expanded and leaked. I asked him what voltmeter had been doing, he said it was wavering before battery completely killed. Alternator bench tested okay. Brand new battery went in. Volt meter on batt terminals look okay when no key in, look okay when turned to run, but as soon as car starts, voltage drops and is erratic between 0.5v - 8.0v. Immediately shut car off. Any suggestions? Thx. |
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#2
10-19-2009, 01:46 PM
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First, when measuring the battery voltage with a multimeter, it should read 12.6 and steady with the battery charged, and about 14.0 and steady when the engine is running and charging.
What may be happening when the engine starts is the voltage at the battery is oscillating for some reason and the display update time on the meter is too slow to follow the oscillations, giving you the strange readings. The only way I can think of that the batt voltage could actually get that low instantaneously is if there was a very bad short circuit in the wiring causing the bat to discharge, but that would almost certainly be accompanied by a lot of smoke, etc. Perhaps the regulator in the alt is bad causing the voltage swings. The voltage must have been high at some point to cause battery swelling and leakage. Also check that the large harness plug that plugs into the alt is in good shape and the terminals tight, etc. Poor connections there may cause strange things to happen. George
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"...out to my ol'55, I pulled away slowly, feeling so holy, god knows i was feeling alive"....written by Tom Wait from the Eagles' Live From The Forum |
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#3
10-19-2009, 06:17 PM
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thanks for the ideas George.
i was thinking same thing about a short in the wiring, and looked to make sure the fire extinguisher was handy. i'll put a scope on the voltage and take a closer look. i found some potential similar issues using Google about a BCM - Body Control Module that can cause electrical problems including voltage. i always thought it was just the alt doing that job. always learning i guess. i'll update any progress, and watch out for any other ideas. |
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#4
10-24-2009, 09:59 PM
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problem solved....
turned out that where I was probing for voltage was poor contact, so it was reading normal when car was off, and super weird when fired up. a good probe found that voltage was swinging around between 14.0 and 14.8 volts. new alternator, and voltage is rock steady now. |
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