which battery post do i disconnect to check alternator
Neither one
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The alternator should be tested with the battery connected using a volt meter/amp meter/load tester. If you disconnect the battery the engine will stall, you won’t be able to test alternator output with the battery disconnected.
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Suggest to never run engine with battery disconnected, could cause damage to vehicle’s electrical system you definitely don’t want.
Here’s my basic battery/alternator diy’er test. You are welcome to post your test results here for more ideas.
Connect volt meter’s probes to the battery posts. Before first start of the day, the battery should measure about 12.6 volts. Then immediately after starting engine, 13.5 - 15.5 volts.