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Tech Talk Technical Discussion About The Nissan 240SX and Nissan Z Cars

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Old 10-17-2017, 04:20 AM   #1
Join Date: Nov 2006
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strange S13 alternator test results

Was driving along recently and car just cut out. Was told by the roadside assist that alternator has packed it in and got a tow home. Decided to try to confirm the bad alternator and after charging battery up got 12.6 volts across battery with engine off and 11.7 volts with engine at 1500 rpm so looks like alternator isn't charging. Fan belt seems tight.

I tested the voltage between the battery neg post and the casing of the alternator - got a steady 0.01 volts at 1500 rpm so all good there.

Then I tested between the pos battery post and the pos terminal on the alternator - it went from 0.02 volts one second then all the way up to 8.1 volts and then back to 0.6 volts then 8.1 volts !!! What the hell is going on there? I tried to bypass the cable with another one but it didn't make any difference to the readings.

I tried to re-measure several more times (car was quite warm by now) and then suddenly it felt like something kicked in and I got a chirp from the fan belt and the battery started reading 13.9 volts instead of the previous 11.7. What suddenly kicked in - the regulator? Is the regulator faulty or maybe the alternator itself? Any ideas?
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