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**hatzie** · 09-14-2011, 05:24 PM

It's probably a bum voltage regulator or failing diode in your replacement alternator (not unheard of).

If one of the diodes are flaky (likely) then there will be AC voltage at the battery. Set your Digital Volt/Ohm meter to 20 volts AC and check the battery voltage. You should have ZERO volts. If you are showing AC voltage the diode plate is leaking AC into the DC side of the system. Tell the guys you bought it from you were getting AC voltage at the battery and you want another one with good diodes.

NO AC volts? Get your alternator bench tested at an auto electric shop (not AutoZone) to check that the regulator is operating properly.

If it's not the regulator here's a real general idea of how to chase it. Get a cheap analog volt meter. A fluctuating needle is easier to read at a glance. You see how much the voltage is increasing or decreasing without thinking too much. An oscilloscope will work too but not many folks just happen to have one. You will need a Factory Shop Manual... especially the electrical section. Get the factory wiring diagrams if they were a separate publication for your year.

Pulling fuses one at a time and watching the voltage is probably the most direct way of narrowing it down, assuming it is NOT diode, regulator or battery related.

Sometimes pulling fuses is not an option, especially if the circuits are related to the computer, fuel pump, ignition, etc. In that case, troubleshooting becomes a bit more involved, but still pretty straight forward.

Pull the fuses on any non-essential circuits to at least narrow which essential circuits, if any, may be involved.

A bad regulator will show pretty much the same range of measured fluctuation through all of the wiring.

If it's not the regulator, the range of fluctuation will increase as you move away from the battery/alternator toward the source of the load causing the fluctuation... unless the cause is very close to the battery. You're looking for the circuit where the fluctuation range is increasing. Fluctuation ranges of circuits that are parallel to the affected circuit will not increase.

Sorry to be so long winded. I hope I helped and didn't confuse.

09-14-2011, 05:24 PM	#2
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 5,020	Re: Electrical Q It's probably a bum voltage regulator or failing diode in your replacement alternator (not unheard of). If one of the diodes are flaky (likely) then there will be AC voltage at the battery. Set your Digital Volt/Ohm meter to 20 volts AC and check the battery voltage. You should have ZERO volts. If you are showing AC voltage the diode plate is leaking AC into the DC side of the system. Tell the guys you bought it from you were getting AC voltage at the battery and you want another one with good diodes. NO AC volts? Get your alternator bench tested at an auto electric shop (not AutoZone) to check that the regulator is operating properly. If it's not the regulator here's a real general idea of how to chase it. Get a cheap analog volt meter. A fluctuating needle is easier to read at a glance. You see how much the voltage is increasing or decreasing without thinking too much. An oscilloscope will work too but not many folks just happen to have one. You will need a Factory Shop Manual... especially the electrical section. Get the factory wiring diagrams if they were a separate publication for your year. Pulling fuses one at a time and watching the voltage is probably the most direct way of narrowing it down, assuming it is NOT diode, regulator or battery related. Sometimes pulling fuses is not an option, especially if the circuits are related to the computer, fuel pump, ignition, etc. In that case, troubleshooting becomes a bit more involved, but still pretty straight forward. Pull the fuses on any non-essential circuits to at least narrow which essential circuits, if any, may be involved. A bad regulator will show pretty much the same range of measured fluctuation through all of the wiring. If it's not the regulator, the range of fluctuation will increase as you move away from the battery/alternator toward the source of the load causing the fluctuation... unless the cause is very close to the battery. You're looking for the circuit where the fluctuation range is increasing. Fluctuation ranges of circuits that are parallel to the affected circuit will not increase. Sorry to be so long winded. I hope I helped and didn't confuse.