FJ80 Alternator putting out 10 A at idle normal?

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Apr 14, 2008
North of LA California
Dec 2018 I upgraded to a toyota reman FJ80 alternator by doing the body mount upgrade etc.

Details of that here:

My 62 has been sitting for a year and I'd occasionally drive it with an old battery that gets a trickle charge via solar panel. I am wondering if the dead battery was stressing my alternator. I recently put in a new AGM (last one was also AGM) and i'm getting the alternator light intermittently coming on when I drive. Last night I put a meter on the battery and it's getting 14.4 volts at idle and I put the meter clamp on the alternator line and it's pushing out 10 amps.

I revved the motor to 2,000 RPMs and it's still only putting out 10 amps. Is this something I can fix (ie some kind of voltage regulator) or?

Maybe I need to turn on the headlights and see if it puts more amps out? I don't mess with alternators too much so looking for any insight.

Looks like I should have checked the FSM first. It has a load check for higher amps.


Most alternators don't put out their full loads until a higher rpm. Its usually a pretty steep curve of increasing output to RPM but have to see specs of the alternator to find the idle amperage.

heres a plot from a forum and referring to a specific brand alternator. But it also reminds me that if you want more charging at idle you could try and source a smaller alternator pulley. This will increase the rotational speed of the alternator but it will also make the engine work a little harder to turn said alternator and if its too small the belt might have trouble gripping in high load conditions.

If the voltage is 14.4V at idle and the alternator is putting out 10 amps, the alternator likely is charging the battery too. The engine doesn’t need 10 amps to run.

Maybe fully charge the good battery (not an old battery) to absolute full capacity with an AC battery charger, then start the engine and measure the alternator current. Likely it’ll be less than 10 amps after a while.

If you then flip on the headlights, (65 watts x 2 + marker lights) that should pull another 10 amps at least with a fast idle.
The voltage regulation on older vehicles is pretty simple and stupid. Basically it has a set system voltage it wants to achieve, and it will push the alternator as hard as it can to achieve that voltage. Once that voltage is achieved, it reduces alternator current to the exact amount necessary to maintain that voltage.

14.4v is a pretty typical "battery fully charged, engine running" voltage for a vehicle. So if the battery was completely charged, voltage was regulating at 14.4v, and the vehicle was only presenting a 10a load to the system, then 10a is all you are going to get no matter the RPM.

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