FJ40 alternator upgrade

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Mar 12, 2010
Auckland, NZ
I've been reading around the various alternator upgrade threads for FJ40s on the internet, many of which are here. I'm hoping one of you can help me out.

I've installed an upgraded alternator, a 100amp beast. I used a grinder on the bracket to make it fit then ran new 160amp cable to the battery and a lead to the ignition wire.

The terminal types on the alternator are B+, D+ and W. From reading around my understanding of it is that the W terminal is optional and is used to provide RPMs to the tachometer.

The B+ terminal goes straight to the positive terminal on the battery.

The D+ terminal SHOULD go through a 2 volt warning light and then to the ignition, with the idea being that the voltage differential of ~14v coming from the battery and the opposing flow of 12V from the alternator cancels out, so the light doesn't light and the alternator doesn't turn on. When the battery voltage drops, the alternator kicks in and charges the battery. Finally if the alternator dies and can't produce 12v back, the light then lights up. (I read this here:

I've tried to test the alternator to make sure it's charging. Putting a multimeter across the battery gives me about 12.6V. Once the engine is running and everything is drawing power, this becomes about 3V. With the engine revving, this bounces around a lot, between 3V to 12V. I've been told that this means it probably isn't charging and it should be a steady ~12V. Does that sound right?

I've wired it up WITHOUT the light because I didn't have one to hand. I'm guessing that this is why I can't get it to charge, if indeed it is not.

Can anyone comment?
What year are you working on ? Is it stock ? One thing for sure, any alternator will push the battery voltage up to 13.5-14.5 volts, depending upon demand. The voltage regulator determines low voltage and the need to charge. Even my old 1969 puts out ~14 on the battery with all electrical stuff on.
It's a 1979 FJ40. The alternator isn't stock, it's a 100amp denso-style one, I forget which exact brand but it's a fairly generic reconditioned alternator.

When I've been testing the alternator it's been with a fully charged battery (wall charger), but once the vehicle starts the voltage across the battery drops (voltage returns once the vehicle is switched off). That SHOULD cause the alternator to kick in and start charging, but I'm guessing that it is not. I'll keep researching.

I should also mention that the new alternator has an internal regulator, hence the different setup to the old one.

This thread was also informative:

The only thing I'm missing at this point is a warning lamp. I'll get a light and a socket and hook it up and see if that helps.
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