Alternator / Charging Circuit Issue
 

As the title states, I'm having charging issues and I'm stumped.

I have a 2016 Scion FR-S, 160,000KM. The other day I was driving, I did a medium-hard to pull to 6000rpm and right after the battery light turned on.

Using a multimeter, I probed the battery while the car was running. Sure enough it was around 11.9V - 12.2V. I tried turning on accessories, headlights, blower etc. but the voltage would not increase. I used a DeWalt battery charger to test the battery and it tested healthy, even in -10C weather I'm able to get multiple strong cranks out of the battery and it still has plenty to give.

I also checked continuity on every fuse in the box under the hood. The ones that I thought were related to the alternator (like ALT-S 7.5A) I pulled out, visually inspected, and replaced anyway even though they looked fine.I also tried checking the fuse box insde but I didn't see anything obviously relating to charging. I pulled out ignition related fuses in case it was related and checked them.

I found a used alternator from a 2014 BRZ, it had 80,000KM. I swapped the alternator along with the serpentine belt but the problem persists. Same symptoms, battery light on the cluster and no voltage higher than ~12.1V. It's possible that both alternators are bad but I can't say for sure.

Now that I have the original alternator off the car, I thought I'd try to diagnose or run tests on my own, but I don't even know where to start. I know there's a 3-pin connector on the alternator: one pin is for the battery lamp on the cluster, another pin is for the voltage sense, and the third pin is for the ECM to regulate voltage.

Any advice on how to go about solving this issue? All advice is appreciated.

If the alternator was bad, first thing to go would be power steering. Something else is off. If you want to DIY, I suggest starting with the service manuals linked in my signature line. There are several threads, for better or worse, describing odd issues with charging. There's much more to it than the simple voltage regulators of older cars. We are in the age of trying to squeeze every bit of efficiency from the engine as physically possible.

That service manual for the 2016 FR-S was handy, thanks for that. It shows how to disassemble and reassemble the alternator and what to probe for inside of the alternator. I'll try it out over the weekend.

I haven't driven the car since the issue, but from what I witnessed, I did not have a power steering warning light. When turning the car on, all the dash lights turn on as usual (including power steering). Once the engine cranks and is running, all lights go out except for battery light. Apparently power steering only operates with the alternator and not the battery, but apart from my multimeter showing lower than expected voltage, there are other signs of no alternator. Signs such as windows winding up and down slower than usual, wipers wiping slower than usual, DRL and headlights being a bit dimmer than usual, and blower motor pushing air weaker than normal. Maybe I need to drive the car a bit to trigger the power steering warning?

There's also the battery current sensor on the negative terminal of the battery. Mine looks fine but I haven't probed the 4 pins yet. However, from other threads I've read, it seems like even if the current sensor is unplugged, the car operates normally.

Have you checked all the grounds?

Just a random guess, but bad voltage regulator? It's internal to most modern alternators and it will allow the output to read normal, but the battery will only ever read 12 V b/c it's not being charged. The car doesn't really know it's happening because the alternator is still powering everything else - it's just the battery slowly discharging below the minimum value that finally turns the light on (I'm assuming it works that way on this based on experience in my old Celica).

If you have an Autozone or Oreilly's they can test it for you for free and tell you right away (at least they do Stateside).

You are describing the GENERATOR section. Never mind that. The CHARGING SYSTEM section maps out the troubleshooting process.

https://i.imgur.com/8HZmjNB.png




I suggest reading that whole section. It's nice to have Techstream.

Update / Solved
 

Update for everyone:

I used the repair manual to troubleshoot the car, couldn't find any faults. I ended up disassembling the original alternator and used the repair manual to troubleshoot. I found a fault with the rotor assembly; I am not getting resistance between the two slip rings; it's an open circuit. No continuity. I also found a bad bearing, the one closest to the backside of the alternator. My guess is somehow things got hot in there and the rotor and rear bearing died from heat buildup?

I ended up ordering and installing a "new" (remanufactured) alternator, everything works perfectly.

As for the used one I bought and didn't work, I guess I was scammed.

Thanks everyone for your help!

Bravo! I've seen slip rings burned because of worn brushes but I never would've "guessed" that direction. Congratulations!