Alternator wiring help...worked great for 2 days, now nothing!
 

Hello,

1986 GXL with S5 turbo swap and megasquirt. I think I have two issues going on and I'm confused!

Stock alternator was never wired correctly, appears to have only had one wire running to it from the fuse box. I was getting 12v with the car off, 12.5v with the car idling, and 13.5v at cruise, with some weird spikes of 8v and 18v thrown in for good measure! I decided to start fresh with an S6 alternator swap.

Picked up an S6 alt and machined my S5 pulley to fit it. I added completely new wiring for the plug. L terminal runs back to the W/B wire on the FEM-01 connector inside the car, near the original ECU. The S wire runs to a 15 amp fuse, which gets it's power from the circuit breaker. B+ on the alt is fed from the stock underhood fusebox, which gets its power from the positive cable on the starter. (and from there, the circuit breaker).

After the swap, the car idled at 13.5V and ran at 14.7V at anything over 1500RPM. Not quite what I was expecting, but worlds better than before! I spent about 4 hours driving around and re-tuning the car without any weird voltage issues.

Upon returning home, I turned the key off and the car kept running, which has been happening every once in a while since I added the IAC jumpers to the bottom of the megasquirt board for some reason. (It stopped doing it for a few days after I seperated the megasquirt and coil/injector battery feeds into two seperate relays, but started again after the alternator swap!) Usually, I just reach over and hit the circuit breaker when this happens and it kills the car. Today, the car was still running with the circuit breaker and the key off.

Unsure of what to do, I turned the circuit breaker back on, which killed the car.
Now, I have 11.8V at all times with the engine running. No loose wires or blown fuses.

What's going on here? Did I kill the alternator?

I would go back over the basics. Make sure everything is landed correctly and on the right terminal. Check the FSM for how the stock system is supposed to be and replicate that and you should be good.

Now hopefully someone else hasn't done other "fixes" to the wiring if the alternator was wired correctly from the jump.

Well, alternator is dead. Unsure if due to factory defect or my wiring.

The white/black wire runs to a dash light, right? Shouldn't I get a battery light with the ignition on and the engine off?
Should there be a diode on either of the wires on the alternator plug to prevent back-feeding?

Yes. There are no diodes contained within the wiring itself. The W/B wire has a ground (or just a couple of volts or so) w/key to on, engine off and the alternator plugged in (if alternator is unplugged then it would have 12 volts.) W/engine running the voltage is 12 volts.

The Yellow/Blue wire at the largest CPU plug runs to the unit in the dash. W/key to on and that wire grounded the lights in the dash unit should light up.

Thanks, very helpful!

I will try adding a diode to the W/B wire at the alt and check out all the voltages as you've described above when the new alt gets here.

The alternator places a ground on the W/B wire w/key to on and engine off, or when the alternator is dying/dead (everything hooked up/plugged in). This is what causes the idiot lights to illuminate.

Just to close this out, I fixed it!

Got a replacement alternator and found the battery had droppped to 2v after sitting for a week, unplugged....probably part of my problem too!

New battery and alternator, I ran the L wire through a diode and relay and then to switched ignition INSTEAD of to the idiot light. (I think I am missing the CPU as described in this thread: https://www.rx7club.com/2nd-generati...oblems-809791/ May run to yellow/blue wire at some point to get the idiot lights back, but for now, it WORKS.)

Car idles at 13.8 and runs at 14.4! Now to finally get tuning!!

I would suggest replacing the battery unless it is an AGM or deep cycle.

The abuse from the bad alternator and the low sitting voltage means it won't be long for this world and could kill your new alternator.

Battery was replaced.

Going through the megasquirt logs, I'm still getting a very occasional 8 or 21v spike! New battery, alt, and wiring and it's still happening! Any idea what can cause this?

Alternator wiring help...worked great for 2 days, now nothing!
 

Was it a NEW alternator or a reman?

Even being NEW it could still have issues