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Yes, but nothing to do with an ignition source as ou described.
I'm out. Good luck getting this sorted out.
Yessir. Looks like on a PLFS alternator (vs a PLIS), the F-terminal is a bidirectional communication path with the factory PCM to allow the PCM to monitor/command the alternator’s rotor field current.
All great info!
The summary in post 16 remains accurate for PLIS alternators. The only change to the summary for a PLFS alternator would be to leave F-terminal blank with a standalone ECU such as my situation.
Since I have data logs at idle and while under load, with only the L terminal hooked up, I’ll record another data log when I connect the S-terminal to the battery distribution lug and I’ll post the data. Hoping to knock it out this weekend. Unfortunately I’ll only be able to get an idle data log with some revs since the roads are bad here.
I think we’re getting somewhere.
Last edited by Cory51191; Jan 2, 2026 at 09:17 PM.
The F terminal is just a signal sent to the PCM, no control is possible.
The L just needs a switched 12V source with a resistor or a light bulb in series. Make sure it's switched. There is no "magic" 5V required, the PCM does output about 10V when not connected but the PCM has an internal resistance. I can't recall the connected voltage with the key on before starting but it doesn't much matter as long as you have a resistor or bulb to limit the current.
Obviously, it's kind of working since you're seeing over 13V, but it could be better since it should get over 14V to fully charge the battery.
Sounds like there was a communication issue on how the car was going to be used. Seem to me, it was set up just for the drag strip, then all your problems will be gone.
Based on the pics, it looks like you're missing the second wire that should connect to the alternator. The single wire going to pin B is probably the sense wire for the battery voltage. You need another wire connected to the alternator output post for it to properly charge the battery. I'd recommend checking your wiring diagram and making sure all the connections are correct. A faulty alternator connection could definitely cause the battery to die and the car to shut off unexpectedly.
Based on the pics, it looks like you're missing the second wire that should connect to the alternator. The single wire going to pin B is probably the sense wire for the battery voltage. You need another wire connected to the alternator output post for it to properly charge the battery. I'd recommend checking your wiring diagram and making sure all the connections are correct. A faulty alternator connection could definitely cause the battery to die and the car to shut off unexpectedly.
Based on the pics, it looks like you're missing the second wire that should connect to the alternator. The single wire going to pin B is probably the sense wire for the battery voltage. You need another wire connected to the alternator output post for it to properly charge the battery. I'd recommend checking your wiring diagram and making sure all the connections are correct. A faulty alternator connection could definitely cause the battery to die and the car to shut off unexpectedly.
It wont hurt to add the S-terminal pin. That’s the plan today. I purchased a connector and de-pinned one of the terminals to use.
I did test the alternator output (14.85v) and the battery (14.65v) while the car was idling.
I still need to test for parasitic draw with the car off to rule that out.
Once I get this S-terminal pin wired up, I’ll record a data log with S-terminal disconnected and then with it connected to see if there’s a difference. I’ll post later on.
Ok, but that is not normal operation and your car had an issue with the charging system. If your IROC was stock, it should have had 3 wires to the alternator .....one routed to battery, and two wires in a connector for the regulator.
No, it wasn't stock. Some kid had put a carbed 350 in it with no computer. Lord knows what all he did to it. I mainly bought it for the IROC-Z gold and silver wheels, which were stolen off our nice 1985 IROC-Z. Those damn wheels cost $375.00 each from GM in 1992!! I bought a set of Cragar SS wheels, drove it for a couple of years, and sold it. It had a 350 turbo trans with a B&M ratchet shifter. I had a speedometer shop make up a different ratio cable drive for the speedometer, as the speed sensor and 700R4 were long gone.....
Okay so a few notes before moving on to the data logs:
This data is through the Holley Terminator X Max, which reads voltage at the ECU’s switched 12v source, not the alternator or the battery.
First screenshot is prior to some wiring changes I made over the weekend. I found that the Holley's switched 12v source (red wire w/ white stripe) was sharing a switched 12v source with my radiator fan relays and my MAC valves. The Holley needs a "clean" switched 12v source. Relays kicking on and the MAC valve's PWM signal would be considered a "noisy" signal and is no bueno according to Holley's directions and their tech support. So, I fixed that nest.
The Holley was grounded to the sheet metal between the floorboard and the shifter. I moved that ground to one of the engine block's grounding cables.
You can clearly see the alternator behaves differently with each configuration. In order to determine if the S-Terminal is actually beneficial, I think I need data logs while driving. I also need to test at the alternator/battery output lug with a multi-meter while applying a larger electrical load. Unfortunately, I won't be able to do this until the springtime.
If you compare just the first two data logs, you can see how the alternator's behavior dramatically changed with just the wiring changes I mentioned above.
The data logs below are in this order:
Old "noisy" wiring
After wiring was cleaned up as described above (no S-Terminal)
With S-Terminal lead connected to battery output terminal (under-hood fuse box). You will notice a couple of dips in voltage towards the end of this log because I was putting one of the windows up. I did not do this in the other logs.
1. Old "noisy" wiring:
2. After wiring was cleaned up as described above (no S-Terminal):
3. With S-Terminal lead connected to battery output terminal (under hood fuse box):
Last edited by Cory51191; Jan 6, 2026 at 09:54 AM.
