I want to preface this post with its kinda long and a lot of information. If you have seen any of my smart *** replies on here, it's usually due to the LACK of information people give when they ask for help. This would not be that type of post.

I've had this problem for a while and I've tried several things that I honestly thought would fix it.

Car is a 2000 Firehawk, SBE LS1, TFS heads, Huron Speed APS style TT kit, dual Hellcat pumps, small aftermarket stereo setup

Stock diameter ATI balancer, stock diameter alt pulley, new Gates belt, Katech manual belt tensioner.

At idle, voltage is right at 14.00volts at the battery, HPT always show about .50 volt lower.

At WOT, the voltage drops down, HPT shows mid-low 11volts

I replaced the stock style alternator with the 145amp truck alt. and replaced the 4 pin connector pigtail there was no improvement.

I then did the big 3 upgrade with 2 gauge wire and showed no improvement.

I scoped all of the PCM power and grounds with a scope and everything looked good. I also scoped the L terminal wire and it was approximately 11.4 volts at the alternator and the PCM. According the GM service manual anything over 10 volts is supposed to be good.

Last night I took the car out, logged it and scoped the battery voltage and the L terminal.

There's no way for me to log RPM with the scope along with battery voltage and the L terminal(wish I had a 4 channel scope)but I can see when the alternator stops charging.

While driving the battery voltage is a pretty constant 13.9v and the L terminal voltage 11.3v

At WOT, the battery voltage drops to 12.6v and the L terminal drops to 10v.

In the HPT logs from last night, the voltage shows about 13.5v and above 5000rpm it starts to drop down to 11.9v(I've seen it as low as 11.3v in other logs but it had the stock alt)

I have a few questions.
1. I know the PCM sends power through the L terminal circuit, which I believe is a discrete circuit, to excite the regulator to get the alternator to charge. What exactly determines what voltage it charges at? For example, my alternator is about 13.9 volts, why not 14.6?
2. On the L terminal circuit, is it an OFF/ON kinda deal, or does more voltage means the alternator charges more?
3. The picture of the scope, you can see the voltage differental between the battery and L terminal remain about the same the entire time even when the battery voltage drops. Is the battery voltage dropping because the L terminal is, or is the L terminal voltage dropping because the overall system voltage is dropping.
4. On the L terminal circuit, I understand that it can't be true battery voltage(amps) because it will smoke the regulator, in the old days, they used a bulb(resistance)to drop the amps going to the regulator. I've read in newer vehicles they use a resistor, 479 ohms is stuck in my head for some reason. Do our cars have that or is the PCM actually out putting a lower voltage.

I've watched several YT videos of car audio guys using the flat 4 pin, 1 wire alternator, that add a diode to the S terminal on the 4 pin connector to force the alternator to charge at a higher voltage. Is that safe to do?

I know this was a long post, but I appreciate anyone who took the time to read it.

 

 

 

Following as I believe I will be in a similar situation once I activate my stereo system (3 amps about 550RMS watts so it will put a high demand on the alt). I have followed some aftermarket alternator threads and one common thread I see is the "overspin" of the alternator at or above 5,000 RPM. From the description is this possibly the behavior of alternator/PCM-ECU "shutting down" the exciter signal to prevent damage to the voltage regulator or battery? With the performance high output alternators I have seen people talk about changing the pulley size to keep charging at higher RPM but the down side is at lower RPM the exciter wire seems to not activate the alternator until 1,500/2,000 RPM (depending on the diameter of the pulley). As a layman on modern car electronics I am just grasping as straws here but the High Output GM truck alternator (I have one, a 4 pin, I haven't installed yet) was placed in an engine that wouldn't rev as high as a performance car so maybe it was designed to working within the 900 to 4,700 RPM range?

 

For testing purposes, I've thought about running power through a switch with a resistor to the L terminal to see if it makes any difference at higher RPM.

I will have to go back and look at some older logs, but IIRC the stock alternator didn't cut out till 6k rpm.

My personal laptop died a while back and I lost a bunch of logs. I need to check what logs I have on my work laptop.

 

I looked at some logs with the stock alt/wiring and the voltage starts to drop around 5 grand just like with the current truck/big 3 upgrade.

 

What's the condition & age of the battery?

Have had an older battery w/weak or shorted cell cause problems etc.

 

I swapped out the battery from another car yesterday and it did the same thing.

 

Is there some possibility that the belt is slipping on the alternator pulley at high RPM ?
I had a lot of belt shriek noise on full throttle 1-2 uphshifts 6500 rpm.
Solved the problem by using a larger diameter 6 rib pulley on my 4.8...
I think I used about 64 mm diameter alternator pulley, slowed it down some 20% and gave it a bit more traction on the drive belt.
Using the larger 145 Amp alternator, 4 pin regulator plug. Still makes 110 Amps at low idle, and 145 rated at 1000 rpm.

 

I don't think the belt is slipping. It's pretty damn consistent in the logs where it stops charging at. It acts the same as stock as upgraded.

I thought about the larger alt pulley but was worried about it not charging at idle/low speed.

 

I have a question about this. I read my "battery voltage" from the PCM with Torque Pro. It's The PCM voltage is usually higher always lower than what a volt meter shows at the battery itself. The PCM might show 13.9V or 14V with the engine running, but a volt meter at the battery will rarely go over 13V, even with the engine running.

My question is, where does the PCM get it's "voltage" for live data in the OBD-II data stream? Does it read the voltage on the "battery supply" pin at the PCM (the one that provides constant power to maintain codes and LTFT and other "permanent" data? Or does it read from one of the other ignition switched power pins?

On my particular "problem car," we had about 12 feet of battery cable from the starter to the battery in the trunk. We've actually had to remove the spare tire well where the battery was sitting and move it even further back (about a foot and a half further). We had 2 gauge cable running from the battery to the starter. I'm upgrading that to 1/0 now. And there's an 8 gauge (+/-2) wire from the 145 amp truck alternator (it's a truck 5.3 engine) to the starter post at the front. I'm considering 4AWG there, or maybe even 2AWG.

If the battery was fully charged (or topped up with a battery tender for the week), there wasn't any issue with getting enough juice to the starter with the 2 AWG battery cable we had (at ~12 feet long). But if you drove it for a few hours, restarting it was sometimes a problem, and even when it started, it sound labored and the voltage at the battery was lower than it should be (usually just under 12.0 volts, in the 11.9x range).

The PCM harness has two power input ring lugs on the starter post that lead to some kind of fuse/relay/power distribution piece that has the fuses and relays for most of the PCM related wiring. I'm now wondering if I need to run the constant (unswitched) battery power to the PCM from a wire directly back to the battery in the trunk. Will that make the PCM "battery voltage" track better with actual battery voltage and improve charging system performance? Is the voltage drop from the alternator to through the starter post to the battery so great that the PCM is getting confused about what's required to keep the battery at optimum charge?

I'm hoping someone on here has some expertise about this. I'm redoing all the wiring back in the trunk (including fuel pump and fuel gauge wiring) over the next week or so. If I need to run power to the PCM "constant battery" power pins from there, now is the time to do that as well.

 

I don't know which circuit the PCM looks at to determine voltage. I wondered the same thing when I was checking all of the power and grounds.

 

The first thing I'd do in your situation is clean the PCM related connections in the fuse block and any PCM related relay sockets and the blades on the fuses and relays. You don't seem to be getting full voltage to the PCM.

What kind of OBD-II device do you have to use with HP Tuners? Is it a Bluetooth dongle? Can it work with other applications (like Torque [Pro] or Car Gauge [Pro])

Or can you get a terminal opened to the device and shoot raw commands at it? It's likely an ELM327 device, and if you can send commands directly through an "ELM terminal" type app, send ATRV and see what the voltage is at the OBD-II port, and compare that to what you're seeing in the data stream from the PCM.

Torque Pro will show the adapter voltage (at the OBD-II port) in the "adapter information" screen. I think there's also an option for a gauge on the display for OBD-II adapter voltage in Torque Pro. If the OBD-II port is getting better voltage than your PCM, you definitely have an issue in the power circuits to the PCM.

Also, I'm pretty sure the PCM completely controls the alternators in these cars. The small wire to the connector is where the PCM supplies a voltage to the VR in the alternator, which determines the charge current out of the alternator. Basically, the PCM lies to the voltage regulator/alternator about the state of the battery charge, and can tell the alternator to go to full charge (when you see 14V or higher system voltage) or to no charge at all (you'll see battery voltage for the system voltage, usually 12.xV), or anywhere in between. The PCM has various "alternator operating modes" and "charge strategies" it can use depending on driving conditions and battery status. This is common in most later GM cars as well.

So "deactivating" the alternator at WOT is either operating as designed (by GM), or possibly operating as tuned by your tuner. Either way, if you're drag racing, you can go 1/4 mile without any current from the alternator, so why slow the car down charging the battery for those few seconds. The same logic applies when you pull out in a passing zone on a 2 lane road and stomp on the loud pedal to pass the farmer on his tractor. In those situations, the PCM will send full voltage (or more) to the alternator voltage regulator input to tell it "the battery is fully charged," and the alternator will step back to almost no charge current.

In other situations, the PCM will send a lower voltage to the alternator/regulator, to make it send more charge current to the battery to charge the battery quicker than it would with a conventional regulator setup with battery voltage supplied to the regulator input on the alternator.

So, cruising for miles on the cruise control, the PCM might "turn off" the alternator by sending it a higher voltage as a "fuel saving" measure. When the battery drains to a certain level (usually around 12V), the PCM will then drop the voltage to the regulator input on the alternator down to 10V to 11V, to make it charge hard, with high current from the alternator for a short time to get the battery back up to 12.6V or so. Then it goes back to "fuel saving" with the alternator basically turned off.

 

I am using the MPVI2 that's hooked directly to my laptop,

On these cars the PCM does control the alternator to an extent. It's either on or off.

What you are describing is on late model stuff. The later models have a PWM alternator which utilizes a sensor, (low amp clamp for lack of a better term)to control the duty cycle of the alternator.

I have looked in the tune and can not find anything related to what kills the L terminal circuit to disengage the alternator.

 

Is your original style car alternator a rebuild, original factory or new? The reason I ask is a couple of years ago I had a rebuilt ac delco alternator do the exact same thing, I only replaced the alternator because I already had everything torn apart for an engine swap and figured I might as well.... it charged normal until around 5k, would stop charging until roughly 6 or 7 seconds after I let off and then charge normally like nothing ever happened.... I reinstalled the original alternator and it charges at idle, cruise and WOT up to my 6600 redline like it should. I think some kind of rpm governor was built into that rebuilt alternator because it cycling on and off with rpm was like clockwork

 

USB cable connection? Usually that sets up a "virtual COM port" and you can shut down all the HPTuners software and open a terminal (Windows Terminal, Hyperterminal, Putty) and connect to that port. Try that and send "ATRV" and a return (enter). If it's ELM327 on the other end, it'll respond with the voltage at the power pin for the OBD-II connector.
I think you're correct. I was probably getting confused with late C6 Corvettes and our own 2012 Cadillac CTS (V6 3.slow), which has the current sensor and the fully computer controlled alternator.

Now that I think about it, these alternators may be turned on by the PCM at some RPM threshhold, or on a timer after starting, but I don't think it'll turn off, even if the PCM goes open circuit (no voltage supplied) on that terminal at the alternator. I think the terminal is bridged internally once voltage has been applied to energize the alternator, and it won't "turn off" until the shaft stops spinning.

It's also possible if your alternator isn't original that it's got a "safety feature" like @speedfreak440 described. I've seen that for safety reason in some rebuilt alternators, and I've seen other rebuilders who do that when rebuilding them to cut it off at a lower RPM for autocross or drag racing, to reduce the "parasitic HP loss" from the alternator above 3000 to 3500 RPMs.

 

And to your original problem/question. When the PCM is reporting significantly lower voltage than a volt meter at the battery, it can indicate poor connections or high resistance in either the PCM power connections or the PCM ground connections. If the PCM reporte voltage falls at higher RPMs, the grounds are even more suspect. All of the injectors and all of the coils ground through the PCM ground, and at higher RPMs, the ground current for those items increases a lot. If there's resistance in the ground path, the voltage at the ground pins on the PCM can "creep up" and reduce the voltage difference show by the PCM as "system voltage."

 

This weekend I'm going to do some voltage drop tests to pinpoint where the voltage is dropping before it gets to the PCM, then clean the PCM connectors.

I've read quite a few threads about aftermarket alternators not charging at higher RPM's. Is there anywhere that sells a OEM regulator?