First post here, I have a 2008 Base Manual Coupe with 91k miles.
I got a "service charging system" message a few weeks back, driving on the highway and my voltage was at 11.8V. When I pulled off the highway and came to a stop, my voltage rose back up to 14.3.
Driving it around, the car drove and acted perfectly normal, with 14.3 volts cruising around and 13.7 or so at Idle. The issue repeated itself when I went over around 2500rpm; The voltage would drop to about 12.2V, and continue to drop until I came to Idle, where it would shoot back up to 14.3 or so.
After a few weeks of dealing with it, I've established a few things:
-The issue occurs once I get over 2500-2700 RPM, no matter how much throttle I give it.
-no matter how long I drive or how low I get the RPMs, the voltage doesn't go back up until I reach idle.
-When reversing, I've noticed that the Voltage will drop and stay around 12.5, even if I'm just idling in gear.
-after a few cycles of Voltage drop-Idle Voltage rise, the voltage will only rise back up to 13.5 or so until I've been at a cruising pace for a while, at which point it goes back up to the normal 14.3

To preface, I bought this car stock with 52k miles last June, did clutch master, headers, thermostat, and coilovers last December, and in May of this year, when I hit 80k, I did a Balancer, Cam, new lifters/valvetrain, timing set, and replaced every seal/gasket on the motor, save for the rear main and the rear cover. When doing the cam, I made sure to thoroughly clean as much of the block and subframe as I could, I checked all the connectors and cleaned all the grounds.

When it came time for the first startup after my cam, the car would go straight into RUN, with no cranking over at all. The starter would work fine when we jumped the fuse, we were getting fuel and spark, there was nothing to keep it from turning over. After looking into the forums, someone suggested it might be some sort of syncing issue between the BCM and ECM, and to leave it on a charger while in RUN, after we tried that a few times the car started with no issue, but ever since then, anytime I disconnect the battery, or anytime I flash a tune on HPTuners, I need to either bump-start the car or try my luck with the charger trick (which only works maybe 3/10 times).

My first step in trying to diagnose was to take the battery and alternator to three separate places to get them tested, all three came back good on both. I checked all the grounds and the voltage difference between the alternator and battery at idle, The Alternator was at 14.4, the battery read 14.2, and the fuse box read 14.1.
Even though I'd had my alternator tested, I figured that the machines that tested them didn't test at the RPMs that I was having issues at, so I took off my alternator, took off the voltage regulator/rectifier, and checked everything out. The slip rings were in good shape, the solder on the diodes was solid, and there was no corrosion or play anywhere. I cleaned off all the connections, cleaned all the carbon off of the brushes on the regulator, and put it all back together and on the car. This didn't fix my issue, it did raise my idle voltage at the Alternator to 14.6, but the battery and fuse box were still at 14.3 and 14.2, respectively.

The next thing I did was check the resistance between the alternator post and the battery, where I came out with 0.9 Ohms. I have heat-wrapped headers, but no heat shield on the starter, and 30k miles sitting next to the headers in the Oklahoma heat could've affected the wiring. I have hot start issues, but I attribute that to a still-in-progress Speed Density tune and ambient temps being 95+. Instead of dealing with removing my headers and starter, I looked at some Audio threads where people had done the "Big Three" wiring upgrades, and I decided to run some 4GA from the alternator to a 150A fuse, then to the fuse box post, while keeping the stock alternator-starter wire. I also Redid some existing Block-Chassis grounds with 4GA and added some new grounds.
This was the closest I'd come to fixing the issue, there was a 0.5V difference between the Alternator, Battery, and fuse box. I was still having the same issue, so I plugged in HPTuners to read the Voltage, which was labeled as "ECM Voltage", and I had a buddy hold the throttle at different RPMs while I went around with a multimeter so we could compare readings.
At idle, all the readings were very similar to each other.
At 1500 RPM, we started to notice a difference between the ECM voltage at 13.9V, and the Voltage I was reading on the fuse box/alternator, at 14.1V
At 2000RPM, the Battery, Alternator, and Fuse box were all reading relatively steady at 14.3-14.4V, but the reading on the ECM Voltage was jumping around like crazy, from 13.3-14V.
After 2500RPM, the dash gauge dropped to 12.2ish, like normal. The alternator/battery/fuse box were all reading the same at 14.4V again, and the ECM Voltage reading was losing its mind, going from 11.7V all the way to 13.9V, jumping around everywhere. We dropped the RPM to 2000, then 1500, then around 1000, and it didn't stop jumping around until idle, where it climbed back up to a pretty steady 14.3V.

My last-ditch effort before coming here for help was to clean all the BCM connections. All the fuses were good, the two big blue connectors were fine, the red connector was fine, and there was no corrosion or bent pins anywhere. After cleaning and putting it back together last night, I'm still having the same issues. With the Starting issues, the intermittent "Service Active Handling" message, and the weird voltage issues in reverse (I saw a bunch of important-looking connectors labeled REVERSE on the BCM), I'm looking at my next option as a BCM replacement, but before I pulled the trigger on that I wanted to come here to see if anyone could spot something I'd missed or if anyone had some advice. I've absolutely scoured the forums looking for people in the same boat as me with no luck, so I've tried to include as much detail as I can.

 

Sorry but after the 3rd paragraph I couldn’t concentrate on what you were saying…if the voltage drops like that at higher RPM you need to do a voltage drop check on the positive and negative side of your charging system…do the test at 2500 RPM with the electrical system loaded…lights, AC, blower, etc…the part about starting the car after the cam install and going straight to run I have no idea what you mean…what “fuse” did you jump ??..is it a crank no start or a no start ??…I’m confused here…the charging issue may just be a poor connection at the starter solenoid where the alternator B+ and positive battery cable join up…if bad connection the battery will not charge…you just need a DVOM to do a voltage drop check .. oh, and welcome to the Forum !!

 

Yeah I'm sorry I know it's a jumble, I just wanted to be as thorough as possible.

I'm fairly certain The issue isn't at the starter solenoid, I ran a length of wire directly from the alternator to the fuse box to rule that out. With all the lights on and AC blowing, at 2500RPM the fuse box, battery, and alternator were all reading normally at 14.4, the dash was at 12.2, and the reading I got from HPTuners was jumping around all over the place, from 11.7-13.9. I took that to mean that the issue wasn't with the charging system, but somewhere between the fuse box and the BCM.

The part about the cam install was that when I tried to start the car, instead of cranking, it just put it into "RUN" mode, the same as if you hold down on the switch for 5 seconds normally. When we disabled VATS and jumped the starter fuse it cranked over fine, but to get it to actually start when I pressed the start button I had to leave it in "RUN" mode for 15 minutes while on a charger.

 

What reading were you taking from HP Tuners to see that voltage ??...the difference between the actual battery voltage and the DIC should only be 0.4 to 0.5 volts and not 2 or more volts...the starter fuse 30 feeds the load side of the crank relay so I have no idea how you got the starter to turn over...you normally jump pins 30 and 87 at the relay to get the starter to tun over....do you have a 12 volt test light ??...what is your battery voltage when you try to start it ??...let me check where from the BCM the voltmeter gets it reading from…I don’t see how you were getting fuel and spark by just jumping the starter ??....also you NEED to check the negative side of the charging system....connect a DVOM between the alternator case and battery negative and see what your voltage drop is...you should see around 0.2 to 0.3 volts.

 

BCM powers and grounds below.

 

The channel I was reading from on HPTuners was labeled "ECM Voltage".
To get the starter to turn over we jumped that relay, I incorrectly called it a fuse.
I've got a test light, a multimeter, and an oscilloscope.
When I try to start it the Voltage reading on the dash is 12.2V, and it'll slowly drop as the car stays in RUN mode.
When I said we had fuel and spark, I only meant that I knew everything was connected properly. The issue on that first startup was that with the brake pressed, clutch in, and key in the glovebox holder, the car didn't even make an effort to turn over. I would press the ignition switch and the gauge would come on as if it were in RUN, but there was no signal going to the starter. We jumped that starter relay to make sure that it was connected properly, not as an attempt to start the car.

Those pictures that you sent are very helpful, thank you. I'll work my way down the line of that starting sequence and try to find where the signal is being interrupted.

My power drop from the alternator case to the battery negative after I added that 4GA directly to the fuse box was right at about 0.3V, and it was fairly consistent throughout the RPM range, it never went over 0.5V difference.
Is there a possibility that there's something in either the ECM or the BCM that's cutting off power once I get to a certain RPM? Maybe a faulty overcurrent protection kind of situation?
I'll need to go find diagrams for the ECM and the Charging System wiring, I don't see where the DIC is being sent from on those BCM diagrams you sent. I think that my next step is going to be to find where the voltage reading is coming from and trace that back as far as I can to check fuses and look for any obvious issues.
How would you go about testing the BCM connections? It's my understanding that you don't want the battery connected while you've got the BCM connectors unplugged, so how should I check to see if the pins are getting a signal?

Electronics aren't my strong suit, so I appreciate you being patient and trying to help.

 

Ok, let’s focus on the RUN/CRANK Relay…remove Crank Relay 43 and with a test light connected to ground probe terminal 86…DON’T shove the tip of the test light in there.. that is the terminal closest to the windshield and closest to the fender….with the ignition ON the test light should light…that tells me the RUN/CRANK Relay is working…that tells me an input is being sent to the BCM (Crank Request) and key FOB is good..does your HP Tuners show and BCM inputs like this crank request and whether the clutch is pushed in…if these inputs are missing the car won’t start…you need a factory level scan tool or equivalent aftermarket.

 

Crank Relay

 

Rob, where can I find that Circuit Description in post #5. I just converted from the old ALLDATAdiy to their new system, and I haven't figure out how to use it efficiently.

 

Just type in starting system in the search box and then starting system in “description and operation” !!

 

Thanks. That's a lot easier.