I have spent countless hours researching this, and am still somewhat stumped. I just got the car back together from being down for about 2 years for a twin turbo build. Running a Holley Dominator with the stock ecu piggybacked. Car fires up and runs good, everything seems to be working as it should, and all trouble codes have been tuned out via HP tuners.

From the first start, the car was throwing a P1637 and P1638, along with the charge system fault on the DIC. Factory 2002 Z06 alternator. Double checked I had everything still pinned correctly at the ecu, and it was all correct. I had a brand new 145 amp AC delco alternator on the shelf I bought a few years ago and never used. Decided to swap it in in thoughts that maybe my stocker was done. I did tear the stocker apart and coat the housing, so thought maybe I didn’t get it back together right(yes, the alternator and bracket mounting points were masked and are metal on metal).

New alternator in, same thing, then after the car ran for about 5 min around 11.8v, suddenly the idle changed, voltage went up to mid 14v, and the charge fault warning went away. I thought it was fixed, cycled the motor a few times, and all good. Disconnected the battery, went back out a week later to work on the car, and same issue as before.

I decided to turn off F and L monitoring on the stock ecu, along with the trouble codes, and hook the alternator L terminal up to a switched 12v with a 470 ohm resistor in line to attempt to get the alternator excited and begin the charge. Same thing, sits about 11.8v and drops as the car idles over time. I have tried it with both the sensing wire connected and not connected, have checked for voltage drop across the alternator post to positive on battery, only .07v. Alternator voltage matches within .1v when I check voltage direct on the battery, and when I check the post and use the alternator case for ground.

I am stumped, is the alternator on the C5 looking for something special over the 470 ohm resistor to excite and begin charging? I know these cars are finicky with the alternator, and with everything turned off now, I prefer to keep it that way, feed it what it needs to work, and not have to worry about the stock ecu talking to it, if I can get away with it.

Sorry for the long post, any and all thoughts welcome.

 

Strange that with both alternators installed the voltage is dropping...one would think a large voltage drop on the positive and negative sides of the charging system but they look good...if you can back probe the connector on the F terminal you should see 2.0-8.5 volts KOEO or KOER...on the L terminal KOEO you should see 0.5 - 1.3 volts and KOER 8.0-11.0 volts...the S terminal should have battery voltage KOEO or KOER....since the L terminal is PWM it would be good to put a scope on it and check the duty cycle...below is the duty cycle for my 01 running with no load and loaded...31% no load and 75% loaded....this could be a bad PCM possibly.

 

F terminal has been deleted, and the L terminal I am using a 470 ohm resistor, back probing with the engine running on the L, I am seeing 1.04 volts, grounding to the case. Nothing in the alternator is tied to the factory ecu now. It was originally, and even when it did finally start charging it was still coming off the ecu. But after it quit charging again I decided to try and run it stand alone not using the ecu. The less I have the ecu doing the happier I am, it’s only purpose in life now is to keep the gauges, bcm, ac, and fans happy.

S terminal is a mirror image of battery voltage at all times. I have a 2 gauge cable running from battery, through a 200A fuse, to the starter, then 2 ga from starter to B post on alternator, and the factory fused link wire running to S terminal from the starter. Everything was out of the car during the build, and the positive terminal is clean and tight.

 

I recently had a similar problem on a stock motor where the voltage would sit low between 11-12V and would jump between those values, however, it would also sometimes sit at a healthy 13.8-14V, albeit a bit unstable. Ran fine with the voltage high, not so much when it was low.

I did find the problem, turned out to be the battery. Try hooking your battery up to a battery charger( a digitally controlled one if you have access) and leave it on a 1-2A trickle until it reads full. Drive it around afterwards and see if the symptoms alleviate. It will be a temporary band-aid if the battery is causing your issue, but it will at least give you some evidence that it is or is not the problem.

I wish I knew exactly why the battery was causing the chaotic behavior, but I do not know lead-acid battery design to know what happens. Someone else might chime in on that to give an explanation. Regardless, swapping the battery for a new one solved the issue and the car runs as it was intended with the voltage barely fluctuating between 13.8 and 14V

 

Have you got a test light? If you do, try connecting the wire to the positive battery or alternator post and then putting the probe to the L terminal. While the engine is running. See if the light stays on or goes off and it begins charging.

The alternator is rather dumb, despite some historic postings you might find here claiming it's some super special unique design. The alternator is not controlled by the PCM, as in the PCM has no control over the charging voltage. The main issue people have replacing the alternator is that the L terminal from the PCM doesn't provide enough voltage or current to excite the regulator in some replacement alternators. I would have expected 470ohms was enough for a stock alternator but maybe it isn't. Hence testing it with a test light which will do it.

 

Thats a good thought. I decided to swap to the ICT Billet bracket and go with a truck alternator. Haven’t had a chance to test it yet, but I’m sure it will fix my problem. Truck alternators work well with a 470 ohm resistor to excite them.

 

Fired the car up today on the truck alternator, charges great. Turned off the codes in HP Tuners, no charge fault codes, definitely a good option for anybody wanting to move away from the standard C5 alternator, and having to pay the vette tax on a aftermarket alternator.

Here is a shot of it while I was finishing up the install. I was able to tap the front c5 spacing bolt hole to a 1/2-13 and mount my cold piping to it just like it was designed.