1999 Trans Am Battery Charging Problem...Please help

[1999TransAm]

I've been having a problem with my 99 Trans Am for over a year now. I keep getting DTC P1637 Generator L-Terminal. I've replaced the battery 2 times, the alternator 2 times, new battery cables, even a new PCM and still not working correctly.

When the engine is running, I am measuring about 14.4 volts at the battery which is normal but when I turn the car off, I am only measuring about 11.8 volts at the battery. I don't understand how it can show the battery charging but it doesn't charge. If I hook up a house battery charger, it will charge to 12.6 volts no problem.

I checked the L terminal wire and it is not shorted or open. There is good continuity from the PCM to the generator.

I checked for parasitic drain and the car is only pulling 150mA when the car is off.

Any help will be greatly appreciated...I'm pulling my hair out with this problem.

Thanks in advance!

[wrightz28]

Sounds like you've covered pretty much everything, nice work

You say there's continuity, but perhaps there's too much resistance?

[1999TransAm]

I found the problem. I installed a Diablosport Performance Programmer and it was causing the P1637 error. Even after doing the Diablo uninstall, it would still give me the code so I took my car to a dealer and they reflashed the computer with the stock GM calibration and the code doesn't come back anymore. The battery and charging system are working great now. No more code and the battery is charging to 12.7 volts!