2012 cadillac srx luxury fwd 3.6 code P0621

Recently got service charging system message and battery light on dash.  No charge from alternator.  Replaced battery and alternator.  still no charge.  Problem is intermittent.  When vehicle is cold alternator works for a while then drops out. sometimes while driving it will work for a short period of time, then stop working

Per alldata diy I tested the gen L circuit by disconnecting the connector at the alternator.  Ignition on - voltage at the X1 terminal 1 connector  circuit 25 brown wire and I've got 4.9 volts
On my scanner for the charging system datastream  "Generator L terminal circuit low voltage test status is malfunction".  any help greatly appreciated
 

Hey basshollow! Thank you for including the pinpoint testing you did at the alternator. That rules out a dead short to ground on the Gen-L circuit.

On my scanner for the charging system datastream  "Generator L terminal circuit low voltage test status is malfunction".  any help greatly appreciated

Does this data PID show 'Malfunction' all the time, or only when the battery light is on (or when the P0621 is shown as Current)? If it's only when the battery light is on, you can use that PID to test further.

Get the battery light to come on with the engine running, check that the low voltage PID says 'Malfunction', then unplug the alternator. If the low voltage PID now says 'OK' or 'No Fault', then the alternator was shorting the Gen-L circuit to ground. From service info:

The ECM should detect low voltage on generator turn on control circuit when the ignition is ON and the engine is OFF, or when the charging system malfunctions. With the engine running, the ECM should detect high voltage on the generator turn on control circuit.

After that, I'd reverify no excessive voltage drops from the alternator to the battery on the B+ and ground circuits. Make sure the belt is tight, and that the crankshaft pulley isn't slipping. Then, it's time for another alternator. What brand of alternator have you installed previously?

Hi Tyler, Thanks for the reply This one is really giving me a fit !!! I’ve not been able to check the datastream when the charging system is working. It only charges for a short period of time. I’ll get the scanner set up before I start the car and see if I can catch it while its working.
I’ll try unplugging the alternator next time I have it running as well. I did test for voltage drop on both battery positive and ground. Drop was .1 volt.
I’ll look for belt slip as well. Would there be other symptoms if that were the case? Everything else on the vehicle is working fine. The replacement alternator is an Oreilly remanufactured. I removed that alternator yesterday evening and took it to Oreilly. They tested it and said it tested good. I talked them into doing a warranty exchange anyway. I’m in the process of installing the 2nd replacement now.
A couple of other things. At one point I thought the ECM was causing this so I bought a refurb ECM from Flashmasters. I sent my old one to them and they cloned it, then sent back both my original and the replacement ECM. The charging system fails exactly the same way with either ECM.
I did a couple of other tests after my 1st post yesterday. I backprobed pin 53 on the ECM. With key on engine off, alternator unplugged I’ve got 4.99 volts. With key on engine off alternator plugged in I’ve got .75 volts on pin 53. When I start the vehicle with the alternator plugged in I get about 3.6 volts on pin 53 for the brief time that the charging system works. When the charging system stops working I get about .6 volts on pin 53 with engine running, alternator plugged in. I also did a voltage drop on the generator L circuit with engine running, alternator plugged in. Voltage drop was -0-
I’ll post again when I get the replacement alternator installed. Thanks again for your help !!

I’ll look for belt slip as well. Would there be other symptoms if that were the case? Everything else on the vehicle is working fine.

Oh sure, there'd definitely be other symptoms. Weak power steering, overheating, no A/C, like that. I just figure it's better to ask.   

I did a couple of other tests after my 1st post yesterday. I backprobed pin 53 on the ECM. With key on engine off, alternator unplugged I’ve got 4.99 volts. With key on engine off alternator plugged in I’ve got .75 volts on pin 53.

Good stuff. I'd say this shows normal operation.

When I start the vehicle with the alternator plugged in I get about 3.6 volts on pin 53 for the brief time that the charging system works. When the charging system stops working I get about .6 volts on pin 53 with engine running, alternator plugged in.

Even better.  This pretty much spells out the problem, IMO. Only other thing left to do for that 110% confidence is to do the unplug test with the engine running like we talked about earlier. 

Let us know if the second alternator does better.

2nd alternator solved the problem no more charging issues. thanks for looking into this for me. your advice was spot on Rick