915is generator start sequencing AND Battery chargeissues

I believe alternator B is not producing power, because either alternator B or voltage regulator B has failed.  This is what’s I think is happening:

You start the engine with battery power via the Start Power switch or the Emergency Power Switch.  Once running, the engine electrical power should be initially provided by alternator B, because alternator A does not come online until you reach 2500 (or maybe 2600) RPM for a few seconds.  If alternator B does not produce power, and Start Power is removed before Alternator A is online, the engine will stop just as you describe. 

The failure of alternator B (theory) is further supported by the fact that when you do get the engine running by starting above 2600 RPM, then the battery does not charge if either the start power or emergency battery switch is not closed.  This makes sense because alternator B is normally charging the battery after alternator A connects to the engine (at which time alternator B is assigned to the airframe). With alternator B failed the battery will not charge. However, when you then close the emergency power switch you are connecting the airframe/battery to alternator A.  Alternator A is not intended to charge the battery because it’s dedicated to the engine loads and normally isolated, but it will do that short term as long as power requirements are low.  

Your successful starts (above 2600) are possible because at that RPM alternator A is almost immediately assigned to the engine, likely before Start Power is removed.  Follow the prescribed procedure to test Stator B and Regulator B.  

I agree with Jeff B. -- likely a failure in the Stator/Regulator B system.

To troubleshoot: Disconnect the stator harness plug from the regulator and check voltage on the pins in the stator connector (pins 1-2, 2-3 and 1-3) with the engine running (BE CAREFUL!).  You should see an AC voltage on all three pairs of pins (measured voltage will vary with engine speed).  If you do, then the stator is likely OK and your problem lies in the regulator, or in the Fuse Box (blown B system fuse, bad relay, etc.).

If your engine has had SI-915i-006 done (replacing the factory stator/regulator connectors with Amphenol ecomate RM connectors) then here is the assembly/disassembly instructions for that connector (in case you need to remove pins to access them for measurement):

https://www.amphenol-sine.com/pdf/Assembly-Instructions/ecomate-rm-Assembly-Instructions.pdf

If your engine still has Deutsch DTM connectors on the stator/regulator harness, the instructions for that connector are here:

https://www.deutschconnector.com/technical/deutsch_dt_and_dtm_connector_user_guide/

Just one thing to add to Eric’s troubleshooting.  Be sure you check each of the three Stator B Wires to engine ground.  The reading should be infinite (no continuity to engine ground).  I’ve seen some stators that still make some voltage but have a grounded stator coil.   You can do this test first without the engine running (safer), and if you find a grounded stator wire the voltage test need not be done.  If grounded, the stator must be replaced.  

Excellent point, Jeff.  Thanks.