Cummins 6bt shut off issue

I wanted to get input for the following issue with my starboard engine. I recentl had the alternator rebuilt.
I have an on/off key with a push start button at the helm.
With the engine not running and when I turn the key on the power goes to the screw in type fuel solenoid and retracts the internal plunger.I get a good reading of 12v.
With the engine not running and when I turn the key off the fuel solenoid shuts off and I get no 12v reading.
Here is the kerfufffle.
When I start the engine and have it running, then shut the key off the engine does not stop or shut off. I have a reading of 12v at the fuel solenoid with the key in the off position.
If I unplug the fuel solenoid the engine shut off. Hence the fuel solenoid is not sticking. I have looked for a plug in diode but do not see one in the wiring harness
These are 1988 cummins 6bt.
Help!!!

Just to be clear, before the alternator rebuilt worked fine, next startup after rebuilt, not fine?
Are the alternator connectors hooked up the same?
Who did the alternator remove and install?
Anything else worked on at the same time?

Yes before the alternator rebuild the engine shut off fine with the key.
Thank you for your reply.
John

Sounds like a bad diode or incorrect hookup on the alternator. Without the diode the alternator will power the fuel solenoid.

A resistor or diode is missing from the alternator. Probably a rebuild error. Exciter power is backfeeding the ignition lead somehow.

You should have a large wire going to the alternator and either 1 or 2 small wires. The small wires may either plug in or attach on a stud. Remove the 1 or 2 small wires, tape them off so they don't short out to anything Start the engine while looking at the gauge and pay close attention to the gauge panel. If it stops with the key and voltage is below 12 volts, its a problem in the alternator. If it stops and the voltage is above 12 volts (charging) it's the wrong alternator (single wire self exciting).

Ted

Update,
I removed the alternator and brought it back to the repair shop. At first the owner was questioning my findings. He said it was highly unlikely that it was the alternator. I told him the engine shut off with the key before his rebuild of my alternator. I told him I had a 12v reading from the fuel solenoid when the engine was running and the key was switched to off. Furthermore when I unplugged the field wire with the engine running, the engine was able to be shut off with the key.
I asked him to do his bench test while I was there. He did it and sure enough the field terminal was producing charge.
He offered his apology and is replacing the diodes in the alternator. Problem fixed.
Thank you all for the input.