1984 GMC Van Starter continues to run after key is released

Engine Mechanical problem
1984 GMC Van V8 Two Wheel Drive Automatic

Greetings
1984 GMC Van with 6.2 diesel engine.
Starter continues to run after key is released.

TEST: I tested wire that goes from ignition switch "start" position to selenoid "S" terminal, (test performed with wire disconnected from selenoid "S" terminal).
TOOL USED: Digital multimeter
RESULTS: Key on start position=12 volts
key released to run position=0 volts

TEST: Bench test starter and selenoid (off vehicle) as follows: 12 volts to battery lug on selenoid, jump with screwdriver between battery lug on selenoid and "S" terminal of selenoid, thereby providing 12 volts to "S" terminal of selenoid.
RESULTS: Screwdriver touching "S" terminal= starter engages and runs.
Screwdriver removed (power terminated to "S" terminal)= starter stops.

These tests indicate to me that wire to "S" terminal of selenoid is OK, that selenoid is OK.
Somehow, power is going to selenoid after key is released, causing it to stay engaged.
I have been told the starter itself has a short, but no one can explain how or how to test it.

I am 62 year old cancer patient living on $674 per month disability and can not afford to replace starter without being absolutely sure it is at fault.

Same reason prevents me from making a contribution, I am sorry.