2005 M2 106 no start

I just had the truck at frightliner for brakes and steering column issue. Now the truck won’t start. Any suggestions?

 

I have 12 volts at the left most large wire with ignition on. Nothing at the other three

Trying to rig up a way to see what is what while cranking. I don’t have anyone to turn the key for me

 

Can i place a wire between D and B?

While ignition on
C = 0 volts
B = 0 volts
A = 0 volts
D = one time it was 12 volts and now it shows 10 volts

With the key in start position
C = 0
B = 0
A = 12 volts
D = 0 volts

Battery is at 12.4 volts on the terminal. Dash shows 13.1 volts

CEL light comes on and goes off when you turn the ignition on

Gauges sweep as normal

Ac blower comes on

Radio works

 

Why would D drop to 0 volts when the key is in the start position? I’ll see if I can find the fuses

When I turn the key to start the volts drop to 0 and when I let the key go and it turns back to run position the voltage comes back. Now it’s only to 10 volts vs 12

 

What is the purpose of C on a Diesel engine?