If I'm looking at the correct wire diagram for a 2155,
the blue wire for the solenoid joins up with red and orange wires in the engine connector.
The orange wire brings the voltage into the engine connector after it passes through the brake safety switch and the PTO switch.
The red wire feeds voltage to the fuel solenoid and the PTO.
I would pull the plug off of the fuel solenoid and the PTO and see if the voltage still drops at the starter solenoid when the key is operated.
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