[bestgenman]

Mike, I concur with Steve,  the cooling fan is 240V and wired directly to the output of the generator (ahead of any overcurrent protection such as a breaker).  The Martin control box looks for 240V in order to hold the solenoid in.  My bet is a broken wire on the AC output of the generator since you have voltage on one leg.  It's highly unlikely you have voltage regulator failure.  you'll need to look in the generator termination box rather than the junction box on the wall of the generator set's / Newell box.  

Yanmar manufactured engines of your vintage using  a two wire solenoid with a third wire not used.  Shortly afterwards, they switched to the three wire "hold in / pull in" version.   Your solenoid is working fine since it pulls in and doesn't have a seperate hold in circuit.