I'd double check battery connections, battery condition, and grounds and then have someone hit the start lever when you're checking voltage at the starter. Many years ago, I chased the similar problem on a mid-70's (told you it was many years ago) Honda CB550 motorcycle my uncle gave me to get to/from college classes. I had 12v to the starter and starter solenoid but nothing happened when I pressed the start button. Every where I checked, I had 12v where 12v was supposed to exist, and no voltage anywhere in the ground circuit. But when I hit the start switch, nothing. After many frustrating hours and cuss words testing with a voltmeter, the only difference I could find was that the 12v at the solenoid went low (4-5v IIRC) when I pressed the start button. Turned out the problem was the starter switch itself which had corrosion in the contacts. So when the starter button was not pressed, 12V (battery voltage) was present at the solenoid because it was an incomplete circuit. When I pressed the start button to complete the solenoid circuit, most of the voltage drop occurred across the corroded start switch because that was the high resistance and little voltage drop occurred across the solenoid which was low resistance when the trickle of current flowed in the now complete circuit. So while you hear the solenoid on your M818, you still need to chase where 24v is not getting to the starter when you hit the start lever. Could easily be a fried connection somewhere that allows voltage but no current.
Here's the engineer in me explaining my test methodology: When using voltmeters to check voltages in systems, the assumption is all connections in the circuit have no resistance. That means that all connections are tight, corrosion free, grounds are actually grounded, etc. otherwise, you can have significant voltage drop at the connection instead of the DUT (device under test). The way to test this is to measure the voltage of the DUT (positive lead at DUT with negative lead on a known good ground like the negative battery post) while someone hits the starter button for you and see where you have a change in voltage but the DUT still isn't working. A drop in voltage at the DUT but with the DUT not working probably means you have a bad connection somewhere in the overall circuit that's letting voltage pass in an open circuit but won't let enough current pass with a closed circuit to activate the DUT. Or it could mean your DUT is shorted internally (fried) and all current is going straight through without any resistance which would still cause a low voltage. At that point, it's a matter of isolating the DUT from the circuit and testing each to verify if your circuit or DUT is bad.
A good example is to loosen a battery terminal so it has a lot of play but it's still touching the battery post. I bet you'll easily measure 24v at your starter but the moment you flip the starter, sparks will fly and one of two things will happen: 1) nothing as either the battery terminal or post melted away and there is no longer contact between the two or 2) the engine will start as you melted your terminal to the post for a really, really good connection. Ask me how I know about 2)....
