Sounds like a few things it could be.
1) Battery condition. A weak battery will not produce enough power to spin the starter fast enough to raise the bendix. Load test the battery. A local auto parts store can do this for you.
2) Stuck bendix. Check to see if you can raise the bedix by hane with only spring pressure giving resistance. Lightly grease the worm gear under the bedix and shaft with graphite, lithium or any other light grease.
3) Poor battery connection. I usually put a battery load tester on the battery connections at the motor to test this. If it holds a load at the motor, the cables are good.
4) Poor connection in the starter solenoid. Take two BIG screwdrivers and cross the two big studs on the starter solenoid. If the starter spins the bendix up and spins the motor, the solenoid is bad.
5) Bad starter. Get a known good battery (like the one in your car) and a set of jumper cables. Hook the ground to the frame of the starter and touch the positive to the stud on the side of the starter and see if the strter will spin up and engage the motor. If not, get the starter rebuilt or get a new one.
I cannot raised the bendix by hand. Is it best to load test the battery before proceeding? I had trickle charged the battery, so it should be good, right?
i have the same motor: to raise bendix, turn it the opposite way.
90% of starter problems are bad connections. remove and clean both ends of the battery cables, so that they are shiney, also the cable from the solenoid to starter. check for nicks in the cables. and make sure the connectors are on the wire good. the cable tend to corrode from the inside out, if nicked, corroded wires, and connections, heat up and cause resistance to the follow of electricity, thus the starter doesn't get enough. you can also take jumper cable pos battery post to large post on starter. with a good connection, the engine should spin. then if the starter is good clean everything and retest. then trouble shoot solenoid. starters can be rebuit at a starter/alternator shop, much cheaper than a new one.
also have the battery load tested at the auto parts store, free, i've had new batteries go bad
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If you can't raise the bendix by hand, you have isolated the problem. Its obviously seized to the starter shaft. Remove the starter, remove the bendix, clean up the shaft and install a new bendix. DO NOT replace the solenoid. It is not the problem as it obviously spins the starter. The starter is also not likely the problem. It makes no sense to do electrical tests when you have identified a mechanical issue.