Delco Starter Problem
 

I'm working on an A4 in a '78 Catalina 27. Symptoms were no action or a dull thunk when the key was turned to Start. The starter is a Delco.
I found the key switch to have a large voltage drop and some other connections to be less than ideal. I replaced the key switch and cleaned up up all the high current positive and ground connections between the battery(s), motor, and starter solenoid.
I also added heavy gauge wires between the starter switch and solenoid, sistering the harness and connector, in keeping with the Moyer wiring kit instructions.
Now the solenoid makes a loud clack when the key is moved to the Start position, but the motor still does not spin. The clack sounds to me like it is full stroke and that the starter fork is being moved. Voltage drops are low and more than 12 volts is maintained between the the solenoid positive post and motor ground during the Start attempt.
I intend to pull the solenoid and examine it's contacts. I'd like to isolate the problem to starter or solenoid before I order parts. I assume I will be able to examine the solenoid internal contacts and see if they are the problem.
1 - Anything else I should look at or do?
2 - What is the current path from the solenoid Bat post to the starter windings?

Delco starter problem
 

Had exactly the same thing happen last fall just before laying her up for the winter........put a new solenoid on it....problem solved!!! Easy, inexpensive fix!!:)

Yes, pull the end cover off the starter and check the brushes. Definitely an area subject to corrosion (high bilge water?) and wear. Are there any brushes left? Do they move freely in their slots? Do they make good contact with the commutator? Is the commutator reasonably clean? Worn brushes and broken springs are not unusual.

Sometimes a solenoid will actuate the fork and drive the gear into position but the contacts are so dirty insufficient current flows to spin the starter. Replacement of the solenoid is a good first step unless you like taking stuff apart and cleaning contacts. The other more serious possibility is that brushes are not making sufficient contact with the commutator. This is caused by frozen pivot pins holding the arms the brushes are on. The remedy is to disassemble the starter and free things up. Notice the two pins each carrying two brushes on arms.

Thanks. Will pull starter.
 

I was hoping to avoid pulling the starter. The space around it in the Cat27 is almost non existent. Sigh. I'll gird my loins and pull it.