on the ZD28 click-click-click-click-start its usually in the wiring and/or relay.
check voltage at the starter's trigger wire while trying to crank-you want to see as close to battery voltage as possible. Usually when that complaint arises, the voltage is way down, IME, typically 10v or less, oftentimes down around 7-8v. This is enough to make the solenoid click, but not enough to drive it hard enough into the contacts.
to diagnose it you have to use the voltage drop test method. Or at least that is the least intrusive, and most accurate, fastest way. Or you can replace stuff like a lot of people do, one at a time, until something fixes it. I call it "firing the parts cannon".
