how to check starter/engine ground

starter whirls when taken out and hooked to external battery. when its in only the relay closes. its got power to the primary and the solenoid. and its a brand new starter. gotta be the ground? theres no fuses or safteys in the primary power supply line. i herd the pcm grounds the starter? there is no ground wire it grounds via the casing on the engine block. im goin nuts tryin to figure this out.my dash lights and all work its an 87 dodge dakota 2.2 no turbo, manual tranny. the motor turns if i pop it. just clicks no starter engaging.

Take a volt meter and check the voltage at the connection to the starter when somebody tries to start it. If the voltage drops under 11 volts check for dirty connections. I've changed antifreeze and spilled some on the wires. I had to use electrical cleaner to get the car started. A bad battery is also a major cause of these type of problems.