When you turn the key to start, the solenoid/relay on the starter is energized. This does 2 things, it closes 2 contsacts which apply battery power directly to the starter (this is the heavy, thick wire you see running from the battery down to the starter) and it shoves the starter into the flywheel to engage the ring gear teeth.
Where that heavy gauge wire from the battery connects to the starter is the solenoid, off the other contact you will see a heavy wire going ino the starter motor.
First make sure that solenoid is closing. a loud click is a good indication.
If you connect a volt meter to the terminals of the solenoid and turn on the key you can check if you are getting full voltage (at least 10) at that point.
I suspect you have a bad connection in that heavy cable. I would take it off at the battery and clean it with baking soda, make sure if the cable uses a clamp to hold the wire that you clean that too, if it looks corroded it may be best to purchase a new cable.
You should also check where the cable connects to the solenoid and make sure that is not corroded. Some fine sandpaper will shine up the terminals, and the stuff they give you for new batteries should keep it from corroding.
The last possibility is the solenoid itself could be bad.
To know for sure you have to use the voltmeter to check the starter side of the solenoid, it should have the same voltage as the battery side.
Have at it!
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745-4+OD-16v, 744-4+OD-16v, 245SE auto, 242 4+OD Turbo
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