Have a 1975 chevy nova 350, starter will not engage to flex plate. Had starter and soleniod tested and both passed, seen and heard it myself. Still have head lights and interior lights. With key in the ON position: has battery source voltage (12.62) at soleniod, battery source at ignition plug in at fuse block under dash (has 0volts with key in OFF position), and source voltage at distributor plug in. Wires seem to be all connected properly. Dropped starter with soleniod all wires still attached and tried cranking, still nothing happened. Even tried the old trick of bypassing soleniod/starter with screwdriver to engage starter ring gear, sparked but didn't engage. Buzzer/seat belt warning light is still heard and seen when key is turn to the ON position. About a week before it this happened noticed that vehicle would not fully enage into PARK. Vehicle would roll on sloped roads, as if it was in NUETRAL. Would have to hold or jiggle/shake column shifter and then it would egage. Column shifter does seem to have some play/loosness. Maybe the source of this problem is the PARK/NUETRAL safety switch? didn't check this bacause I do not know where it is. Please give me any suggestions, I'm stumped.
Hope this discription narrows the problem down, Thank you in advance.

 

First of all you cannot make the starter work while its hanging by the wires unless you have added a ground wire to it.
Before you go testing and tearing have someone turn and hold the key into the crank position. While they are holding it you wiggle or twist every connection from the battery to the starter including the ground wire to the block. If there is a simple bad connection you will find it this way.
Okay now working from memory since 73 is the year I graduated highschool I think I can remember that far back. Make sure the purple wire going to the solenoid (small bolt) is on the one markedS (starter)not the bolt marked R (resistor). The S bolt is the one closest to the engine block.
Next make sure the fusible links mounted to the larger stud along with the battery cable are not burnt out. (use a standard test light)
If theres no loss at the fusible links then go to the neutral switch located on top of the column under the dash. The switch will have 2-10 gauge purple wires and If I remember right 2-14 gauge green wires. The purple wires are the neutral switch and the green ones are for reverse lights.
By using a jumper wire like a paper clip jump the purple wires then try to crank it over.
If it was me I would go into the wiring harness behind the brake booster and open it up to find the 10 gauge red wires and the one 10 gauge purple wire. Next I would take something like a dart or test light and probe clear through one of the red wires and then into the purple wire making a connection between the two. If the engine cranks over its defineately NOT a fusible link or starter or solenoid or battery connection and leaves you with either the ignition switch (also on top of the column) or the neutral safety switch.
If my antique memory is working I am thinking the neutral switch was a problem back then and we usually ended up bypassing it by splicing the wires. Good luck John

 

It just dawned on me that in 73 GM came out with the HEI ignition system and some cars had it and some didn't.
My point is if yours didn't have HEI the (R) stud on the starter would have a wire on it running to the distributor. This wire fed the points with 6 volts (resistor built into the solenoid) so after the engine started the power to the points was cut in half (12 volts cranking 6 volts running) to keep from burning out the points so fast.
If yours is apoints system make positively sure your wires are not turned around at the solenoid.