1973 Ford Mustang Just Put New Starter Seolinod Start Could Wrong

Does the starter crank? Have you done any voltage testing, anywhere? Battery has a full charge? Battery connections all clean and tight, positive and grounds ok?

Turn on your headlamps, turn key to crank, any reaction from headlamps. If they stay bright, no reaction, possible open circuit to starter. If the lights go out or almost, usually the battery or connections.

You should be able to jump the relay to see if starter will crank. I use two big screwdrivers to jump the relay. Those cables carry a lot of amps. You have to be sure the tranny is in park or neutral and the e-brake is set. You don't want to crank in gear.