I have had more than one mopar that will not start with the key, but will start with a jumper on the starter relay. more than one have been 1974 a bodies. what is the most common cause?

Im not sure about the year but mine is a 75 and you have to have the seatbelt locked to start the car.

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Ronnie
http://www.cardomain.com/ride/2714941

Yes, the seatbelt-ignition interlock is a problematic system that was mandatory for a very brief time ('74 and early '75). Once the congressmen who'd voted it into law bought their first new '74-5 cars and had to live with these problem-prone, obnoxious devices, they quickly rescinded the requirement and made it fully legal to disconnect the interlock system. See here.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.