Get a copper brush, remove both battery terminals from the battery and clean both terminals and posts inside and out. Also, separate the fusible link box from the positive battery terminal and clean that connection well too. Put everything back together and try again. If you still have the problem than it's either the starter (see Offroader's post) or it could be a bad battery cable or connection at the starter.

It's not likely an engine control issue. The starter is hotwired directly to the battery along with the ignition switch.

Good luck,

Powell


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