Whi h battery terminal to unhook ???'s

[Ribbedroof]

Just for clarity, remove the negative first on negative ground systems.

Positive first on positive ground systems (commonly older 6 volt systems). Usually pre 1955 for automobiles. My 57 Ford tractor 6V system is positive ground

 

[clean8485]

If you read service procedures in GM service manuals, usually the very first step says "remove battery negative cable".

 

[86LK]