Aye Roll nailed it. It's that under-engine-front harness. Only three wires, but the black tube around them gets hard over the years, allowing the metal tabs that hold it in place to chafe a hole into the wires and ground them out.

Grounded oil pressure sensor => oil light comes on.

Grounded alternator exciter wire => amp light and all the otherlights come on.

Grounded heavy red wire => fire, but this verrry seldom happens.

Check the archives, there is a fix for this involving putting in new wires, leaving the old ones in place. New wires routed differently.

Good Luck,

Bob

:>)
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240s: 1986 244GL, 1988 244GL, and 5 others.