What is an idiot light?

The positve battery cable clamp might need to be replaced or replace the entire cable. The clamp corodes badly where it attaches to the cable. If this is the case, even jump starting won't work. If you don't have a battery post and clamp cleaning tool (two brushes in a holder), use sand paper on both post and the negative clamp. As suggested, be sure the battery is charged and the smaller red wires attached to the + clamp are making good contact. If your battery has fill plugs, check the water level and fill as needed with distilled water. jp

"no juice at all. the dome light, the idiot light, radio ...nothing worked. "
Could be just corrosion or looseness at batt post. Use vaseline to coat posts and tighten.
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1980 245 Canadian B21A with SU carb and M46 trans

Wouldn't be a 1986 240 would it? These are notorious for the wiring harness becoming brittle. Could be the wiring harness area on the driver side firewall.

In addition to the Frenchman's tips, I would also loosen and scrape clean the Battery (Red cable) connection at the starter (Be SURE to disconnect the Negative Battery cable first!).

That is a critical spot, because the other (smaller) cable there is from the Alternator. This is a junction point where Alternator voltage is fed back to the battery to keep it charged — and it is exposed to lots of road slop.
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Bruce Young,
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.

This is what I would do in this situation:
-Check battery posts for clean and tight contacts. Check your main ground connections. That is: the two blue cables going from the negative battery post to the engine compartment and engine block.
-Also,make sure the battery is charged: 12 to 13 Volts between posts. If not, you may have a problem with the charging circuit: Recharge the battery, start the car if possible. Check the voltage again while the engine is running. You should read 14+ Volts between battery posts. If not, the voltage regulator may be bad. It is fastened to the alternator. You need to remove the alternator for easy access to the regulator. It has two carbon brushes. Those brushes wear out. If they are about 1/4 inch or less in heigth, you need to replace. An aftermarket regulator is about $20.00 . AT the same time, check the condition of the slip rings that make contact with the brushes, inside the alternator. Those surfaces must not be grooved by wear due to the friction with the brushes.

Let me know what you found out.

Richard