As my headlight suddenly stopped working recently, (see two posts below) I spent the afternoon in the garage for troubleshooting. I got a very good troubleshooting guide from a fellow Cadillac enthusiast from the USA.
Glen wrote the following:
“If you take the panel under the steering column off I think you can reach up and disconnect the connector from the light switch. Or you maybe able to get a meter probe up there to check voltage. The battery connection to the switch for the headlights is on the 14 gauge yellow wire. This only supplies the headlights and front parking lights. If you have 12 volts there then your problem is in the switch. There is a circuit breaker in the switch and it may be bad or the contacts in the switch are bad. IIRC these switches have a vacuum switch on the side that operates the headlight doors. This makes them hard to find. They do not interchange with the 68 Eldorado because in 67 the parking lights only come on when the switch is pulled out to the first position while in 68 the parking lights come on with the headlights also. If you do not have voltage on the yellow wire then you should check the fusible link on the starter. It is an 18 gauge dark green wire that changes to a 14 gauge red with white tracer before it gets to the connector. Check for 12 volts at the connector and check the contacts in the connector. “

Glen was spot on with his diagnose and it turned out that the green 18 gauge fusible link was the culprit for my headlight problem as it came apart for unknown reasons.

the green wire once was a fusible link - it fell into pieces when I slightly pulled on it

As a follow-up Glen recommended the following:

“You might want to look closely at the break in the link. See if it is melted or did was it broken by mechanical means. If it was just pulled apart or cut then you have to check what caused that. To trouble shoot this if it was actually melted due to over current, I would unplug the headlights and front parking lights, and temporarily connect a headlight or other big light bulb, in place of the fusible link. You can then try things without blowing fuses. With the troubleshooting light in place it will light up brightly if there is a direct short. With the headlights and front parking lights disconnected the test light should not light up at all under any condition. You should be able to move change the position of the headlight switch, switch from high to low beam (light switch in the headlight position); wiggle the wire harness anywhere and not light up the test light. If it does you have found the short.”

I did this procedure today but it did not show any shorts. I then reconnected the front lights and everything did work again. I will now have to find a replacement fusible link which is very uncommon over here in Europe. I will replace it together with the positive battery cable where the clamp is almost falling apart. I hope the issue wont come back. It looks like the fusible link maybe only fell apart due to age as it did not show any signs of burning. Its located very close to the starter and the exhaust manifold - so maybe 42 years of dirt and heat just made it crumble into pieces....

the completely removed positive battery cable harness
the original battery clamp - its already missing the spring part

The starting circuit diagram is showing the battery cables - Click for larger picture!

The Iginition and Starter Circuit Diagram for the Eldorado (693) and the other Cadillacs of 1967.
Click for larger version!

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