The other night I was driving home in the dark when I noticed my the lighting for the gauges starting to fail. Soon, all I had left was the lighting on the speedometer, and it failed shortly after that. Fortunately I had a GPS unit running, so I could monitor my speed, but I was filled with dread thinking that my parking lights also went out.

I checked the fuse and found it blown. I replaced it, but still no lights. I checked the connector in the steering column. It had suffered heat damage. (Note: The wires are not sized for a 20A fuse.)

I replaced the connector (Thank you, Vintage Connections!) and tested. The ammeter I had on the battery was running over 20 A. I quickly shut off the switch. I looked for corrosion in the front side markers. They were fine. I returned to looking at the dash lights.

I noticed the current was normal after removing the speedometer. I tested the wiring on the speedometer for a short. There were no shorts. I finally checked the GW & RL wires in the dash harness. The resistance was way too low. I unplugged all of the gauges, and no change. After looking at the FSM for other ideas, I unplugged the hazard switch. (The hazard switch in the 74 has a light.) The short went away. I pulled the switch out and found the cause. When the RL wire came loose from the light, it made contact with the other wire. When the speedometer was plugged in, the RL wire was then connected to ground, completing the path for the short.

That only took 3 hours to find.

 

Good find. That wire got a little warm...

So did the wires on the 9 pin connector in the dash. I'm planning on de-rating the fuse on that circuit. I'd rather blow a fuse than burn up the wiring harness.

1 hour ago, SteveJ said:

So did the wires on the 9 pin connector in the dash. I'm planning on de-rating the fuse on that circuit. I'd rather blow a fuse than burn up the wiring harness.

Yes, absolutely!

(Not surprisingly) great sleuthing there SteveJ!

What about the root cause? Why do you think that wire came off the back of the socket? Heat? Vibration? Mechanical overstress? Something else?

I'm not sure. The socket was not in good condition when I pulled out the bulb. All I know is that I can leave that light dark without any problems.

I found the missing piece to the socket on my garage floor. The wire runs up through the button in the socket and is soldered in place. Somehow the solder joint weakened, and the wire came out. I removed the old solder from the button and ran the wire through it, soldering it back in place. If it holds for another 40 years, it will outlive me.

I've seen that type of failure a number of times in the past on old equipment. Both on wires pulling through tabs like you had there and the solder ball on the bottom of bulbs. I suspect that even though the operating temperature isn't high enough to actually "melt" the solder, it is high enough to just slightly plasticize the consistency.

I'm thinking that the constant pressure or strain combined with the plasticized hardness is enough that it very slowly creeps over the years. Sometimes resulting in a failure like you had there. I'm sure you've seen this too in other applications.