Dave Stribling
January 28, 2019

A customer brought me a ’66 Mustang with a unique electrical problem: The turn signal switch kept burning out. In diagnosing the problem, I found several shorts and a couple of minor problems, but nothing that would cause the switch to self-ignite. I finally tracked the problem down to the front parking lamps. They were an economy brand and had been wired wrong from the manufacturer. I must admit that I have had a lot of experience with awful aftermarket parts, but I have never experienced this manufacturing failure of a new component. We swapped out a quality-built reproduction and voilà, all is well in the world. There are several points I would like to make about this problem that you need to consider for your restoration project.

1. When components are melting, you have a direct short to either ground or battery power. The circuit wires are designed to carry a specific current load, and the components (e.g., light bulbs) act as a resistance to that load. By design, the wire itself is not supposed to have much resistance so the components using the power work properly. In this case, the wire that was supposed to go to the turn signal element in the socket was in fact connected to the body of the parking lamp housing, making a near-zero resistance path to ground. The brass connection in the turn signal switch took the brunt of the current surge and made smoke signals (three times between me and the owner). When chasing down system shorts, it may be necessary to isolate components to find where the short is occurring (use your meter, not the electrical power). You can get a full voltage reading through some components, so it is best to isolate wires before chasing them down (e.g., pull the light bulbs from the sockets).

2. The customer did what many of us do—he bought his parts in bulk years ago when he started the project, and they sat on shelves until they were needed. Years later when the bad part was discovered, the part could not be sent back to the vendor. To the vendor’s credit, they did know about the bad batch of parts and did try to contact the customers, but some slipped through the system. Always check all of your parts when you get them; don’t just put them on the shelf and then a couple of years into the restoration pull them out and be disappointed. This was a very unique situation, and I don’t think you need to buzz out all your electrical parts when you get them in, but it might not hurt.

3. Which brings me to my whine: Don’t buy parts based on price! I go to swap meets and watch people walk out the door with the cheapest, most awful parts with a big grin on their face, not knowing that it may actually cost more to make the cheap parts work than if they bought the Ford replacement or the better reproduction parts. Some of the cheap parts are also damaging to your car. Just because it is stamped Autolite doesn’t mean it is good for your car. There are manufacturers who care about their parts, and they sometimes go out of business because we buy based on price, not quality. In this case, it cost the owner several dead turn signal switches and the labor for me to track it down.

I am sure that I would have smoked at least one turn signal switch if this had happened to me—this was a defect you just don’t expect when you buy new components. But by inspecting parts right when you get them and buying the better reproductions (or if possible, Ford service replacements/N.O.S.), you can avoid a lot of costly headaches in your restoration.

On the left is an original front turn signal socket, center is an “economy” brand unit, and on the right is the Scott Drake unit. The economy unit not only has the wrong color wiring, it was also wired wrong. The blue wire, which should be brown, connected the ground signal from the headlight harness to the turn signal element; the brown wire, which is supposed to be blue, connected the turn signal element to ground (direct short). The black wire, which is supposed to be blue with a black stripe, was connected correctly. The Scott Drake part is a quality piece, looks closer to the original, and works perfectly.

Photography by Dave Stribling