Last month we discussed the basics of your car's 12-volt electrical system with the goal of understanding enough to trace problems to a particular electrical component. This month we're going to delve into the diagnosis and repair (or replacement) of some switches and relays.
Typical "completes-a-ground" switches include doorjamb switches that operate interior lights, the horn switch in the steering wheel, and Neutral safety switches. A regular switch completes the circuit between the power source and the accessory, allowing the electricity to flow to the accessory, through it and to the ground. With a completes-a-ground switch, power flows through the accessory/relay, into the switch, and then to the ground.
This shows the simple insides...
This shows the simple insides of the emergency flasher switch. The underside of the "lid" is on the left, the mechanical part of the switch is in the half on the right. When assembled, the bumps (contacts) are inside the box across from the metal bar. The sliding bar in the box moves when the switch's lever is moved, touching the bar to the copper contacts in the base. One of those bumps on the lid was dirty, and that was the switch's only problem. It was cleaned with 600-grit sandpaper and now works like new.
Relays are small boxes that contain an electromagnet which, when activated by current flowing through it, pulls down a bar that completes the circuit and allows the current to flow to the accessory. Relays are often controlled by completes-the-ground switches. When a relay that's getting power no longer works, the problem is frequently corroded points or a broken wire.
Some switches and relays should just be replaced when they quit working. If the headlight switch on your car is bad and only costs $13, replace it. However, not all switches and relays are cheap or easy to come by, and if you have the correct original part, you may not want to replace it.
To illustrate the repair of some switches, we'll use the emergency-flasher switch and a headlight switch on a '69 Barracuda. The right front turn signal wouldn't light up when the emergency-flasher switch was turned on; however, the light did work as a right-turn signal or parking light. The problem was traced to a bad emergency-flasher switch, which we tried to repair before replacing it.
The flasher switch has four small metal tabs that hold the top casing to the bottom part. The tabs are pried up and the switch is carefully pulled apart. Be careful when pulling switches apart, because many of them (this flasher switch included) have springs that force the contacts tightly together. If you're not careful, the springs will go flying. The flasher switch is a simple sliding bar with copper contacts that touch other contacts when the switch lever is moved, completing the circuit. The corroded contact is easily visible and quickly cleaned with 600-grit sandpaper. Then the contacts are greased with a little di-electric grease, the switch reassembled, and the tabs bent back down. It works great, and some money is saved.
This shows proper testing...
This shows proper testing of a headlight switch. The fused jumper wire is used to connect the switch's running light terminal to the battery, and the non-fused black jump wire jumps from that terminal to the headlight Power In terminal. Use the circuit tester to test switch terminals based on the knob's position.
Some switches, like the headlight and windshield wiper, are held together with rivets instead of bend-over tabs.
If the switch is the problem, you can replace it or attempt to repair it. To get riveted switches apart, drill out the rivet heads and separate the switches very slowly-these are more complicated and may have several loose parts inside them. However, they operate on basically the same concept as the emergency-flasher switch. Clean everything with some 600-grit sandpaper, apply a little di-electric grease to the contacts, and reassemble the switch, using screws or thin bolts in place of the rivets if you can't find replacement rivets locally. (Try industrial supply companies for the rivets and setting dies.) If the switch still doesn't work, you're no worse off than you were before, but there's a chance it will. And it may just be the little kid in us, but it's fun to see what makes them tick!