Classic Mustang's Overloaded Headlight Switches - Death To Blinky

Horse Sense:
Overloaded headlight switches are not uncommon. Even vintage Mustangs are known for their faulty headlight switches. In the case of '87-'93 Mustangs, you can solve the problem by either repairing the circuit following the instructions in TSB 89-17-11, or you can hard-wire your foglights to an aftermarket relay.

You're cruising home in your '88 GT, and you've decided to take that nice, winding back road and stretch the car's legs a bit to relieve some of the day's trials and tribulations. Your only concern is that it's getting late and darker by the minute. No problem-all it takes is a flick of the foglamp switch to throw some much-needed additional light down the road. You're hitting the apexes and grinning from ear to ear when all of a sudden your world goes dark. If it weren't for the distant moonlight and an occasional street lamp, you wouldn't have a clue where to turn. As you reel in your GT and look for a place to pull over, your GT's lights come back on. What just happened?

What you just experienced was your headlight switch's circuit breaker kicking in to stop a current overload in the system. Much like a circuit breaker in your home, the breaker is tripped when the current flow through it exceeds the safe load of the circuit. And, as at home, it's better to have a circuit breaker than a fuse that can blow, as a simple reset is all you need to get back to business. The circuit breaker in the headlamp switch goes one step further and is self-resetting.

So what causes this problem in the first place? The current overload comes from the foglight circuit tying into the main headlamp and multifunction switches in your GT. The circuit wiring is too small to carry the heavy loads of the additional lights. Factor in that many people often increase their foglamp-bulb wattage and you can see how there'd be a problem. Oftentimes you can touch the gauge-cluster hood next to the headlight switch and practically burn your hand! Ford had a fix for the problem in Technical Service Bulletin 89-17-11 and then fixed the problem on the assembly line for '90-and-newer Mustangs by installing a heavier-gauge wire in the circuit between the headlight switch and the multifunction switch.

Depending upon the damage inflicted, you may need to purchase a new headlight switch, a multifunction switch, and matching harness connectors and terminals to fix any existing heat damage. Even though the '90-and-later models have this heavier-gauge wiring preinstalled, it is still best to bypass the factory headlight switch and its integral circuit breaker for a stand-alone relay design. With some simple wiring and a standard automotive relay, you can ensure your lights will not overtax the headlight switch anymore. The results of this modification will allow you to run your driving/foglights independently of whether the headlights are on or not, just as many GM models do.

While we are rewiring our '87 GT for safer foglight operation, we also decided to replace our damaged foglights with new KS Reproduction units. These lamps feature a smoked plastic lens and have a chrome reflector to match our previously installed smoke headlamps. We ordered the set from AGE Classic Design. AGE has several foglamp options from which to choose, including clear and smoke lenses, glass or plastic, and direct plug-in or "cut-and-splice" models for most any year and model Mustang. Read on to see how we fixed "Blinky" and made our GT safer.

Relay Road Map
The relay used in this article is a standard 30-amp automotive relay. You can find these relays at Radio Shack or just about any auto parts or hardware store. Check the electrical section of the store as sometimes they are packaged as high-current relays for lights and stereos.A relay is used as a switch by using a low-current load to pull the relay's windings closed, which allows the high current to flow through the other terminals of the relay. The use of a relay in this repair and upgrade takes the high-current foglamp feed circuit and removes it from the headlight, multifunction, and foglamp switches, allowing the relay to carry it. Wiring the relay the way we did puts the low-current side of the relay on switched 12 volts, meaning the foglights will shut off when you turn off the key. This is a good thing as the headlight-warning chime is not part of the foglight circuit and the low mounting of the foglights means sometimes you don't see they are on. You can easily use the factory foglamp switch as we did to control the circuit. The low-current side of the relay is grounded through this switch. Push the foglamp switch, and, providing the key is on, the relay is grounded and the foglamps will illuminate.