A little modern technology can be good for old cars, but you've got to know how to integrate and use it properly. That's a lesson we learned recently when we replaced the standard-issue 1157 taillight bulbs on one of our rides with new-fangled LED-or Light Emitting Diode-bulbs. With LEDs, we reasoned, we'd get brighter, longer-lasting taillights without the heat of high-intensity bulbs. Several benefits in one-how could we lose?

Everything was going fine until we got the LEDs installed and went to test the turn signals in the driveway. Nothing-no blinking, no flashing, no nothin'. Perplexed, we retraced our steps, double-checked our connections, and tried it again. Still nothing. Time to call in the experts.

Standard bulbs vs. LedsOur quest led us to Greg Schlup at Hotronics, who sat us down for a little lesson. It seems our problem was a common one rooted in the difference between incandescent light bulbs and LEDs.

Incandescent light bulbs generate light with a filament-actually two of them in an 1157 bulb (one for the taillight, another for the brake light). Light is generated when electricity flows across and heats up the tungsten filament. By contrast, LEDs have no filament. They produce an electrical arc that jumps across a gap. A reflector behind the arc directs and focuses the light, which is what we see glow.

In addition to operating on different principles, LEDs are much more efficient than incandescent bulbs. They're so efficient, in fact, that they don't draw enough amperage to activate the typical metal canister automotive flashers. That explained the sudden lack of blinkers on our car.

The FlashAs part of our lesson, Schlup explained flasher fundamentals, too. Standard automotive flashers, such as the 552-type used on many cars, work in a similar fashion to the old mechanical voltage regulators from the '50s and '60s. When you activate the turn signal, electricity flows through the flasher across a thin bimetallic strip. As the strip heats up, it bends away from its contact, cutting off the electric flow and making the lights blink off. The strip then cools off and makes contact again, completing the circuit that turns the lights on, and the process begins anew. It's repeated dozens of times per minute, providing the blinking lights we use to signal turns.

Standard flasher function requires a certain amount of electrical load to properly heat that metallic strip. LED lights, however, require so few amps that the electrical load typically won't activate a normal flasher. There's a chance it'll still work if you keep incandescent bulbs in your front turn signals and only use LEDs in back, but if there are LEDs at all four corners, forget it.

Rodders have come up with several solutions for making LEDs function as flashers. One is to wire in a load-equalizing-type shorting resistor, a method Schlup does not recommend, likening it to connecting jumper cables to a battery and touching the other ends together. Such resistors, Schlup says, "short circuit the electrical system, drag the voltage low, create extreme amounts of heat, and have set fires and melted wiring."

A better option, Schlup says, is to use a no-load flasher such as the PG-3 unit sold by his company. Unlike traditional flashers, the PG-3 is a solid-state, 100-percent electronic device that has a built-in relay for switching independent loads. It only needs a very small electrical load to recognize a complete circuit and start the flashing process. Thus, it can be activated with something as small as an incandescent dash indicator bulb or small motorcycle signal lights like those many rodders use on the fronts of their cars. Once it's on, the unit's internal relay pumps out 5 amps at full system voltage to the LEDs and carries current through the front, rear, and dash bulb directional circuits. In other words, your LED lights get full system voltage, so they're as bright as possible.