How to wire the sidelights to also act as turn signals

Last update: 6/30/08


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Background

Normally a Locost's front turn signals are partially blocked for viewing from the sides by the bezel hardware. I mounted a pair of 1995(?) Nissan amber sidelights just in front of the foot wells on either side of the car. In studying an old American car wiring harness I found out how the manufacturer wired the front sidelights to also act as turn signal indicators.



Caution
  1. The sidelight HAS to have both leads to the bulb isolated from ground and available to do this modification. Your sidelight can NOT be wired as a turn signal and a running light using this method if it has only one lead with one lead of the bulb grounded. The Nissan and most newer cars do have isolated grounds on the bulbs. So check out the sidelight you plan on using -before- you wire it up.
  2. The main turn signal lights DO have to have one lead of each bulb grounded.  Normally turn signal lights are wired with one lead grounded.
  3. The sidelight bulb has to be a bulb that draws less than 3 watts.(See theory [below] why this is necessary.)  The Nissan sidelight uses a 2 c.p. #194 bayonet bulb.


How to wire the light

Wire one lead of the sidelight bulb to the "hot" parking light lead and the other lead from the sidelight is connected to either the left or right turn signal "hot" lead depending on which side of the car you are wiring at the time. That's it!



Theory

The left and right turn signal circuits use hi wattage bulbs to give a bright turn signal illumination. Normal "parking" lights use bulbs that are generally 4 watts or more each so they can't be wired as turn signals using this method. The [normally amber] sidelights use a rather small wattage bulb in comparison to the turn signal bulbs on the circuit. By wiring the #194 sidelight bulb as described above, the sidelight bulb can be illuminated by the active circuit (either turn or parking light) and uses the turn signal circuit to "ground" the #194 bulb through the unlit turn signal circuitry.



Examples

Lets assume you turn the parking lights ON and at that time both the left and right turn signals are OFF. The sidelights see 12 volts on one lead of each bulb. At the same time the -other- lead of each sidelight is connected to their respective turn signal circuits that are not energized. The bulbs of the turn signals present a low resistance path to ground. This causes the current through sidelight bulb to illuminate through the turn signal bulb grounds. The turn signal bulbs don't light because the sidelight bulb can't pass enough current to allow them to light but the turn signals can pass enough current for the sidelight bulbs to light.

Now let's assume you have the parking lights ON and you now turn ON your right turn signal. When the right turn signal circuit has 12 volts on it to flash the turn signal bulbs the right sidelight will have 12 volts on BOTH leads. This will cause the right sidelight to flash OFF because there is no voltage drop possible across the bulb. The left sidelight is unaffected by the right side flasher and stays ON.  The same action applies when the left turn signal is used.

Now let's assume you have the parking lights OFF, and you turn ON one of the turn signals. When the turn signal circuit has 12 volts on it to flash the signal the same 12 volts will be on one of the sidelight wires and the other wire of that sidelight is grounded through the parking light circuitry.

The interesting thing about this circuit is that when the parking lights are OFF the sidelights flash in cadence with the turn signals. When the parking lights are ON the sidelights flash ON when the turn signal lamps flash OFF!


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