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My 86 has side turning lights that turn on and stay on when the headlights and turn signal lights for that side of the car are on. It is a fairly bright light that turns off as soon as the turn signal light shuts off. (I hope that explains what light I am talking about) The problem is the right light works fine but the left light stopped working. I put a meter on the wire w/o a bulb and there is 12 volts there. But with the bulb in the voltage drops to “0”. Thinking it might be a grounding problem, I traced the wire, orange I think, and with the meter attached to ground I had 12v. So I spliced in a bulb and when I grounded the bulb to the frame the voltage dropped to “0”. Is there a relay somewhere that works this light or is there a switch somewhere that needs replaced? I tried to follow the circuit in the FSM but it makes no sense to me. Any advice will be appreciated.
The front marker lights go to the headlight switch (brown wire in my 87) and the other wire (blue) goes to the adjacent turn signal filament (turn/parking light has two filaments). The marker light goes out alternating with the turn signal (when activated) because when the turn signal is on, 12v is on both wires to the marker light (sum is 0 volts). With the turn signal off and the headlights on, the marker light gets 12v on the brn wire and very close to 0 volts on the blue wire because the blue wire goes to the turn signal filament which is grounded. The turn signal filament requires much higher current and will not light with only the marker light current flowing through it. Look on your service manual (exterior lights turn/hazard/stop/park/front marker), you will see that the marker goes to the headlight switch and to the turn signal lamp.
The light I am referring to shines to the side of the car, (cornering light) lighting the area you are turning into. The light on the right side of the car stays on until the turn signal clicks off. The left side was working but has since stopped.
Assuming this area has not changed much over the years (mine is 1996) there is no relay. The light is turned on directly by the turn signal switch. There is a procedure listed in the service manual for troubleshooting one or both cornering lights not working. Hopefully someone will chime in with the information for your 86.
In general your problem sounds like a bad connection. Open circuit, there is no current draw and your meter reads battery voltage. When you complete the circuit by trying to power the bulb the voltage is all lost across the bad connection. The trick is to find the bad connection. It could be anywhere from the switch itself to the bulb socket.
WOW, I missed that page in the FSM. But this makes sense; I had the steering wheel lock pin spring break a few weeks ago. I fixed the broken spring but must have damaged the switch or wire because it only worked for a short while. Thanks again, I'll check it out.