2019 Issue-2
My brother has an old MG that he likes tinkering with. He wants to use the side lights to indicate when the turn signal is on for drivers that are on either side of him. Normally, the side lights turn on and off with the headlights.
In his case — and to reduce peak power consumption to the blinker circuit — he would like the side lights to turn on when the signals are off, and vice versa (when the turn signal switch is turned on). Additionally, if the lights are on, the side lights should give priority to the turn signal, turning on and off, opposite to the signals in the front and rear, and returning to full on when the turn signal is not engaged.
I think the simplest way to do this would be with solid-state relays and the use of some logic gates (for each side). I have built hundreds of logic circuits, but have not really dabbled much in automotive applications. I know the electronics have their own temperature and performance specs and are noisy environments, not to mention the notoriously bad reputation that English cars have for electrics. Can you recommend a circuit for this application? (This car is a NEGATIVE chassis).
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This is what we do in the DeLorean community for a similar effect. We also have a negative chassis but our side markers are not grounded to the chassis, but rather through their own separate wire. I am not sure how an MG does this. If the MG side markers are grounded through the chassis, perhaps you could find a suitable socket with two separate wires. Maybe also switching to an LED if wanted/needed.
Simply disconnect the ground wire of the side marker light and connect it to the positive side of the parking lights.
When the parking lights are off, the side marker will flash in sync with the turn signal lights, using the parking light bulb as a ground path.
When the parking lights are on the side marker will light up, using the turn signal bulb as a ground path and blink off, ie. out of sync, with the turn signal.
Solid state relays are not ideal for this application. Automotive relays are cheaper, handle more current and are designed for the environment.
A 4 pack of automotive relays including the wiring harness from Amazon (for $9.97) will do what you want: https://www.amazon.com/Pack-EPAuto-Relay-Harness-Bosch/dp/B072QXDZRD
The diagram shows the wiring for the left side only. You need to duplicate it for the right side. RY1, when energized by the turn switch, gives priority to the turn signal. RY2 allows the side lights to light when the turn switch is on and the turn bulb is off. When the blinker turns the turn bulb on, the side light turns off, and vice-versa.
Add a 1N400x diode across each relay coil as shown in the diagram.
I have wired many cars this way as car makers also do this on some models.
The side light can not be grounded. I have replaced the sockets in the side makers to a 194 bulb socket available at all auto parts stores.
In the day time, the side lights alternate blinking with the turn signals and ground through the filament of the park bulb.
At night, the side lights come on with the running light, flash opposite the turn lights, and ground through the turn light filament. (when park and turn are both on you power on both sides of the side light).
You can see this on many cars and trucks at night when turning, if you are observing things at night. I have also done this at rear of car when possible.
Only a special part may be a side light socket. If it’s a bayonet type socket, it has to change to a 194 insulated socket that is readily available.
My 1988 Dakota has side lights like that. They’re just 194 type bulbs (you could use something like a 721 if you wanted really bright), connected across the turn signal hot and the parking light hot.
When the parking lights are off, you turn the turn signal on, the current flows through this bulb, then through the parking light filament, and it lights up. When the parking lights are on, and the turn signal is off, current flows from the parking light lead through the turn signal filament and the bulb lights up. When the parking lights are on, AND you have the turn signal on, both leads are at the same potential, and the light turns off. This will be complicated by LEDs, but for incandescents, it’s bog cheap, bog simple, and does what you’re asking for.
