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L320 LED Taillight Mod


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I have a 65 Datsun L320 pickup that uses a separate wire for the Tail Light, Turn Signal Light and Stop Light. It uses a dual filament 1157 bulb for the Stop and Turn signal, and an 1156 single filament bulb for the Tail Light, and I wanted to convert it to LED tail lights for brightness and reliability. (I should mention that mine is already converted to Negative Ground!) This write-up really applies to any vehicle that uses separate wires for stop/turn/tail where the owner wants to use LEDs with only two wires, so maybe it should be posted in some other forum for more general interest?


The LED lights that I am retrofitting are such that the Stop Light and Turn Signal Lights are combined on one wire (RED) and the Tail Light is by itself on one wire (BLACK). There is a third ground wire (WHITE) that has to be grounded to the vehicle chassis. These are actually made for trailers, but I am inserting them inside the tail light housings and behind the stock taillight lenses on my vehicle. I could easily have used a Dremel tool to cut off the red plastic lens from the LED lights if it turned out they were not bright enough under the stock lenses, but they were OK as is. Outward appearance will still be the stock housing, with illumination provided by LEDs.


The Flasher

The first step was to buy an electronic flasher unit that uses a recycling time delay relay internally. The stock flasher is thermally operated. When the circuit between the flasher and the turn signal light is completed, a bimetal strip inside the flasher heats up and bends, opening a set of contacts and the light goes out. When it cools back down, it reestablishes contact, and the light illuminates. As long as the circuit is complete, the light will alternate between the two states. The problem with using the stock flasher with LED lights is that the load of the LED bulbs is not enough to trigger the flasher in most cases, or if it does, the load is so minimal that the flasher cycles very quickly, giving you an abnormal extremely fast blink rate.


The electronic flasher I got is a two pin model, since my stock setup uses a two pin flasher. The flasher unit has power provided to it all the time, and the power is switched through the turn signal switch in the steering column to the center OFF position, or to either the left side or right side light circuit, depending on which way you want to turn. The electronic flasher actually contains a small relay along with a capacitor, resistors, a diode and a transistor arranged to provide a consistent time delay between each cycle.




The main issue that has to be addressed is the combined Brake and Turn Signal wire. Here are the desired scenarios:

Brakes OFF You are not stepping on the brakes.........................Brakes ON You are stepping on the brakes

No signals active Both lights are OFF ......................................No signals active Both lights are ON

Left turn active Left light blinks while right light is OFF............. Left turn active Left light blinks while right light is ON

Right turn active Right light blinks while left light is OFF........... Right turn active Right light blinks while left light is ON


You can buy an off the shelf converter unit to adapt the existing 3 wire circuit (Separate Stop, Turn and Tail wires) to a 2 wire circuit (Combined Stop/Turn wire and Tail wire). The units are anywhere from $15 and up. They are potted in epoxy, so essentially are black boxes. If anything goes wrong, throw the whole thing away and buy another one. There is nothing to troubleshoot or repair.


Rather than buying the converter unit, I wanted to make something work out of stuff that I already have on hand. I have a number of ultra-common (and cheap) standard 12V SPDT automotive relays with sockets.



The circuit I used incorporates 3 relays, one for the Brake circuit, one for the Left Turn circuit and one for the Right Turn circuit. The good thing about this type of relay is that they are so common that they are freely available almost anywhere, whereas the converter units are a little less common. Plus, you can fix this if it conks out on you. If you think these are obsolete and old school, look under the hood of your new car, and you will see relays very similar to these, mainly for switching high power loads like lights, heated seats, fan motors, etc.


One handy aspect of the relays that I used is that the sockets hook together, so you can make up a small assembly of multiple relays to simplify mounting. The sockets came with the wire pigtails already installed, so wiring them up is made much easier. Strip the ends off, solder and heat shrink the connections. End of story.


Here are the three relays ganged together, labeled for Brake, Left and Right. This pic was taken just before installation, so all of the interconnections between relays are made, and all of the vehicle connections are labeled to make the installation as easy and error free as possible. I did bench test it before installation with a 12V power supply to make sure it worked as it should, and it did. I had to use a test lamp to simulate the front turn signal to provide enough load for even the electronic flasher to function. Without the test lamp, bench testing would not have worked. Ironically, it would have worked in the truck because the load from the dash indicator light and the front turn signal are there, but I might not have gotten that far if it failed the bench test.




Relay Functions

These are electromechanical relays. They use an electromagnetic coil to switch the power. (You could buy solid state relays that have no moving parts to switch the power, but they are a lot more expensive). When the coil is at rest, the relay is said to be de-energized. When power is supplied to the coil, the relay is energized, and it opens one contact while closing another. In its simplest form, when energized a relay will either turn something ON that was OFF, or turn something OFF that was ON, depending upon how it is wired. So relays only have two states to worry about, energized and de-energized.


The relays used in this circuit are SPDT (Single Pole, Double Throw). The Pole is the Common connection, or terminal #30. The Common © terminal is connected internally to the Normally Closed (NC) terminal (#87a) when the relay is in its normal state, or de-energized. When power is supplied to the coil, the relay energizes, and opens the internal connection between the C terminal and the NC terminal, while closing an internal connection between the C terminal and the NO terminal. This circuit uses the relay contacts more as a selector to route the signal among two different pathways.


The Circuit

The following schematic or ladder diagram is the one I used to make this circuit work for my application. This schematic represents the circuit condition when in its normal state. Brakes OFF, Turn Signals OFF. Note that the brake switch should be a pressure switch for my truck and the symbol would be different, but it was bypassed with a regular mechanical switch operated off the brake pedal by the previous owner.




Truth Table for Inputs and Outputs


Truth tables show the state of each input and output for all possible conditions. True = 1 and False = 0. In this case, possible states are ON, represented by a one; OFF, represented by a zero; and BLINKING, represented by a B.




Connections to the Relays

In my case, I was able to connect the LED lights to the existing turn signal wires, after I found the right spot to cut them. I was able to leave the front turn signal wiring in place and still use regular bulbs in those, so I only had to be concerned with the wires feeding the rear turn signals. You could run new wires if you wish, but that increases the complexity of the job. I do recommend installing new connectors if yours are old and corroded. I like to use crimp connectors, but remove the plastic sleeve and solder them for insurance. Not knowing for sure how well the crimp worked out with that plastic sleeve in the way is an issue for me. Heat shrink tubing seals the deal just as well and provides a little bit of protection from the elements. Just remember to put the heat shrink over the wire before you crimp on the connector.


You really need a good wiring diagram of your vehicle with color codes to make it easier to find the wires. The original brake light wires that used to run from the brake switch to the lights is no longer used, since the brake switch is now controlling a relay instead, so that wire was disconnected at both ends, but left in place in case I need to route some power to the rear of the vehicle in the future.


Here is a chart showing a summary of the relay connections. It worked for me the first time when I got it all connected up. The Ground goes to a good chassis ground. You can connect them together at the relay block, then run a single wire to ground to simplify the wiring. I used a self-tapping sheet metal screw to ensure a good ground.




Here is the relay block all pre-wired ready for installation. You can see how I tied the wires together. Do not wire yours by looking at this picture, draw out a wiring diagram and follow that instead.


The wire colors in this pic are what came with the pre-wired pigtails and have no relation to the table above or to the schematic diagram. The critical thing is that the wires attach to the proper terminals on the relay connectors.




Here you can see the diode that I mounted across each of the relay coils. The diodes inhibit the electrical spike that could backfeed into the electrical system from the coils. I pulled the connectors out by releasing them with a small screwdriver, then soldered the end of the diode lead to the spade connector, then slid it back into place in the relay socket. Make sure you get the polarity right on the diode if you want it to work.




The LED Tail Lights

The LED trailer lights were about $20 apiece. Wally world had similar ones cheaper ($15 each), but they only had one in stock, so I bit the bullet and paid a little more for them to get a matching pair. I believe they have 8 LEDs each.


This is similar to what I purchased. The rubber trim ring was not needed, so I threw it away and just used the light itself. You can get fancier ones with more LEDs and chrome trim rings if you want to spend the money on it.



My original lights were in pretty bad shape. The lenses were OK but the lamp sockets and wiring were pretty badly corroded and would need to be replaced anyway. Rather than go through all that, I simply used RTV silicone to adhere the LED trailer light to the back of the lens assembly. It can be reversed simply by cutting through the silicone with a razor knife, if I should ever want to return them to stock. The lens screws are no longer needed to attach the lens to the light, but I will put the screws in with a small dab of clear silicone to seal up the lens holes. The original metal light sockets are no longer needed.




I forgot to take a photo of the spacer that I made to fit the lens assembly, so a drawing will have to suffice to give you the idea. It was necessary to make sure the light was at the right depth in the housing. It is made of PVC lumber from Home Depot. I made a slot in the middle for the wires to pass through, and drilled a couple of holes in it and counterbored them with a spade bit so that the elevator bolts would be flush with the surface, then glued it to the back of the LED light. The protruding threads are spaced out to fit right into the stock light mounting locations in the taillight housings.




These are elevator bolts. They are used on garage doors so the heads of the fasteners will be flush on the outside of the door (and on elevators I guess).


Relay Block Mounting

This is the relay block as installed. I plan on adding some tie wraps and such to neaten it up as time permits. This is on the right kick panel just under the glove box.



Finished! Here are some pics of the lights working. They are bright, even in sunlight. Much brighter than the stock lights! On a side note, this truck has been painted 5 times it appears, after stripping some of the grey off on the tailgate. Looks like dark grey, lighter grey, white, blue and the original orange or whatever that color is supposed to be.


Running Lights Only



Running Lights with Right Turn Signal

(turn signal is same as brake light).



Brake Lights only



Left Turn Signal only



Hope yours comes out as well as mine did! Let me know if anybody has any thoughts or suggestions for improvement. The images are mostly copyrighted by me, so don't republish them elsewhere without permission. Thanks!

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  • 5 years later...

I'm thinking about adding these 3/4" mini round tail lights where the stock reflectors go:




Good idea, bad idea?


I'd like to add a little more of a signal on the ass end but want to keep the stock feel. 

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What is the website for your parts you purchased??? Sorry new to this stuff

The relays were from EBay, just standard SPDT 12V automotive relays.

The LED lights were from WalMart and the flasher unit was from AutoZone. The elevator bolts were from Lowes.

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