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Switchbacks - an Easier Way

Note:  This article was originally written as a post to the gm-volt.com forum.  It is both a description of what I did, and a tutorial to show others how to do the same thing.  Although I re-wrote portions of the article, there are still some of the sections where I am clearly instructing others how to do it.

I recently replaced my front turn signal / running lights on the Volt with switchback LED bulbs. I had to look up "switchbacks" when I first heard the term.  They are a form of a combined running light/turn signal LED bulb with built in logic.  The logic turns off the running light when the turn light is flashing.  There are two types of this logic.  The simpler merely turns off the running light during the time the turn light is on.  The result is that during a turn the light is alternating between white (running) and amber (turn).  The other logic form turns off the running light during the entire period of time the turn is flashing.  This results in the light being dark between flashes of the turn.

Changing light bulbs on modern cars is nothing like in the old days!  You used to be able to reach behind the bulb, twist the socket out of its mount, change the light bulb, and be done in a minute or so.  Now the bulbs are buried deep withing the structure of the car and bulbs are usually considered a dealer task to change them, often requiring body parts to be removed for access.

There are a number of threads on the GM-Volt forum describing changing the front turn/running bulbs, which basically describe two techniques. The driver's side is not bad for access, but the passenger's side has the charger in the way, and you have to make enough room to work around it. The first of the two primary methods I have seen are to pull the screws holding the fender liner then disconnect the right side of the bumper and pull it forward. The other is to have an assistant hold the fender liner after the screws have been removed, then work one handed in the limited space available. As I am basically lazy, and I don't have easy access to an assistant, I figured there must be an easier way.

There actually are two problems to solve: 1. Get access to change the bulb. and 2. Install the load resistors to prevent hyperflashing. Hyperflashing is the rapid blinking of a turn signal when a bulb is burned out.  It is required by federal regulations.  The flashing circuit senses the load of the bulbs, and if it is too low, hyperflashing results.  As the LED bulbs draw much less current than incandescents, they will hyperflash if the load is not built back up the the old level.  Resistors are used for this, usually 6 ohm 50 watt.

To change the bulb, I pulled the fender liner screws and several screws holding the short air dam to be able to move the area just under the bulb about 1/2 inch. I made a tool from a length of 3/4 PVC pipe with a slot in the end just over 5/8 wide and about 1 1/4 inches long. Using this tool I went up from under the car, (which I had driven up on blocks) and pressed the slot of the tool over the end of the light socket. It was then easy to twist the tool to remove the light and its socket from the car and let it drop down through the opening behind the bumper.

On the left you can see the tool on the light socket. The right shows the socket hanging down and the LED bulb installed.

I had some trouble trying to reverse the operation, as the tension of the wire kept pulling the socket out of alignment. I then removed the socket from the harness (it is attached with a connector), used the tool to replace the bulb and socket into the housing, then I used the tool to plug the harness back into the socket without any problem. I replaced a few screws and the passenger side was done!

A word of caution: Be sure and try the light before replacing it in the housing. These bulbs are polarity sensitive and will only work in one orientation. If it doesn't light, turn it around in the socket.

The bulbs I used are from Super Bright LEDs. I used them since I know they make great products, they have the correct type of bulb for our vertical orientation (no lens pointing up), they are relatively short for our limited space, and their bulbs totally blank the white light while the amber is flashing. Some brands alternate white and amber while flashing and don't show up nearly as well.

This is how the new bulb looks on the passenger side.

About the resistors: The wires for both sides of these lights pass through a connector on the Driver's side. Since there is much better access on this side, I decided to locate both resistors there.

Once an engineer, always an engineer! I was worried about the temperatures the resistors might reach under sustained use of either the turn signals, or more likely for long periods of the emergency flashers, so I did some experimenting. I mounted the resistors on a heat sink I had. It was 4 1/2 by 4 3/4 with lots of fins. I hooked one of the resistors to a 10 volt supply which simulated 50% power like a flashing light would average. I recorded the temperature at intervals up to 15 minutes and plotted them. I then did the same for the resistor mounted to the heatsink using thermal compound. I then tried it with the resistor hanging in air with no heatsink as the vendors often tell you to do. After pulling the driver's side fender liner back and checking out my actual available space, I made a heatsink of 3/16 aluminum about 3 3/8 square, I also ran a temperature graph of this.

As you can see below, the resistor really gets hot hanging in mid air, but any of the heatsink combinations keep it relatively cool.

No comments please about the fact that I only energized one resistor. I know it will get hotter with both energized during emergency flasher operation, but my test was really to show if a heat sink would make much difference, and it obviously does.

I discovered when working on the driver's side that I actually had more room to work with the wheels turned to the left. When they are turned to the right the tire limits how far you can move the liner. Of course removing the tire would give you the best of both worlds, but as I said, I am lazy.

I decided to mount this smaller heatsink on the metal structure just above the horn. There is adequate clearance from any other parts to avoid any damage from the heat - even though it is much lower in temperature, it is still hot!

I then had to tap into the harness coming from the 6 pin connector. On my MY2013, the right turn was a light green/violet wire, the left turn light blue/white, and the ground, black.  I opted for Posi-taps, which are expensive, hard to find, and reliable! They make one that is rated for the approximately 20 ga wire used in this location.  These worked well, and in my opinion are much more reliable than the typical red Scotch-locks we all use.

This shows the taps into the harness wires on the left, and the cleaned up version on the right, which also shows the mounted resistors.

I am very pleased with the final results: The switchbacks  give a little extra close illumination at night and are a little more distinctive when showing a turn signal, but most importantly, they look cool!

R. S. Mason 1/8/17