GMC ’98 Sierra
CK Series Truck Tail Light Upgrades
By Gawain Tomlinson
San Diego , CA

Disclaimer: Any use of the information contained in this document is at the user’s own risk. The author does not assume any liability for actual or consequential damages.

The GMC 1998 vintage trucks have badly engineered tail lights. The 3157 lamps are too high wattage for the tail light circuit board lamp sockets, resulting in burned up sockets and circuit boards.

Dorman makes a set of replacement tail light circuit boards, available at most auto parts stores for about $30, PN 70001 that fixes the problem. It is an identical parts replacement resulting in a temporary fix.

I decided to replace the 3157 lamps with LEDs to permanently fix the problem. At the same time I decided to install an off-road LED utility light in parallel with the backup lights, since I couldn’t see when backing the truck with a shell in a dark parking lot.

I bought a couple of 3157 compatible Sino-Chino LED lamps on EBay for less than $4.00. I plugged then into the brake/turn lamp position. I watched as something on the inside of the lamp caught fire. After the fire went out, the brake/turn LEDs still worked, but the tail light LEDs were toast. Another problem was that the brake/turn LEDs were completely inadequate, being only slightly brighter than the tail light filament on a 3157 bulb. The LED lamps also caused hyper-flash of the turn signals, which I expected.

Murphy’s Law is true. Nothing is as easy as it looks. If anything can go wrong, it will, at the worst possible moment.

After some further research, I discovered that GMC in all its wisdom wired the sockets in the “CK” configuration instead of the “Standard” configuration. “Standard” wiring puts both grounds at one end of the socket. “CK” wiring puts grounds on one side of the socket, or at both ends of the socket (depending on polarity). My LEDs were “Standard” wired so they caught fire.

I now had three problems:
1) Find LEDs that are bright enough to be truly compatible with 3157 bulbs,
2) Find “CK” compatible LEDs for the brake/turn lamps,
3) Find load resistors to fix the hyper-flash problem.

My son recommended DDM Tuning, http://www.ddmtuning.com/ . I found most of what I needed there at very competitive prices:
(2) Load Resistors for LED Bulb 6ohm 50W $4.21 each
(2) 3156/3157 LED Bulb Standard wired HP18 Red $5.99 each
(2) 3156/3157 LED Bulb Standard wired HP18 White $5.99 each

I still needed two CK wired LED bulbs for the brake/turn lamp position. I found those at http://autolumination.com/3157_3156.htm :
(2) 3157 Specially Wired 20 LEDs SRCK Red $14.99 each
I got them in and tested them on the bench with a 9V battery. They were wired correctly, but the tail light connections seemed awfully bright. On the bench, I couldn’t tell the difference between the tail light intensity and the brake/turn intensity.

I put them in the truck. The brake/turn signals worked fine, and the intensity was good. However the tail light LEDs in those bulbs did not come on at all. At least there was no fire. I decided to install them anyway, and think about the problem, since I had taillights on the upper bulbs anyway with the DDM LEDs.

After some consideration, I decided that I could make the brake/turn LEDs do double duty as tail lights, if I put in a dimmer circuit powered by the tail light circuit. A couple of blocking diodes would prevent cross connection between my brake/turn circuit and the tail light circuit. When the brake/tail lights were on it would override the low intensity driver. An advantage of doing this would be that in the future I could use “Standard” wired LEDs by just clipping of the inner leads on “Standard” wired 3157 compatible LEDs, such as DDMs.

One way to accomplish a LED dimming function is to simply wire a properly sized dropping resistor in series. This has disadvantages due to variation in LED specs, and some bulbs have built in voltage regulators which could cause unknown results.

The reliable way to dim LEDs is a Pulse-Width-Modulator (PWM). This flashes the LED on and off at a rate that is to fast for the human eye to detect. The intensity depends on the on/off proportion. This is very reliable, and is not dependent on LED specs.

I considered building my own circuit using a 555 Timer IC and a driver. But that is a pain, requiring circuit boards, enclosures, heat sinks, etc., and takes a lot of time before it’s done.

Then an Internet search gave me an off-the-shelf solution on EBay:
http://www.optimalighting.com/optima-lighting-1-channel-dimmer.html ,
OPTIMA LIGHTING 1 Channel 12V DC 8 amp LED Dimmer $12.99 w/ shipping .
I was unable to determine if the dimmer would work with both the inpu...

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