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 Wagon LED third brake light

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Posts : 401
Join date : 2009-01-06

PostSubject: Wagon LED third brake light   Wed Jan 29, 2014 12:31 pm

For my third and final installment of the week I will how to retrofit LED's to the glass mounted CHMSL. The reasoning behind this isn't just to have faster acting brake light action (though it has been shown the reduced time to bright does give following drivers more distance to work with) but to reduce the high current draw of the factory incandescent setup. Melted housings and premature failure of the brake light switch as the contacts burn out are symptoms of this condition.

There are many, many ways to convert the third brake light to LED lighting. Some will simply replace the factory bulbs with LED equivalents and call it done. Others will prefer to construct all new boards using the newest high intensity surface mounted devices. This method is a compromise between the two by re-purposing Ford third brake lights mounted on modified wagon internals.

When Cash For Clunkers was in full swing, our preferred yard was so overwhelmed with the C4C vehicles that the normal Chrysler / Ford / GM / Honda / Subaru / Toyota etc. sections morphed into a mix of “Set it down wherever you can find the room” assortment of cars and trucks. And it became obvious that the two most prolific being turned in were Explorers and Grand Cherokees. You couldn't swing a dead cat without hitting one. One Saturday while we were there on a quest for the usual B-body parts and climbing over my 19th Explorer to find them, it occurred to me that the third brake lights on the 98-01’s looked to be just about the right size to fit into a wagon third brake light housing.

After removing one and looking it over, I separated the Explorer housing and found a compact LED strip inside. After retrieving a handful more, I brought them home to see if they could be used in the wagon. I disassembled a wagon brake light and removed existing bulbs by breaking the mounting block off at the glued joint. (I find placing the bottom cover edge-down on the workbench and hitting the block with a mallet pops it right off.)

I wanted to use two Explorer strips if possible to completely fill the lens area with light. A small bracket made from aluminum angle worked to hold a pair of strips and locate them on the factory reflector / mounting bracket. I milled two reliefs on the lens side to match the strips, and then drilled & tapped two holes in the upper side for retaining screws. Holes drilled in the reflector / support bracket completed the machine work. If you wish to duplicate the setup, you could get pretty close with a just small piece of angled stock cut to size and the creative use of foam double-stick tape.

I then epoxied the LED strips to the angle bracket, and soldered some leads to the factory power strip.

The Explorer brake light assembly uses a 19 ohm resistor to reduce the voltage supplied to the LED strip, instead of reusing the resistor and trying to find room for a pair of them (remember we are using two complete LED strips) I just wired the pair in series and eliminated the resistors. It is slightly dimmer than using the original resistors, but it eliminates 2 components for very little loss of output.

I then cut a new lens from a scrap piece of faceted recessed lighting material, though new lenses aren't required as can be seen below.

Factory third brake light assembly:

Third brake light with outer housing removed:

Bulb support block (note damaged areas near light bulbs):

Close up of heat damage to bulb support block:

Factory reflector / mounting bracket (bottom view):

Aluminum angle bracket:

Explorer LED strips (pair):

Explorer LED strips epoxied to the angle bracket:

Molded brake light power strip:

Molded brake light power strip, with LED supply leads soldered on:

LED assembly installed into reflector / mounting bracket (front view):

LED assembly installed into reflector / mounting bracket (bottom view):

LED's installed in brake light housing:

Factory lens:

Clear lens cut from scrap recessed lighting material:

Completed LED conversion with clear lens:

Completed LED conversion energized with factory lens:

Completed LED conversion energized with clear lens:

Recently it was suggested to drill a hole in the front side of the brake light housing to be able to observe the light working in the rearview mirror. A clever suggestion and a useful upgrade. As I was gathering parts to take photos for this post, I found some parts that can be used to take it one step further: Instead of using RTV to create a lens, consider using one of these mini lenses that are sized to align a standard sized LED. About ¼” in diameter, they are available in several colors and simply pop into a drilled hole to add a factory look to the modification. The ones shown below were removed from a scrapped warning indicator, but they can be found new at most electronic supply retailers.

And something to watch for: When I first did this conversion I built two of them at the same time. I installed the first one and it worked as planned, but when the second one was installed in another wagon, it didn’t work. I knew they were wired the same as I had bench tested them prior to mounting them, so something else must be amiss.

What I found was that the contact polarity was reversed in the second wagon. Wanting to know why, I pulled the inner tailgate panel and found that the plunger connectors can be placed in either position. So you are at the mercy of the person who plugged them in on the assembly line as to what you have. Just swap the leads on the LED strips, or if you didn’t leave yourself enough wire length, open the panel and swap the connectors inside the gate.

One last note: Using two strips means they get crowded towards the back of the lens. While it doesn’t cause any issues, it does make for a blotchier appearance as the individual LED’s can be recognized. I haven’t done this yet, but the next one I may try using just one strip and mounting it further back in the housing. Increasing the distance between the LED strip and lens should even out the light pattern, and lining the reflector with chrome tape should maintain the brightness. Just don’t forget to include the resistor if you try this, as the LED's won’t live long without it.


- J

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Posts : 2052
Join date : 2009-05-28
Location : Huntsville, AL

PostSubject: Re: Wagon LED third brake light   Wed Jan 29, 2014 3:10 pm

I recently tested my reaction time at a science museum - it took me an average of 200 ms from the time an LED lit to the time I pressed the trigger. And that was with full concentration on the light. Given how easily totaled these wagons are in a rear hit, installing a bright LED CHMSL is cheap insurance.
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phantom 309


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PostSubject: Re: Wagon LED third brake light   Wed Jan 29, 2014 8:53 pm

A nice set of LED tail lights like an escalade has would be nice.
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Age : 42
Location : Sandown, NH USA

PostSubject: Re: Wagon LED third brake light   Wed Jan 29, 2014 9:38 pm

Pretty cool. Thanks for the write-up, Jay. Smile

I've yet to have a wagon where I can consistantly get that chmsl to work. I clean and sand the contacts all the time, on both sides, and it will work. Sometimes. Very frustrating.
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PostSubject: Re: Wagon LED third brake light   Wed Jan 29, 2014 11:14 pm

Nice. Going to grab some of these strips tomorrow.
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