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LED Gauge Panel Backlighting Upgrade

Swamp Donkey

The Engineer
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As promised, here's a thread on upgrading the gauge back lights with drop in LED bulbs and lenses, without modification to the original wiring or bulb sockets. While I'm working with a M939 vehicle here, other M series vehicles that use the same bulbs sockets and lens can also benefit from this. Taking pictures of lights in the dark doesn't work well so you'll just have to take my word on some of this.

To start, we need a baseline for comparisons. All of my gauges are new with perfectly clear cases in the back and perfect lenses in the front. Basically, the best possible scenerio for lighting the gauges. If you have not cleaned the back casing and windows for the gauges, this can drastically affect how much light enters the gauge. I had swapped the red lenses out for some green ones, since green lit the gauges better with incandescent bulbs, so that is what I'll be comparing against.

20161001_221611.jpg

The military supplied an upgrade kit for HMMWVs, that included an LED bulb and new lens, in an attempt to remedy the problem of swollen lens caused by the heat of incandescent bulbs. This kit is referenced in PM Magazine and all the parts numbers are included in the screenshot below.

Screenshot_2016-10-02-00-26-47.jpg

You'll need (10) 1/4" spacers and (10) 8-32 tpi x 1/2" long screws for the sockets. After the spacers are installed, the bulb of choice can be used and the new lens is screwed in normally. The seal for the lens with fit as intended against the panel to keep moisture and dust out.

This is what the kit includes. You can also just get the lens or just the bulb.

20161001_033340.jpg

This kit will work with other vehicles, but the lens is longer than the standard lens, so the socket will need to be spaced back from the panel to accommodate this. The longer lens is the key here as it gives you a larger area to let light through to the gauges.

The problem with using LED bulbs with the stock lens is that by design, the LED diodes aren't in the small area where light can exit the lens. There isn't enough "spill" from a diode, since they are so directional, to make a useful amount of light either. The longer lens puts the diodes of the bulbs in the area where useful light can make it out. Here's a comparison shot of old vs. new.

20161002_140333.jpg

The new lens is exactly 1/4" longer, so that gives light 1/4" more room to exit. To accommodate this extra 1/4", I used spacers and longer screws between the back of the panel and bulb sockets to take up the extra room. Without the spacers, the lens just stick out the front of the panel and does nothing to help on the back of the panel. This is the modification part of this: screws that are 1/4" longer and 1/4" spacers. That's all that will need to be done to make the new lenses work. A comparison of a socket without spacers on the left and one with spacers on the right.

20161001_222655.jpg

In this picture you can see the spacers. Don't mind the bulb I had in there. I was doing trial and error, and there was more error with this style bulb. More on that later.

20161001_223815.jpg
 
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Swamp Donkey

The Engineer
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So now that we can effectively use LEDS to light the gauges, we need to choose some bulbs. The design of the bulb you use does matter and I'll explain why below.

Heat, heat, heat. It's the number one killer of LEDs. While the diodes used in smaller bulbs might not produce enough heat to feel, they do produce heat, and it must be pulled away from the diode or the diode will fail quickly. This can be done with heatsinks or, in the case of miniature bulbs and such, the air space inside the light is enough.

The problem with the lens we're using is that it is so small and the bulb takes up most of the air space. This was an issue with the incandescent bulbs and caused swelling of the lens. A miniture bulb that uses higher powered diodes might not swell the lens but will fail quickly instead.

Here's some bulbs and why they are good or bad. All of them use a BA9S base and are 24V.

This is the bulb that comes in the kit. It uses 18 tiny diodes and produces little heat. They used a high quantity of low powered diodes to manage heat. The problem I have with this bulb is that it's more of a yellow green than a true green. Light output is more than incandescents but the color can make this a moot point when it comes to how well you can see the gauges. The color does matter when it comes to visibility.

20161002_140750.jpg

20161002_140814.jpg

Don't trust the camera here. They are more yellow than green and not as bright as they appear.

20161001_234456.jpg



These are some no name bulbs from Ebay. They use 5 high power 5050 SMD LEDs. They create a great amount of light and the light is in true color as opposed to the yellowish green of the one above. High power LEDs do not belong in small fixtures though. These use air space instead of heatsinks to keep the diodes cool. Given the lack of airspace inside the lens, they heated up quickly. The first diode failed in about 3 minutes and the first bulb failed completely in about 10 minutes. The lenses were warm to the touch.

20161002_140703.jpg

Here's some red for the purists. :mrgreen:
Really bright but too much heat. Stay away from bulbs described as "super bright", "high power" or as using SMD diodes.

20161001_232810.jpg

20161001_232319.jpg



These are LED bulbs used in some of the upgraded warning lights behind the steering wheel with the clear lenses. These are not bright enough to be useful here, plain and simple.

20161002_140734.jpg



These are what I went with. I had to order some more because I didn't have enough to do the whole gauge cluster, so I don't have a good comparison shot from the baseline above. These are made by Alpinetech, who are based out of California and make aviation lighting and control switches. The bulbs are available on Ebay for very reasonable prices. They use a single LED and a frosted lens to distribute light. Heat is not an issue with these since they were designed for small places. These give off more light than the original bulb in the kit and the colors are true. I'll be using the green since it is more easily seen and has better color rendition, as opposed to red.

20161002_140721.jpg

I only had 3 bulbs in here. This is the route I'll he taking.

20161001_223636.jpg



This is probably way more information than the normal person would need. I rarely invest in new technology, instead tending to lean to the "tried and true". LED technology is something I've completely embraced though, on my truck and life in general. These 5 lights were the last of the incandescent bulbs on my truck. I find LEDs to be interesting and extremely useful, especially given the fact that when you get down to some of the very basic concepts on how LED diodes operate, you start pushing into the realm of quantum mechanics and theories. And who doesn't like quantum mechanics? :mrgreen:

Maybe my trial and error will spare somebody the same. Hope this helps.
 
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Swamp Donkey

The Engineer
Steel Soldiers Supporter
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Location
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The finished product...

Stock incandescent bulbs with green lenses.

20161001_221611.jpg

Alpinetech green LED bulbs with the long clear lenses.

20161006_225443.jpg

All the gauges are now able to be accurately read with just a glance. No more squinting or leaning forward while driving.
 

therooster2001

Active member
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Search the NSN, they come right up. $20 a piece. I also found them for $14.50 each elsewhere. Again, search the NSN in the PM Mag.

I tried to give you the Ebay link via PM, but you're full.
 
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Swamp Donkey

The Engineer
Steel Soldiers Supporter
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63
Location
Gray, GA
Give me a couple days Jeff. I've had several requests to put together a complete kit with everything needed to do this. This was just a project of mine I wanted to share and I hadn't planned on doing kits, but I might entertain the idea if I can work everything out. I've sourced a large quantity of these lenses and am currently in negotiations for them.

In the mean time it would be good to see what kind of interest there is for complete kits. There is nothing for sale now and no guarantee there will be until I secure the lenses. I don't see a problem doing so though. Kit would include lenses, bulbs, spacers and stainless screws. No need for commitments, just interest. I hope to have everything worked out in the next couple days.
 
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