Better Dash Lighting with LEDs

Better Dash Lighting with LEDs

The W123 chassis cars share what appears to be a common trait: Dim instrument lighting. As in squint and concentrate to read most any gauge at most any time- and forget about reading the odometer when moving. You could be in the next town before getting a good read and have a headache to boot! Ooops, officer, I didn't see that light....I was trying to read my speedometer! After dealing with this for too long I finally broke down and decided to try and do something.

Some weeks (OK, months ago) I bought an old 240D instrument cluster to experiment with. The goal of the project was to improve lighting for the cluster in my car without causing more long term injury to the cluster, as has been reported with some who have replaced the original 3W bulbs with 5 watt versions.

In this quest, I bought some U.S. number 194 bulbs as well as some 'white' LEDs; I already had the materials needed to thoroughly clean the instrument faces, light guide ends and interiors of the 'light boxes' that the bulbs are set into.

White LEDs have only recently become commonly available. I bought them through the radio Shack 'commercial' catalog over a year ago; since then they've started to carry a white LED (5mm) BUT I do not believe these are the high intensity ones I've used. Cost for these through the catalog (or now online at RadioShack.com) was about $3/each compared to about a quarter apiece that one might normally expect to pay for a common red LED.

When I first got the LED's I hooked them to a battery to see what I had; these are rated at 4V max; with two AA batteries in a holder I was using them at roughly 3V. I was impressed by the amount of light these little suckers put out BUT was a bit disappointed that the light had something of a bluish 'tint' to it. Now, if you want to string these up in your Honda Civic with chopped springs, formula 1 wing and coffee can exhaust a blue tint might be just what you're looking for. Not quite what I was looking for. That being said, the LED's cast a fair amount of light and when shone at the instruments while still installed, I decided right then and there I'd have to figure out a way to make these work. The difference was Dramatic.

My cluster:

After disassembling, cleaning and hooking up the cluster on the bench I found little difference in brightness between 3W and 194 (actually about 3.7W at 12V) bulbs with this unit. Shining the LED's directly on the face again was a huge improvement, but because of the shape of the LED light is in cast in a circular beam with about 30 degrees total arc. The result is that from the top of the cluster, shining the LED straight down onto an instrument from above casts an ovoid pattern with most light at the top. I played with placement at the top right pointing left and slightly down. This was better, but there was still a noticeable gradient high/low even if using a 'mirror' on the left (pointing right/down)- more on this later.

Next step was to find a good mounting point. My goal was to keep the location completely unobtrusive with a minimal amount of mounting hassles.

First thought was to mount the LED's in back along with (or even in place of) the original bulbs. Looking at the light collector box on the donor dash, I found the light boxes to be in good condition through slightly yellowed from age/heat. The clear receiving end of the guide was not deformed, but light had to hit the guide straight in for effective transmission. The guide is on a bit of an angle relative the bulb that goes straight into the back of the cluster, so that if the position of the filament is a bit further in/out from designed, much light will be lost. Despite the white plastic used in the box, there is not much reflection- if light doesn't get into the guide, its going to be lost in heat- and these guys DO heat up!

Given the amounts of light lost, I let myself get sidetracked and also experimented with lining the interior of the boxes with aluminum foil. WARNING!!! The contacts for the bulbs are EXPOSED and can contact the foil and cause a short (DON'T ASK ME HOW I KNOW!!) !! Proceed with this at your own risk!! Ahem, now that that's out if the way, I shaped the foil so that it would be retained in place with friction and used a small metal clip to the plastic body to help ensure it stays away from the hot contacts. It took a fair amount of work to cut, form and retain the foil and the results were, again, rather disappointing. There WAS a noticeable difference but not a dramatic one- still could not read the odometer well without really concentrating.

So, how and where to mount the white LED's.

The original hope was the light boxes. However, with the heat seen and the need to get the LED's mounted to point DIRECTLY at the light guide it was going to be a challenge. If using only the LED's I could have fashioned an alternative mount, but because of the blue tint I wanted to have some of the yellow/white of the standards bulbs to balance things a bit. Also, I would have needed more of the LED's than I had to make up for the light lost in transmission. Mounting the LED's next to the standard bulbs would have been difficult without damaging the circuit traces powering the bulbs. Also, heat from the original bulbs may have damaged the LED's. So the back of the dash was out.

Next was somewhere along the 'roof' of the cluster. I saw that there was SOME space near where the light guides exit from- this area is 'shielded' by an existing hood so it was a natural place to look. Just about directly next to the light guide exit I drilled a 5/32" hole outboard of the guide- be careful not to nick the guide as significant light loss may result. The LED will fit through this and allow you to play with position from here. Now, I started with the 5mm LED's but ended up using the 3mm instead- they were easier to position in a smaller hole and gave plenty of light when modified.

Sample LED Catalog numbers (Radio Shack; their online catalog has a great selection): 900-7864- Super white 5mm LED 900-7863 Super white 3mm LED

Sample LED Catalog numbers (Radio Shack; their online catalog has a great selection): 900-7864- Super white 5mm LED 900-7863 Super white 3mm LED

Because of the focus of the LED's I'd have to change the shape of the 'lens' to scatter the light a bit. This isn't hard with an LED- they're just a plastic material. But, in the case of these LED's they're rather expensive and my supply limited so I played with grinding on other LED's first. After playing with a couple of profiles, I ended up with simply grinding the lens from a rounded tip down to an angle of about 50-60 degrees putting in a BIT of curvature to the face.


        **  *
        *    *   
        *     *   <--Ground down surface
        *      *
        *       *
        *        *
        *  xxxxx  *
        *   xxx   *
        *    x    *

When grinding it's a REALLY good idea to get a feel for it by using 'cheapie' LED's first. As shown above, the surface gets CLOSE to the emitter- I do NOT know the implications of this so be aware! Grinding was done in stages, testing the pattern at each step- what you see above gives an idea of what I was happiest with. Now, it's also possible to leave the LED alone and redirect light with another lens but I did not get into that here- time constraints and all.

After grinding the lens I deliberately left it as is- leaving a 'frosted' end with the assumption that it would act as a diffuser. Now, how correct that is I don't know; one COULD polish the end without too much trouble.

With the LEDs shaped I got a much nicer pattern. Now to pull the cluster out of my car, validate, mount and wire.

Upon disassembly of my cluster I found the left light box discolored and slightly deformed; perhaps there had been 5W bulbs here before! Made the appropriate modifications by drilling the holes for the LEDs.

Mounting was accomplished simply by setting with rubber cement- not the ideal, but it worked and I was getting on to 1 in the morning by this time. The LED's will take up to 4V each; to be safe with up to 14V nominal, I wired both LED's in series along with a 220 ohm, 1/4 watt resistor (see note below- I'm going to raise this value soon by about another 150 ohms). IF BUYING RESISTORS, I would have used a 1/2 watt just for better safety factor but 1/4 is what I had. I wired the plus side into the existing positive solder joint for the original bulb so that brightness would be controlled with all lights Then wired the LEDs and resistor and grounded to the case on the right side. Made sure the LED's were pointing where I wanted and fired her up.

Eureka! Light, even if with a slight bluish tint.


1) While into the cluster it would be a good time to clean the potentiometer. If disassembling, BEWARE THE TWO SPRINGS! They may jump out, but are not too difficult to get back in.

2) Also while in there, you've already pulled the speedo so this would be a good time to lube the speedo head- just a few more screws. Stu has recommended graphite-based grease for this purpose. May quiet down any noises, even thought it did not for me in the end.

3) On reassembly, take care to mount the trip odometer reset lever correctly!

4) Since you've already done all of the hard work, this would be an EXCELLENT time to replace all of the bulbs!

5) I have a COUPLE of pics of the mounting positions.

Lessons learned:

- Driving with a LIT DASH is a revelation! Wish I had done this earlier.

- The LEDs are BRIGHT! I'm actually running at only about 1/2-2/3 on the potentiometer. Downside: Since the pot is at a reduced setting, lighting for the rest of the bulbs it feeds (ACC, switches) is also reduced considerably. I'm going to place another resister inline to dim the LEDs a bit. I never thought I'd be saying lights being TOO BRIGHT would be a problem in this car!!!

- If doing this again, I would have put a 1/2 amp fuse at the connection into the 12V feed. For safety, this SHOULD BE CONSIDERED A MUST- not doing so was a bad oversight on my part.

- Again, if doing this again I would have put a small 'hood' on the LED to cut off light at the bottom and below the cluster., I realize that light wasn't really well contained in the original design, with the added brightness there just more than I'd like below- a very minor annoyance. Normally, thought, this is not seen due to the position of the steering wheel.

-Lighting is pretty good and relatively even given the crudeness of the lenses. Only spots I wish had a bit more light are the right and left most margins of the cluster- perhaps 1 inch or so. I can live with that.

- This was kind of a pain to do but worthwhile- I'm glad I've done it, but do not want to do it again!

Hope this makes sense- please feel free to share where it would help others!

Tony Wirtel
'85 300TD

Added notes: The topic of best light COLOR was also raised; it was noted that white light can be detrimental to night vision and that red would be better- this isn't much of a factor since recently most all driving has been in rather well lit areas. This is something to consider if night vision is critical to you.