For a few years I have been doing some tweaks here and there on the car, one of those bigger projects was the LED bulbs all round. At the back of the car the rear licence plate bulb was the biggest pain to source one that would fit in the original factory housing. I tried many variations some were close others not so. The left pic below is the now dedicated draw now allocated for bulbs, the right pic shows the original bulb (copper bottom) in the middle, with the other trial and errors bulbs around it. Some were too long, too wide or the bulb glass (plastic) to bulbous to fit under the lamp holders glass cover itself. Then there was front focussed light, 360o light shine, light pattern, colour temperature, brightness, bottom housing fitting, wattage etc etc. Some bulbs what I thought would look good were in fact quite poor lighting options when plugged into my dummy rig to see what they would look like before I took the holder off the rear bumper and started making a mod to the wiring loom.
I spent many an hour making mods to a couple of the bulbs that had potential with the Dremel in a vain attempt to make them fit. A slight slip of the Dremel grinder ruined a promising bulb that was just a bit to wide, luckily those bulbs came in pairs.
The before and after of the grinding was fine, there was no cover for these LEDs themselves which shouldn’t be to much of a problem as the bulb sits behind a opaque glass bowl if you like. To make the bulb fit under the glass properly it would need more work on one side of the bulb fitting, in doing so it would be getting dangerously close to the LEDs themselves. Before I got carried away I quickly put it back together with the glass cover sort of half in place held by a bit of clear tape to see what it would look like at night, it wasn’t good. There was so much white light I could expect a pull over from the local plod in their donut cars as you’re not supposed to show white to the rear in the UK. This bulb was on another level and would be more like towing a WWII search light behind me.
That bulb was discarded and the original put back in for now. After many, many searches and I eventually found a match same size bulb and voltage. Best of all it was a comparable wattage to the original bulb, I placed the order which was shipped from China (of course) to the UK.
The new bulb was still quite bright, but I figured it should be OK.
This LED bulb thing is all a little bit of a moot point as I tend not to drive at night anyway. However, if I needed to, then the LED bulbs are a significant improvement for safety. The reason I mention all this now is because of a recent trip trip to Mustang Maniac where I watched as they were making a custom fit LED board for a customer’s car. They had to make bulbs from bulbs and then set the brightness. I wasn’t aware that it could be done to that degree and spent some time with Mark to see how I could make their ideas work for me.
I purchased a box of various resistors taking on board the advice I was given. This box is complete overkill of 350 pcs, I only wanted one or maybe two variations of a resistor. The complete box was the cost only a large coffee so I couldn’t complain.
There’s a lot to understand about resistors; basically wattage and resistance describe two different, but connected processes a resistor performs in a circuit. First is Ohms (Ω) tell you how much the resistor limits electrical current, a higher Ohm value means more resistance and less current flow. Second is Wattage (W) tells you how much power the resistor can safely dissipate as heat without burning out. So the Ohms value sets the electrical behaviour, while the Watt rating sets the physical durability. A resistor with the right resistance but too low a Wattage may overheat, whereas a resistor with the correct Wattage but wrong resistance will change how the circuit works. You can also fit a resistor either direction as they aren’t polarised. If you need a specific Ohm rating you can add one after the other to achieve the correct rating.
As the licence plate bulb only has a single live feed, the earth is handled via the bodywork. The live feed just needs an inline resistor so the LED’s bulb brightness is reduced as a result.
In the trunk the live is taken from the rear drive lights feed via a T-junction split and a connector. The wire then passes behind the rear bumper to the lamp holder out of direct line of sight. This will be a quick task to make up an insert male and female fitting at each end to connect it back up again. If I don’t want to use the resistor, I can just remove the inserted resistor and reconnect the original wire back to the normal live feed voltage.
When I rewired the car I didn’t use all the options of the American Autowire kit, that left me with various amounts wires of different colours and a number of connectors to go with them. A short clip from one of the coils of wire I used was the same gauge as the live feed about to be modified.
The short wire was cut in half and each end of the wire was tinned with the soldering iron. My American Autowire crimps were pulled out of retirement and used to secure the male spade connector and protected with heat shrink.
The resistor I chose was a 330 Ohms which should roughly half the output. In order to make the resistor lay flat inline; wrap the wire leg of the resistor around a small jewellers screwdriver the same size as the gauge of wire to be used forming a tight coil. I didn’t realise the picture was out of focus (sorry), but you get the idea. Trim the coil flush to make sure no sharp parts are sticking up and potentially cause a short.
Thread the new wire into the coil and add some flux, fresh solder on the iron and tap it onto the join you just created. Before the last female spade connector is added slide over the shrink wrap and then repeat the process and solder the other end of the resistor. Add a spade protector on the female spade connector and crimp. Once completed and fully heat shrunk, I checked the resistance with a multi-meter to make sure connections were solid and working.
To create the inline project that I wanted, the live feed wire needed to be modified to add the additional spade fitting rather than a direct solder of the resistor, my fail safe if you like. This additional could then be re-joined in seconds to the main loom if needed, or connect another resistor inline back to the main rear end wire harness. I remade the duel spade (T-junction) split along with the new single connection needed for the lamp holder.
With the new inline resistor in place it only added a couple of extra inches so the extra length could easily be hidden back behind the filler cap support. Here the fittings are in place to recreate the original look before I added the resistor.
With everything in place the garage door was closed and turned the lights off. I turned the car’s lights on and I could see that everything was working fine. Once the door was open again I could finish the job by wrapping the wire loom up with some cloth OEM look wire loom tape, but not on the resistor section as that may need to dissipate heat, not that it gets that hot anyway. The resistor section is insulated by the heat shrink and the rubber spade fitting covers.
Until I can get a good view in the dark and out in the open I wont be able to see if the light output is adequate or still too much. The results so seem to be what I’m after and not far away from the original bulb output, all be it a cleaner light.
With that in mind while the soldering iron was still hot and the resistors were out, I repeated the process for another inline with a resistance of 220 Ohms which will allow a little brighter light than the installed 330 Ohms. It will only take seconds to swap out which is the beauty of this idea. If the result still isn’t what I’m after, I can just knock up some more variations until I’m happy. There is no after picture of the rear loom as it’s all hidden away again, and nothing to see.
The only visible difference is the cool white light instead of the dull orange glow of the original incandescent bulb. The change to the lighting type of the cool white light picks out the silver lettering from the black background of the licence plate better for more effect.
Before & after comparison of the resistance hack:
A short and sweet little project from something that was in the back of my mind that needed addressing at some point. It’s funny how a chance conversation can lead to these little projects, all sorted for now.
