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I own two 20+ year-old BMWs. They are cars that I drooled over when I was younger but couldn't afford. Now it's possible to find very nice examples for under $5,000; my 1988 735i was purchased last year for $1,200, and my 1984 528e for $800 two years ago. They are easy to work on and will run forever if properly cared for, and will easily keep up with more modern cars. Parts are readily available for the DIYer to keep them running. They're definitely not for everybody though; if you need to rely on a paid mechanic to do the work because you don't have the the ability or space to work on a car, you're better off with something else, because these cars do require some regular attention. The payoff is being able to drive a car that was state-of-the-art in its day and will make you grin with its great handling and acceleration.

As great as these cars are, they are still equipped with subsystems made up of 20+ year old technology, so there are a lot of minor modifications that can be made to make them even better. One of the more pathetic items is the lighted keys; originally they were equipped with tiny incandescent lamps that didn't light very well and wafer batteries that didn't last very long. I have a couple of little LED flashlights that were given to me as promotional items by vendors; dandy little items that work well but have been cluttering up the junk drawer in the kitchen for too long.

So I thought it would be good to use the LED lights as donors to convert the key lights to get longer battery life and better light. It was pretty easy, didn't take very long, and the end result works really well; well worth the time.

Step 1: First Step...

... is to dissemble the key to see what you have to work with. This key, from the 735i (e32) comes apart by pushing in the button then using a small screwdriver inserted into one of the gaps to pry the light assembly from the housing. Then the screwdriver was used to release two tabs that kept the light assembly together. After that, the parts of the actual switch could be accessed. Of course, I pulled it apart without taking note of how things were arranged, so putting it back together was a bit of trial & error to see what went where. Next time I'll take a photo of it first!

Step 2: LED Fitted

Here's the light assembly with the LED fitted.

The diameter of the LED was just a bit larger than the original lamp, so I used a Dremel tool to round the hole out a bit more so the LED would fit properly & allow the light assembly base & top to fit together. A different cutting bit for the Dremel was used to carve a groove in the plastic that helps to hold the LED in place & keep it from moving in or out.

The brass pieces had to be bent slightly to make better contact with the LED leads. Other than that, the light assembly went back together pretty much as it was originally designed (although it took some trial & error to get everything just right.) 

The leads on the LED had to be trimmed to fit the housing. Before trimming though, it's important to remember that an LED is polarity specific; it will only work when voltage is applied in a specific way. To test which lead is the positive, manually touch the leads of the LED to the positive & negative contacts on your battery. If it lights up on your first try, great; remember which of the leads is touching the positive contact. In my light the battery's positive contact touches the spring, which is in contact with the brass plate at the bottom, which completes the circuit to the shorter leg of the LED. The brass piece that curves around the battery comes in contact with the negative contact of the battery when the button is pressed, and completes the circuit to the longer leg of the LED.

The closeup gives a better view of how the battery, spring and brass contacts all work together as a switch; the battery is positioned on top of the spring with the positive side facing down. The spring provides continuity to the brass contact plate at the bottom. The brass contact plate that curves around the side comes in contact with the battery case (negative) when the button is pressed, lighting up the night!

Step 3: Battery Modification

Probably the most time-consuming step in this was modifying the old battery to allow the new batteries to fit inside & work with the existing mechanicals of the key. Using the old battery wasn't an option — even if it did still have a charge; it was a 1.5V battery, and the LED I used had two 3V batteries stacked together to yield 6V. 

I might have been able to save myself some trouble by buying different batteries that matched the diameter & thickness of the original, but I didn't feel like going out in the cold last night. ;) Besides, these wafer batteries are expensive. 

To do this (sorry, no photos of the actual surgery) I locked the battery in a Vice Grip pliers and used a Dremel cutoff wheel to cut some slits across the negative contact, then pried it apart & cleaned out the 'guts' of the battery, and finally used the cutoff wheel to cut the edge down flat. The end result was a little metal cup that fit inside the key button, and into which the batteries could be inserted.

Step 4: The New Batteries

The new batteries, as they were used in the donor LED flashlight I tore apart, were 3V wafers stacked together. To be used in the new light, they needed to be stacked inside the old battery case. But if the side of the top battery were to contact the side of the case, problems would happen, so I cut a strip of transparent tape and wrapped it around the batteries, leaving the positive and negative contacts exposed. 

The new batteries didn't fit down to the bottom of the old battery case, plus the stack wasn't quite as thick as the original, so I found a steel washer that was the same diameter as the bottom of the case & stuck it in there. End result was just the right thickness, and the battery stack wrapped with tape fit snugly inside the metal case with no movement that might wear through the tape.

Step 5: The Final Product; IT LIVES!

After several tries and several adjustments to the bending of the contacts and the shaving of the case around the LED it all went together, and the LED lit up with the button pressed. As you can see in the photo, the light is much, much brighter than the original. And yes, the one on the right — the key for my '84 528e — is illuminated in the photo. It isn't much brighter at night. I'll mod that one next!
Very good idea! I own myself 2x e34, old but gold ;)
Do you know what batteries these were? Like what model? (Example, the old battery is a CR[followed by 3-4 numbers])
No I don't; sorry. They were just the batteries that came with the LED flashlight. If you've got an LED that didn't come with the batteries, you could just pick some batteries that were the proper diameter, thickness & voltage. Keep in mind that the LED's typically need 6V to operate properly, so a pair of 3V batteries works great.

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