BMW E36 Key Fob Battery and LED replacement

Maxell CR2025 5-Pack 3V Lithium Coin Cell Batteries

 

As I was reading through the owners manual, it showed how to open up the key to access the light and battery. I did, tested the battery voltage, solid 1.5V, and tested the light and it was dead.

I checked for a light as the local electronics store, but couldn’t find one that size. So instead, I took the suggestion of some guys at work and upgraded to an LED.

Parts

• 1 ea Superbright White LED, 5mm dia, 2.5V – 5V input, 3mm will work, but you need a 4 1/2 mm housing for it. $0.99
• 1 ea 3.0 V Lithium Button-cell Battery CR1616, about 15mm (5/8″) dia, 3-5mm thick. $3.00
• Electrical Tape

NOTE: Battery MUST have the one terminal (+ in my case) wrap around the casing and be on the same side as the opposite terminal (-). This is the same design as the original battery, which made contact through the spring and contact ring design.

I would post pictures, but I dont have enough posts to do that. Google picture “lithium button cell cr1616” if you need a better idea what I’m talking about.

Tools

• Small slotted screwdriver
• Knife or box cutter / razor blade
• Wire snips
• Soldering iron (if desired)
• Scissors

Steps

1. Remove light assembly from the plastic frame by pressing and holding the power button and sliding the assembly out. (BMW E36 Owner’s Manual Page 26)
2. Press in the two tabs with the screw driver and pop off the lid. (BMW E36 Owner’s Manual Page 26)
3. Remove the old light and battery.
4. Test the LED with the new 3.0 V battery to verify it works and to verify the + and – leads of the LED.
5. If you are using a 5mm dia. LED (like me), you will need to increase the size of the hole. Take the knife or box cutter and shave off some plastic from the base of the plastic assembly until you can fit the LED into the “trough” where the old light was.
6. If you are using a 3mm LED and 4.5mm casing, you should be able to just slide the LED into the casing and the casing should press into the hole. If not, do Step 5, or trim the plastic casing.
7. Bend one LED lead towards the spring end point that used to touch the OEM light casing.
8. Snip to length and either wrap it around the spring, solder it to the spring, or just let the press fit of the assembly keep things in place. I tried solder, but I couldn’t get the spring hot enough to melt the solder to it, so I just left it touching. When you put the lid on, the LED wont move so the assembled fit should keep things in place.
9. Bend the other LED lead into position where the old OEM light tip touched the brass contact ring.
10. Snip to a proper length and either wrap the lead around the spring the contact ring point, solder it, or just let it be. Again, didn’t try to solder so I just let the press fit keep things it place.
11. Cut a piece of electrical tape and put it on the bottom of the battery (negative terminal).
12. Cut a circle in the tape along the ridge where the positive and negative terminals meet. Make sure the postive termal edge is not cover up, or the postive side of the battery wont make good contact with the brass contact ring.
13. Cut another smaller circle in the middle of the electrical tape, about 3/8″ dia. This electrical tape donut will provide better separation between the positive and negative terminals and prevent them from both touching the brass contact ring.
14. Turn the button cell over, and put 4-5 layes of electrical tape on the back positive (+) side. Dont worry, this side wont be touching any metal. This just give some padding and extra thickness to the battery since it is thinner than the OEM alkaline button cell.
15. Put the battery back on, negative (tape donut) side down, touching the spring.
16. Before you close it up, verify that the light works by pressing on the battery. The small hole in the bottom electrical tape donut should touch the spring. The positive bottom edge and sides of the battery should touch the brass contact ring.
17. Do you see the light??? If not, check your LED lead contact, LED polarity, and electrical tape to make sure you have a working circuit.
18. If all is well, then “put a lid on it”, press and hold the button, and slide the assembly back into the key frame. If the lid doesn’t fit, the plastic where the LED sits probably needs to be trimmed more.
19. And you’re done. The light is brighter than when it was new, whiter than when it was new (or colored depending on your preference), and should last longer with the lithium cell.