E21 Blade Fuse Box Upgrade Kit Installation Manual

Installation of the E21 kit is pretty straightforward, but it's a bit more involved than the E28 kit because of the design of the fuse box.


Basic instructions:
  1. Disconnect positive terminal from the battery.
  2. Remove old fuses and relays.
  3. Remove back cover from fuse box.
  4. Unplug all wires that connect to the bottom of the bullet-style fuse holders.
  5. Remove most of the bullet-style holders (destructive operation) and drill a hole for the ground wire pin.
  6. Put new circuit board in (soldering step).
  7. Plug all wires back in to their original locations.
  8. Wire up the LED ground wire.
  9. Put back cover back on.
  10. Install fuses.
  11. Reinstall relays.
  12. Reconnect positive battery terminal.


Here are references photos for steps 5 and 6.

Unmodified fuse box:

All 6 rows are the same, here's a close upon the bottom row:

You need to remove the upper 3 clips and their support plastic, along with the lower right clip. The lower left and lower middle clips are both connected to the relay socket they are next to and should not be removed.

Here's a photo of a properly modified row:


UPDATES FOR V2 UNITS -------------------------------

Temporary instructions for beta testers, I'll have a better write up when I have production units on hand.

At this point, each row should have two remaining connections for the original bullet fuses. Cut them off as close to flush as possible - you're trying to prevent them from touching anything on the back side of the circuit board. If you have some kind of non-conductive tape to cover the sharp bits that would be good too. A dremel would work well for cutting, I think the connections are too thick to use wire cutters.

Potential alternative: The way these are build is there's a stamped piece of metal with the bullet fuse connections on one end, and it goes over to the associated relay on the other. If you look at the back side, some of these have a blade connection inside the fuse box. If that connection is NOT used in your car or doesn't exist (meaning the ONLY connection to the relay output is the fuses), you can remove the entire stamped piece (medium-ly hitting the blade fuse connections on the top should get you started). Based on the e21 wiring diagrams I have looked at they should be unused, but I haven't looked at every diagram and some cars have been tweaked by a PO. Unfortunately I don't have a box to take a photo of as I'm writing this, so if none of this paragraph makes sense ignore it and just cut the connections off. NOTE: don't remove the connection bar between the 3 middle relays

You still need to drill a hole for the ground pin, it's slightly leftward of where it is in the below photos. You can line up the board and use a pencil to mark where to drill.

Once you have all the original fuse connections removed and / or cut off (and you drilled the hole) you can install the fuse board and hook all the wires back up.


Depending on the condition of your fuse box, you may need to carefully clean the remaining clips so you can solder to them in a later step. You also need to drill a hole for the ground wire halfway between fuses 3 and 6, highlighted in the following photo:


Once you've done all the box modification, carefully install the blade fuse circuit board. You may need to use something to adjust the clip angles slightly so they go in the holes. The upwards facing clips coming out of the fuse box can break free of the plastic if you push too hard and they aren't lined up with the holes in the circuit board (this isn't a huge deal, just makes things more annoying). WD-40 or similar lubricant can be used on the clips on the circuit board, just be careful not to get it on the clips from the fuse box so it doesn't interfere with soldering. Here's a photo of a correctly installed circuit board (prototype shown):

Here's a photo from the side showing correct circuit board installation height (not an exact science, just make sure it's completely seated):

The last required step before reassembly is soldering the fuse box clips to the circuit board. I highly recommend using a soldering gun for this (I used a Weller 9400PKS) - while it is possible to use an iron, you run the risk of melting the fuse box plastic because it cannot heat the joint quickly. Make sure the solder wets to both the fuse box clip and the circuit board all the way around the clip, here's a photo:

Once you've soldered all 12 clips, you're good to go!