Additional Info on Installing Headlight Relays

By: Brian Curry

Thanks to Alessandro Bruno for raising these issues and prompting some elaboration.

Getting a 12 Volt supply from the Starter relay:

The start relay has two large contacts with bolted connections. The wires have red insulation. From memory, the rearmost connection, goes to the battery. Trace this to confirm it, BMW has been known to make changes. Disconnect the battery negative terminal. (This keeps down the sparks if you make a mistake and short something.) Remove the starter relay wiring bolt. Get a crimp connector that the bolt will fit through and that is properly sized for the wire. Crimp the connector on the wire. Bolt the Headlight relay wiring connector with the original connectors onto the relay. (I use some anti-seize on this type of connection.) You now have 12 volts UN-FUSED available when you reconnect the battery negative.
Fusing the 12V supply:
I found my local auto parts store (Pep Boys) had a real nice "main fuse" holder rated at 30 amps. This should be installed in electrical box, in the wire going to the headlight relays. They also had a 30 amp self resetting "circuit breaker" with the same terminal type and spacing as the 30 amp rated fuse holder. IMO, this is a great application. Since getting to a fuse in the electrical box is a PITA, using a self resetting circuit breaker is a very good move. Or if you don't trust circuit breakers, use a fuse. They also make them in 30 amp ratings. This is plenty for the headlights and the size wire you are running.
Headlights and Driving lights:

If you will be powering both the headlight and driving lights, run two leads. That way if one shorts, you will not lose all the lights.
Use colored wires:

Use different color wires for different parts of the circuits. Otherwise you have lots of wires all the same color and no idea where they go. If you only have one color put wraps of tape on the wire ends. Different number of wraps on different wires. I suggest Red for the wire from the starter relay to the headlight relays. Then other colors from there.
Relay mounting:

Silicone rubber/caulk can be used to stick relays on the inside of fairing or any other panels. No more rattling. Velcro also works well. Then they are easier to take off the bike if it is sold. Don't let them flop around!!
Relay numbering:

If you use relays with contacts numbered in accordance with the German Industrial Standards (DIN) here is what the contact numbers mean:

85 Relay Coil
86 Relay Coil

The control switch and ground/negative can go to either contact:

15  Relay Contact (Movable) Positive normally after the ignition switch
30  Relay Contact (Movable) Normally connected to the Battery
87  Relay Contact Closed when energized
87a Relay Contact Closed when de-energized
While the 12V power lead can be connected to either contact, it is nice to follow the convention with the lead from the start relay connected to 30 and the headlight lead to 87.

A lot of relays, even those made and used in the US, are starting to be DIN labeled, so even if you don't get it from BMW or another European car dealer, they may well be numbered. Look. If your relays are not labeled, you will have to spend some time with a meter figuring out what is what.

For those in the US and maybe Canada, Radio Shack has a nice little relay at a good price. It is RS part # 275-226. The price is US$5.99. It could be plugged into a socket, and it has a little tab with hole that could be used for screw mounting. It is rated at 30 amps at 12 volts. The terminals are numbered as noted above, and are sized so that you could use push on connectors. The data shows the coil resistance as 66 ohm, and 160 milliamps which is nothing. It also notes that it will pull in at 6 volts and drop out at 3.6 volts. So, there is a "common" relay source for those in the US.

 

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Last Update: 05 October 2007