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The Enfield Bullet Manual
- by Pete Snidal (C)2002
Relays 101

What A Relay Is, and Why You Need It
A Relay is a sneaky way of switching short and/or heavy lines carrying high current with thin, possibly long lines and skinny switches carrying very low current.

A low-current electromagnet is used to draw down a spring-loaded bar to complete the high-current circuit. This allows low-current switches, such as your stock dip switch, to control high-current devices, such as a Halogen headlamp. Or your low-current-capable horn button to trigger an airhorn compressor. Or just about anything your batteryc can handle.
To wire it in, you need only identify the low-current terminals - they will make the relay "click" when connected to power - and the high-current terminals - they will close when the "click" is heard from the low-current connection.

It allows you to run a really heavy (#12, say) wire from your battery directly to your headlight high beam (via the relay, of course). and then "trigger" this line with a low-current much less direct line from your dip switch. The second "trigger" line would of course be a ground.
Do You Need One?
A relay is only necessary in cases in which the standard wiring is overtaxed. A Halogen headlamp, for example. To test whether this applies to your application, it is useful to do a Voltage Drop Test.
In this test, you look for a small voltage where there shouldn't be one - between the battery "hot" terminal (the one not grounded to the chassis) and the headlight high beam feed wire, as close to the light as possible.
Open up the headlight shell, leaving the sealed beam connected. Strip the high beam feed wire on one side, close to the light, or clip a jumper to the connector on the back of the light if applicable. Using your multimeter on low volts range (0 - 5 V max), turn on the headlight, and check for a voltage between the battery "hot" terminal and the headlight supply line. If a voltage shows, a resistance, or voltage drop, is present, due to power loss in the circuit - from battery through light switch, possibly also ignition switch, dip switch, the thin wires involved, to the light. This may be remedied by direct connection from battery to light with large-gauge wire. So we will run a new, large (#14 Ga.) wire directly from battery to the light, interrupted only by the points in the relay, which we will also install.
Tools and Materials

A Relay - Your local auto supply store will have a selection of small horn relays. Bosch make a nice one.
Wire. Pick up a roll of #14 stranded automotive wire. Get it in red for (-) ground systems, black for (+) ground.
Crimp connectors - some spades, for the relay, and a ring or two, for the battery.
Crimping tool for the connectors
The usual tools - screwdriver, pliers, wire strippers

What Connects Where?
There is often a little teensy diagram on the relay itself, but if you use a small one you can barely see it. There will be 5 connections. So, here's a coping strategy:

First, find the energizer circuit. Which two? We will use a "hot" wire from the battery, and a ground wire. Clip a jumper to each battery terminal. Attach the ground clip to any tab on the relay.
Try a hot wire to each of the others in turn, looking for a "click."
If you don't get a "click," move the ground clip to the next one. Repeat step 2. Keep doing this until you identify a pair which will make the click. (Close the relay) Polarity doesn't matter; either of these two may be grounded or made "hot" to energize the relay. Mark them.
You now have 3 terminals left. One of these is a "feed," or input terminal, and the other two are "load," or outputs. We will only have to identify two of these remaining terminals - those which are connected when the relay is energized.
First, energize the relay by connecting the battery to the energizer terminals. then, using your multimeter on "ohms," find the two that are connected when the relay is energized. These are your new "high beam switch," or "Load" terminals. Mark them. Disconnect the energizer power, and check to make sure that they are now open.

How To Connect It
We will use the relay only for the high beam, and the low beam continues to be fed with the regular circuitry. The high beam feed from the dip switch will be used to energize the relay, which provides high-current feed directly from the battery.
For safety purposes, disconnect the battery ground cable.
Remove the fuel tank. Then mount the relay in the headlight shell.
Cut the wire from the dip switch to the headlight high beam. Connect the high beam feed wire to one of the relay's "load" terminals. Run a length of #14 wire from the headlight shell along the frame tube to the battery. Connect the other relay "load" terminal to the battery "hot" terminal.
Connect the wire you just cut from the dip switch to the high beam to one of the relay's "energizer" terminals. Connect the other energizer terminal to a good chassis ground.
Locate the high beam indicator lamp feed line. This may be connected to the high beam feed line between relay and lamp, or to the energizer line from the dip switch to the relay.
Now, test your wiring. Ensure that no "hot" terminals are in contact with chassis ground, and re-attach the battery ground cable. Turn on the light switch, and check both beam settings with the dip switch. The relay should click when you select high beam, and the light should light.
A Word On AC Lights
A few motorcycles, such as some late models of the Royal Enfield Bullet, use unrectified AC from designated windings of the alternator to supply the headlamp directly. With these, the headlight works only when the machine is running, and then full-time (there is no switch.) Since the connection is direct to begin with, it is unlikely that a relay can be of any help. But in cases in which the dip switch - often the weak link in the high current capability of the lighting circuit - is showing excessive voltage drop, a relay should still be considered. The major difference with the AC system may be that the AC may not hold the relay down. In this case, you'll have to feed the dipswitch with DC from the battery through your ignition switch, and feed the relay input (not the trigger) with the AC line from the AC Regulator.
Using A Relay For The Horn
Some owners install aftermarket horns with high current draw, such as air horns, with an electrically-driven compressor. In most models, the regular horn circuit is just not up to this kind of work. In this case, follow the basic instructions above, but feed the relay energizer circuit with the horn button, and feed the horn with the "load" terminal, connecting the "feed" terminal directly to the battery with large-gauge wire as above.

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