Relay to Cut Off Headlight When Starting

Somebody inquired how to install a relay to cut off the headlight and other electricals when the starter cranks. I made a hand drawing to show how to do it. This is a generic circuit. The battery positive feeds the entire system through the main fuse which is probably a 20 - 25A size fuse. Then it separates into two branches which are maybe 8 - 10A each (each with separate fuse). One branch will be the headlight and maybe some other small lights. The other branch is rear lights, turn signals, horn, etc.

The place to install the relay is in the main line after where it feeds the ignition coil(s) and the electric fuel injection (if it has that). Those circuits must remain active to start up.

Use a large automotive relay with at least 25A rating. Run the lights power line through the normally closed (NC) contacts. That way, the power gets disconnected only when the relay is energized from the 12V line coming off the starter button.

I used a relay with flying leads to go into the line with a male-female spade terminal setup so if the relay went bad, I could pop it out and reconnect the original power wires together.

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My KZ650-C1 has an On/Off headlight switch on the right handlebar control. Could owners of later models just swap their control switch with one like mine so they could decide when the light comes on? Ed

You can just insert a spst toggle switch in series with the headlight circuit and mount the switch anywhere you want.

If your bike has the Kawaski supplied "Reserve Lighting Devise" (usually under the right side below the seat), you'll need a double pole single throw toggle switch as you will have two (2) circuits to disable.

just keep your bike tuned up and it wont be a problem! :laugh:
i always wire my lights to come on when ever the key is on,i want people to see me coming day or night

PLUMMEN wrote:

just keep your bike tuned up and it wont be a problem! :laugh:
i always wire my lights to come on when ever the key is on,i want people to see me coming day or night

Yes (me too), but you want the lights OFF when you crank the starter. Lead acid batteries are excruciatingly sensitive to depth of discharge and actually "wear out" based on cranking current out. Worse, the starter is already a severe load so throwing the lights on top is like exponentially worse.

When I got my bike new in 1979, the first battery lasted 14 months (2 months outside the warranty). Must be a fluke I thought. I bought a REALLY GOOD replacement that lasted 18 months. same for next one.

I rewired the bike to disconnect all the lights at startup and I average about 6 YEARS per battery and they are the JC Whitney cheapo ones.

bikerjim wrote:

You can just insert a spst toggle switch in series with the headlight circuit and mount the switch anywhere you want.

But the relay cuts off:

Headlight
tail light
running lights
instrument/dash lights

The typical headlight is about 45W low beam, but the rest add another 40 or 50W. Taking 90 - 100W off the battery when it cranks the starter is a huge plus for battery life.

650ed wrote:

My KZ650-C1 has an On/Off headlight switch on the right handlebar control. Could owners of later models just swap their control switch with one like mine so they could decide when the light comes on? Ed

My 1979 has the handlebar mounts that still have the lettering for the light switches but no switches. It would actually be a lot of work to install the older ones (with switches) and wire the switches in. The relay is really an easy installation. The lines are all available near the fuse box. It's also "idiot proof" since it restores stock function automatically after starting.

For the record: having the headlight on with the key is also state law in many places so changing it might get you a hassle trying to register the bike or sell it.

PLUMMEN wrote:

just keep your bike tuned up and it wont be a problem! :laugh:
i always wire my lights to come on when ever the key is on,i want people to see me coming day or night

They can see Otto in the dark without any lights....He's got a chemical glow :laugh:

[just kidding otto ]

Seriously though..... A properly tuned KZ starts in about a half a revolution of the motor.....So where is the real need for this?

larrycavan wrote:

Seriously though..... A properly tuned KZ starts in about a half a revolution of the motor.....So where is the real need for this?

It's the design of a wet cell lead acid battery, and the fact that bikes use very small batteries. The starter needs a lot of current which pushes the battery for the internal impedance. As batteries crank out amps, the material on the plates is consumed and gradually falls off and collects in the bottom of the battery (in the sediment tray). These batteries actually are used up based on how much current they deliver and it is not linear. The other thing is as the battery ages (and it's internal impedance goes up) it can stay in service longer since the battery demand during start is less. It basically both reduces battery attrition and also extends the service range since the demand now requires less CCA (cold crank Amps) to start the bike.

I like it. When I get a rountoit I just might try this. Granted for a "properly" tuned bike it may not be a concern but if you've ever flooded or ran the system out of gas because you forgot to turn the petcock on well... we all make mistakes. I agree with BH. Anytime you can lessen the cranking you might save your butt later. Plus I switched up to an automotive headlight and I know it doesn't take long to suck the life out of a battery when your brake light is stuck on and you don't know it. Seems like a worthwhile mod to me. After all, cars cut the headlights too while cranking.