81 XS650 Headlight relay operation

[FloppaEnjoyer8067]

Does anyone here know how the headlight relay functions? As in voltage to what wires/pins closes the relay, and where does it send the power.

Right now I’m trying to diagnose an issue where my bike will not start if the headlight is on. The headlight turns on when you pull the brake lever for the first time, then stays on. When the headlight turns on, the starter safety relay gets voltage, preventing it from sending power to the starter solenoid. From what I’ve read, this is not normal, but the bike has been doing this since I bought it.

Looking at a stock wiring diagram, the headlight gets power from the stator. The one issue is the bike has a PMA kit, and I have no idea how the previous owner wired up the headlight and I’m struggling to figure it out. I’m honestly to the point where I want to rip out the entire harness and rewire it with modern wire and connectors

 

[FloppaEnjoyer8067]

Alright just figured I’d give an update:

I started peeling back the Non-OEM looking harness tape. I found a T-Tap that connects the rear brake light power to a yellow wire that I am assuming comes from the stator on the stock bike. It’s right next to the left side airbox/carbs.

I’m guessing this means when I click the brake, it sends 12V to the headlight relay through the original stator wire “telling” the bike it’s started, which then kills power to my starter and turns my headlight on.

I am now wondering if that’s why my headlight is dim, it’s pulling power off of the brake light circuit and causing a voltage drop.

Honestly I think I’m going to just leave it alone right now, then this winter rewire the whole bike with modern Bosch relays, and removing the headlight module and starter safety circuit

 

[5twins]

The starter/headlight safety relay is supposed to be connected to charging voltage, not battery voltage. That's your issue I think. On the original set-up, charging voltage was sent to the relay on a yellow wire. Once the alternator was spinning enough to produce a charge (even if it wasn't running yet), the voltage sent on the yellow wire would trip the headlight relay and turn the headlight on. But, the starter relay doesn't get tripped until the motor is actually running. I'm not sure how Yamaha accomplished that, lol.

But, back to your issue - I'd fix it, or at least try. I'd connect the relay yellow wire to the PMA output line somewhere.

 

[FloppaEnjoyer8067]

The starter/headlight safety relay is supposed to be connected to charging voltage, not battery voltage. That's your issue I think. On the original set-up, charging voltage was sent to the relay on a yellow wire. Once the alternator was spinning enough to produce a charge (even if it wasn't running yet), the voltage sent on the yellow wire would trip the headlight relay and turn the headlight on. But, the starter relay doesn't get tripped until the motor is actually running. I'm not sure how Yamaha accomplished that, lol.

But, back to your issue - I'd fix it, or at least try. I'd connect the relay yellow wire to the PMA output line somewhere.

I’ll check that out tomorrow. I’ll give it a shot and if it doesn’t fix it, fully custom harness it is

 

[5twins]

The headlight relay has headlight power fed into it on the R/Y wire coming from the headlight fuse. Once tripped, the relay feeds that power out to the RLU on the L/B wire. From the RLU, the headlight power is sent up to the hi-lo switch. If your headlight is good, power is sent on the L/Y wire. If your low beam has burned out, the RLU senses that and sends power out on the L/G wire to the high beam instead, but at a reduced voltage so you don't blind oncoming drivers. Maybe this is why your headlight is dim? I'd test the headlight bulb. Unplug it and connect 12V to each filament to see if they're both still good and working. All that being said, many of us eliminate the RLU. Details are here .....

http://www.excess650.co.uk/tips & tricks.htm

Eventually, if you want to eliminate the headlight relay, that's fine, but I'd keep the starter one. I think it serves an important function. I eliminated the headlight relay on my '83 and added a headlight on-off switch from a '77. Here's how I went about it .....

https://www.xs650.com/threads/retrofitting-headlight-on-off-switch-to-81-83-model.58167/

 

[FloppaEnjoyer8067]

The headlight relay has headlight power fed into it on the R/Y wire coming from the headlight fuse. Once tripped, the relay feeds that power out to the RLU on the L/B wire. From the RLU, the headlight power is sent up to the hi-lo switch. If your headlight is good, power is sent on the L/Y wire. If your low beam has burned out, the RLU senses that and sends power out on the L/G wire to the high beam instead, but at a reduced voltage so you don't blind oncoming drivers. Maybe this is why your headlight is dim? I'd test the headlight bulb. Unplug it and connect 12V to each filament to see if they're both still good and working. All that being said, many of us eliminate the RLU. Details are here .....

http://www.excess650.co.uk/tips & tricks.htm

Eventually, if you want to eliminate the headlight relay, that's fine, but I'd keep the starter one. I think it serves an important function. I eliminated the headlight relay on my '83 and added a headlight on-off switch from a '77. Here's how I went about it .....

https://www.xs650.com/threads/retrofitting-headlight-on-off-switch-to-81-83-model.58167/

I tried wiring one of the PMA phases to the

The starter/headlight safety relay is supposed to be connected to charging voltage, not battery voltage. That's your issue I think. On the original set-up, charging voltage was sent to the relay on a yellow wire. Once the alternator was spinning enough to produce a charge (even if it wasn't running yet), the voltage sent on the yellow wire would trip the headlight relay and turn the headlight on. But, the starter relay doesn't get tripped until the motor is actually running. I'm not sure how Yamaha accomplished that, lol.

But, back to your issue - I'd fix it, or at least try. I'd connect the relay yellow wire to the PMA output line somewhere.

I tried running the PMA output line to the headlight. Now the headlight will flick on basically as soon as I start cranking. I’m honestly not sure if that’s more desirable, because now the battery is under load from the headlight and starter at the e same time.

also, I already bypassed the RLU. The headlight may just be dim since I’m comparing to a bright new LED bulb on my other bike.

 

[5twins]

Yes, that's the way it works. That's why I eliminated it. These bikes don't have a very big battery so there's not much reserve power there. I didn't like the idea of the headlight drawing power. Find a right side switch/throttle housing from a '76-'78 that has the headlight on-off switch and swap it on. Then you can eliminate the headlight relay (well, disable it anyway). You can also use one from an XS500.

 

[FloppaEnjoyer8067]

Just for anyone who stumbles across this thread in the next 5 years looking for a solution, I would not wire the PMA phases to the headlight on. I’m pretty sure it overvolted the relay and killed it, because the relay no longer works and all the connections are still solid

 

[gggGary]

That relay is designed for about 5 volts AC, what the yellow wire on the stock alternator puts out.

 

[FloppaEnjoyer8067]

That relay is designed for about 5 volts AC, what the yellow wire on the stock alternator puts out.

Gotcha. Not sure what the PMA puts out, but I’m guessing it’s more.

I just bypassed the relay for now. If there’s a rainy week I’m going to get a normally on relay that disconnects the headlight with starter button signal.

 

[gggGary]

Gotcha. Not sure what the PMA puts out, but I’m guessing it’s more.

I just bypassed the relay for now. If there’s a rainy week I’m going to get a normally on relay that disconnects the headlight with starter button signal.

PS it's a latching relay. Once activated it stays latched until the ignition switch is turned off.
Wouldn't want an alternator failure to douse the headlight a on dark deserted highway.
Had a BMW 'puter do that to me one night, it decided it just didn't like an LED bulb I had used so after about 10 minutes it shut down ALL the lighting. NOT amusing.