Prince of Darkness = No Headlight

[D. P. Larmee]

Just acquired a new pony for the stable, a 1980 xs11 SG. After a thorough carb cleaning and fixing three of the four coil pickup wires, she is running pretty well. Still have a small electrical issue; the headlight and gauge lights are out. The bike is charging and the tach is working, but I am not getting any voltage at the white wire to the headlight relay. If I jump across the relay the lights come on and stay on and I tried the relay on my other bike and it worked fine, so I am suspecting that the inline diode for the white wire is toast. Does anybody know where this diode physically lives, or if it is even accessible? Also, if I have to run a new wire from the alt in place of this white wire, what are the specs of this diode so that I can add it as well?

Thanks in advance,

Dan

[TopCatGr58]

Hey Dan,

Why bother trying to unravel the wrap on your harness to find and replace the diode? Sounds like a real PITA to me!?

Just run a bypass like you said and hook it up to a switch, saw a fellow put a MICRO switch into the handlebar housings for just this thing, then just flick on ON after you get the bike started!!
JAT!?
T.C.

[skids]

Here is a link to give you some info on a relay to the headlights.

http://home.comcast.net/~sidhansen/relay/relay.htm

[D. P. Larmee]

Hey TC, that's a great idea. I wasn't sure whether or not that diode was buried in the harness. As it sounds like it is, I will try another way. If I go the switch route, does it have to be a constant on or can I just use a momentary burst to trigger the relay? Is there a chance of the relay unlatching once it's triggered?

I also just noticed that the earlier models used a yellow wire from the generator to switch the relay without a diode. I checked the bike and this yellow wire is still on the generator side of the plug, but it seems to terminate there. I may try to just run a jumper from this yellow wire to the relay and see if this solves the problem. Has anyone had success going this route?

Skids, That looks like an interesting read. I have often wanted to be able kill the headlights and go stealth mode should the need ever arise. I miss my old xs750 that had a headlight switch that killed the headlight, guages, and tail light.

I really appreciate the great ideas,

Dan

[D. P. Larmee]

Well after checking the part #s it looks like the stator assembly is the same # between the early and later models, but the relay is a different part number so I am thinking that maybe the earlier relays incorporated the diode into the relay itself. So following that line of reason, if I were to find a 78/79 relay, I would think that I could use the yellow triggering wire from the stator as they did in the earlier models. I guess its back to ebay for a 78/79 relay.

[jjwaller]

Allright, where is physically this relay?
Can't find it on my 78 E!

[D. P. Larmee]

Its under the tank between the frame rails towards the front. It is a squarish metal cube with four wires coming off, black, red/yellow, blue/black, and either white or yellow. It can be sort of tough to see as it hangs from the front cross rail in a little rubber hammock.

[D. P. Larmee]

Just for kicks, I checked voltage at the yellow wire from the stator and it looks like a fairly constant 7 volts with the engine running. Ran a jumper from the yellow to the stock '80 relay and now everything works as it should; switch on = no lights, engine starts = lights.

Does anyone know what differences there are, if any, between the earlier and later relays? Am I going to run into any problems running her this way? If so, I will pick up a relay for one of the earlier models, but as everything seems to be working with it in this configuration, I think I will leave everything the way it is for a while and see if I run into any problems. I will update you all if anything changes.

[jjwaller]

Originally posted by D. P. Larmee
Its under the tank between the frame rails towards the front. It is a squarish metal cube with four wires coming off, black, red/yellow, blue/black, and either white or yellow. It can be sort of tough to see as it hangs from the front cross rail in a little rubber hammock.

Thanks DP:
Finally found it!

[D. P. Larmee]

Well, I finally got around to a road test this weekend, and I think that I found the reason for the diode in the relay feed from the stator. The 8VAC or so from the stator tripped the relay and the lights came on, like I had experiences before, but by the time I got home from a short ride, the fuse for the headlight circuit was blown. This seemed strange to me because these two circuits visibly look to be separate. After a bit more prodding it looks like the relay, itself, also has a diode in it between the blue/black terminal and the white terminal, pointed towards the white. It seems that once the relay is triggered by the white wire, this diode routes power from the, now on, blue/black terminal to the white terminal so that once the relay is triggered, the blue/black terminal will now continue to trigger the relay until the main switch is turned off. This seems to be what provides the “latching” feature to the relay. Without the other diode in the harness, the relay diode sends the 12vdc from the blue/black crashing into the 8vac in the white wire from the stator. Apparently, this is not good.

I went to Radio Shack and picked up a diode to put in the relay trigger from the stator with hopes that this will essentially put her back to stock configuration.

I’m no electrical guru, but if ones relay went bad, it would seem that It would be pretty easy to follow this same logic to rig up a new “latching relay” from a standard automotive relay and a couple of diodes. A 30 amp relay can be had for about $5-8 at most auto parts stores and a two pack of diodes shouldn’t cost more than $2 at Radio Shack. This would be substantially less than a new stock relay.

[John]

You could also use a regular 12v relay and trigger it with a 5 psi oil pressure switch that is normally open. The switch could be installed where the OEM oil pressure light switch is installed by using a T fitting and both switches could be at the same location. If you could find a switch with a NO and a NC circuit for the OP light, then you could do it all with just one switch.

[D. P. Larmee]

Hey John,

That's a really neat idea. I may have to try that if the new diode doesn't work. While I never really intend to sell the bike, I have a bad habit of trying to stick as close to the stock configuration as possible, just in case. I have aquired vehicles with very creative wiring by the PO that have made me pull my hair out trying to get it all sorted. I like to try to save future owners as much confusion as possible. I guess once I come to grips with the fact that me and this bike are going to be together for the long haul, things will become a lot easier.