Headlight relay/not working

[Charles]

Ok, not getting voltage to the headlight. Tested the high/low switch, no power there. Followed the blue/black stripe wire back to what I assume is the relay. This:

The red/yellow stripe wire has power, so I went ahead and jumped the red/yellow to the blue/black, but I still don't have power to the switch. What am I overlooking? The bike doesn't need to be running for the headlight to work, right?

 

[corruptmind]

Make sure the battery has a good charge the headlights suck a lot of voltage. Also make sure the bar under the controls is grounded well, if painted or rusted it'll be hard for it to ground right.

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[drewpy]

i think the headlamp relay is connected to the alternator, basically it keeps the lamp off till the engine is running. This energises the relay to operate the lamps

 

[XSLeo]

I'm not sure on the xs400, but the xs650 has a safety relay. That in your pic don't look like one.
On the earlier models the safety relay was to protect the starter mechinism. It did this byhaving the contacts in the relay closed. with the closed relay power would flow to the start relay and crank the starter. As the engine starts the alternator start to produce voltage. Some of this voltage is sent to the safety relay. As this voltage rises to 2.5 to 4.5 the safety relay is tripped and the starter is turned off.
On the later models the safety relay was changed to have two set of contacts. The one set the same as early, the second were open until the relay trips and power is sent to the dimmer switch, turning on the headlight.
Now if your alternator isn't working the voltage to trip the safety relay isn't there. No voltage = no headlight.
On the xs650 the safety relay has 6 wires, one black, two red/whites, a red/yellow, a yellow.
Power for the headlight comes in on the red/yellow wire, out on the blue/black. if you unplug the safety relay and jump the red/yellow to the blue/black this should power the headlight.
I would first check the battery voltage. With the bike at idle it should be some where from 12.5 to 13.5. As you rev the engine it should climb to 14.5 volts by about 2000 rpms. Then stay at 14.5 volts at any rpms above that.
I think there are wiring diagrams on here, I'll find one and check the colors.
By the way what year is your bike? Knowing that will help us help you. I have one bike listed in my signature. Very helpful. I don't have to write it out everytime.
Leo

 

[XSLeo]

Just looked at an XS400F digram. It doesn't use a safety relay. It does have a headlight cut out relay.
Your pic looks to be one. It should have a red/yellow wire, a blue wire, a yellow wire and a black wire.
The yellow wire is power from the alternator, this trips the relay to turn the headlight on.
Black is ground.
Red/yellow is power after the key switch. with key on should have battery power.
Blue is power out to the headlight and dash lights.
If you have power on the r/y then if you jump from r/y to blue the headlight should light.
If no power on the r/y wire check for a blown fuse. Replace it even if it looks good. Those old glass fuses were crap 30 years ago, they haven't improved any since.
Leo

 

[Charles]

Problem solved! It was three fold. The connector where the relay plugs was corroded so I was getting erratic readings. One minute I'd have 12v, then next 4v, which puzzled me and caused me to trace the wiring back more. When I did that, I noticed that the P.O. added in a switch of sorts, I'm assuming to turn the light off when starting?? Anyhow, got rid of that, went back to the relay connector and noticed 12v. Jumped the red/yellow and blue/black and the headlight turned on. Yay! Plugged the relay in, no headlight. Boo! So for now, I have a jumper in there and I plan on getting a new relay. Also, the P.O. got rid of the headlight fuse. Grrrr! I think I'm gonna rewire all the crap that's been altered.

Off to ride at night!

 

[toofdoctor]

Had a similar problem on the 81 xs400 I am currently restoring. No power to headlight, no power to the headlight relay or handlebar switch. Pulled cover off the relay and noticed a diode inside the relay disintegrated. Replaced with a known good one and still no headlight. Studying the wiring diagram, I noticed a diode symbol in one of the wires running from the alternator to the headlight relay. After much probing through the mass of tape and insulation surrounding the harness, I found a diode burried in the harness surrounded by clear heat-shrink type insulation that was melted with a broken diode inside. After replacing the relay and the well-hidden wiring harness diode, the headlight works perfectly! Mine was located where the harness passes near the left air filter box and frame. May have gotten pinched or shorted over the years. I will post some pictures I have taken during this restoration and an intro when I get near completion.

 

[dgaige]

Had a similar problem on the 81 xs400 I am currently restoring. No power to headlight, no power to the headlight relay or handlebar switch. Pulled cover off the relay and noticed a diode inside the relay disintegrated. Replaced with a known good one and still no headlight. Studying the wiring diagram, I noticed a diode symbol in one of the wires running from the alternator to the headlight relay.

This was great to learn about. I have a similar problem (headlight doesn't work - checked fuses, bulb etc., and no power to switch or light. Need to find the "relay"?, and the other diode? Where did you get replacement diodes?

 

[corruptmind]

Radioshack

 

[XSLeo]

A diode only lets power flow one way. If you installed it backwards, no power will flow.
Try reversing the diode.