Headlight Is Out

[Eastcoaster]

Recently had some Headlight Issues with an 80SG
Cleaned the relay contacts and all seemed fine now nothing

Troubleshooting

Red to Blue jumper (lights headlight)
Replaced relay with known working one (NO Light)
Replaced Lighting reserve (NO Light)
Put a voltage meter on Ground (all good)
Put a voltage meter on White wire running (Voltage when running)

Possible Diode? I have never seen one not sure how far down the harness it is.
Is there a replacement diode available?

Any other possibilities?
I am really stumped on this one electrical is not a strong point.

[3Phase]

(Copied from PM)

The OEM diode is still available from Yamaha but it's expensive:

Partzilla | Yamaha | Motorcycle | 1980 | XS1100SG | Electrical 1:
31 DIODE 3H5-81980-00-00 $14.31 $9.34

Another option is your local Radio Shack, they should have a package of 1N4007 diodes.

1N4001 diodes will work if you can't find the 1N4007.

The harness diode is just aft of the Headlight relay and you'll have to peel back some of the wrap to find it. I traced the White wire back to the Regulator/Rectifier connector and marked it so I could find and test the diode if I needed to check it again..



"The Diode" 3H5-81980-00-00


The Headlight relay also has a diode inside it that is a 'latch' to keep the headlight on if the voltage drops or the engine dies.



To test the relay on the bike, turn on the ignition switch and jumper 12V to the White wire up at the relay or down at the Regulator/Rectifier connector. The relay should click and the headlight should come on and stay on even after you remove the jumper until you turn off the ignition switch.

B = Ground
R/W = 12V from the 10A Head fuse
W = wire from the alternator and has the diode inline
L/B = 12V out from the relay to the RLU

[Eastcoaster]

Scott

Thank you for the relay test I put 12v to the white wire with the key on relay clicked and light lit right up like you said.
I am assuming the diode is bad? Also thank you for the part source I am hoping this rules out anything else and will order one today.

Gordon

[3Phase]

That's a pretty good indication but you should test the diode before you buy a new one and dive into the harness.

You can test the diode with a meter using Ohms or Volts.

For Ohms, disconnect the relay and Regulator/Rectifier connectors and set the meter to Ohms. Put the test leads on the White wire at the relay and harness connectors. The meter should show low resistance in one direction and when you swap the test leads it should show Megaohms or infinite resistance in the other direction.


To test the diode with Volts, start the bike and check the voltage on the White wire at the Headlight relay.

Set your multimeter for DC voltage. If the diode is working there should be somewhere between 5 and 7 volts DC.

If there's no DC voltage at the relay then the diode may be shorted, blown open or broken.

To check for a shorted diode that's basically turned into just another piece of wire, set the meter for A/C voltage. If the diode is shorted there should be somewhere between 5 and 7 volts AC.

If there is still no voltage, the diode is blown open or broken.

[3Phase]

Oops....

Actually, strike that. The DC voltage with the engine running and the relay 'latched' should be 10 to 14 volts DC.

I forgot about the diode inside the relay that straps 12V to the relay coil from the Red/White wire after the relay is engaged and the headlight turns on.

After it the relay is engaged, the inline harness diode blocks DC voltage from the Head circuit and the relay so it doesn't feed back down the White wire to the alternator windings and the Regulator/Rectifier.

[Eastcoaster]

No Resistance

checked for resistance from the white wire on the relay and each of the 3 leaving the regulator nothing at all either direction of the +- leads on diode test. Diode is ordered keep you posted in a few days

[Eastcoaster]

Not sure I did the diode test correctly.

I tested DC running I could not get anything above 6-7 volts at the relay white wire and 6-7 volts at the white wire at the voltage regulator. It will latch fine and light with a 12volt jumper. I tested the battery running and got 14volts so it appears to have charging output. I have cleaned the connections. Could the voltage be to low to latch the relay? Yet the battery is charging at 14 volts? Voltage regulator have any bearing on this?

[3Phase]

There may be a bad ground.

The Black wire from the relay is the ground wire for the relay coil so check the Black ground wires on the frame at the Regulator/Rectifier and make sure they're clean and tight.

Another way to check the diode is to unwrap the wiring harness and unplug the diode, then put it on the end of your 12V jumper to see if the relay works. One way the diode should let the relay close, the other way it should not.

[skids]

The best solution IMHO is to install a switch for the the headlight with a relay switch.

[3Phase]

I dunno, a switch will let you forget to fire up the light when you ride and you cruise along all fat dumb and happy thinking the headlight's on.

I put a jumper on the connector and got rid of the Rube Goldberg relay, I don't need it unless the battery's half dead or about to go tango uniform anyway.

Hail the kicker!

[Eastcoaster]

Diode

The new diode arrived just installed It did the trick!

3Phase your write up was spot on!

Thanks!!

[3Phase]

Glad it's working for you again, hopefully it'll last another thirty years!

Remember to 'look' at the Head warning light every now and then when you ride, it has a dim glow during the day that's hard to see.

The warning light only turns bright if one of the headlight filaments quits working and the RLU kicks in but the only warning on the instrument panel that the headlight gimcrackery failed to turn on the headlight at all is a dead warning light.

Being my usual observant self I rode around like that for some unknown part of a day until it started to get dark and I finally noticed it was staying kind of dark in front of me too. "D'oh!"