Topic: Headlight dead - tested everything - strange results

[Dr. Noisewater]

1975 CB550. I noticed sometime last week that my headlight wasn't on and flipping the hi/lo switch didn't trigger any response, not even the hi beam indicator on my speedo.

I've been searching like crazy and have tested practically everything. Here's what I know...

- Fuse is good, tested for continuity, swapped in a new one, etc.

- Tested the bk/r wire from the starter button to make sure it wasn't cutting voltage full time. Checked out.

- Ran a jumper directly from the (+) battery to low & high beam leads and headlight works just fine.

- Unplugged the headlight connector and tested voltage on female end of low and high beam leads through the connector... Roughly 12 volts.

- When the headlight connector is unplugged the high beam indicator works as it should.

- Plug the connector back into the headlight and the high beam indicator stops working AND I lose voltage entirely on both low and high beam leads. I repeat, everything works as it should until I plug in the headlight.

- Tested with a brand new, identical headlight from the wife's bike and still nothing.

- Unplugged ground supply for headlight and jumped it directly to a solid ground and still nothing.

I have no idea where to go from here. I found similar threads but they either went unsolved or the poster realized it was actually one of the things I've tested. Please someone help. Thanks guys.

[Frostyboy]

- Ran a jumper directly from the (+) battery to low & high beam leads and headlight works just fine.

- Unplugged the headlight connector and tested voltage on female end of low and high beam leads through the connector... Roughly 12 volts.

I think you've answered the question right there. Reattach the connector to the bulb and measure the available voltage. What you'll be looking for is voltage drop. Put your meter on the supply wire to the fuse first. If battery voltage present, measure the voltage at the other end of the fuse, or more importantly at the wire coming off the fuse block. If there's a substantial difference, there's your answer.
To measure the drop across the fuse block, place the + lead of the meter on the supply side of the fuse block & the - lead on the wire out of the fuse block. Any voltage you read will be the drop across those connections.

[goldarrow]

Follow wiring color codes if your harness is still stock. Schematic is helpful.  Color of 40 yr old may look off also especially light blue, light green

[Dr. Noisewater]

Frosty, I checked the voltage on both sides of the headlight fuse and there isn't any substantial difference. The fuse box is the updated Hondaman version with about 1500 miles on it and a new fuse makes no difference.

goldarrow, I've gone through all the wiring on this bike a few times. Cleaned all connectors and the wires themselves to bring the color back. I tested power at all connectors between the fuse box and the headlight. Everything seems to check out.

This is what I can't understand; how voltage checks out at the headlight connector with the bulb disconnected, but I lose voltage once I plug it back in. Even when I test it with a brand new bulb.

[Thamuz]

Can you try a different plug? The one off the other bike?  If it gets juice until you plug something in, there might be a issue??

[Don R]

Aliens, it has to be.  Can you eliminate the switch from the circuit? It's almost like something changes under load.  Did you plug in the light part way and check the voltage at the connector?

[Don R]

Does a 75 have a headlight switch or is the headlight always on and the starter button shuts off the light when the starter is engaged? My 76 needed a new right switch, I know it doesn't make sense though.

[Frostyboy]

Frosty, I checked the voltage on both sides of the headlight fuse and there isn't any substantial difference. The fuse box is the updated Hondaman version with about 1500 miles on it and a new fuse makes no difference.

Did you check it with the bulb connected? Remember that you won't find where the voltage drop is occurring unless you have a load (the bulb) connected.

Humor me, connect the bulb & turn the light on. Measure the voltage at the Br/R wire going to the switch.
If the voltage is near battery voltage, place one end of your meter on the blue or white wire (whichever you tuned on) & the other end of the meter to the Br/R wire into the switch. If you read a voltage, that will indicate a drop across the switch contacts.

If the voltage at the Br/R wire is low, start working back towards the fuse block to find the bad connection.

[bjbuchanan]

If voltage drops significantly with load that is a classic sign of a bad battery. IDC how old it is, it can still be bad. Otherwise you have seriously corroded connections

[Dr. Noisewater]

Don, yes the 75's headlight is always on and the start button cuts it to direct power to the starter. I found some threads where TT suggested testing the start button so I did and it checks out fine.

Thanks Frosty, I'll give that a try tomorrow.

[Dr. Noisewater]

Well, I tested everything with the bulb connected and caught a break midway. I turned the key on to check the voltage from the black wire going to the starter button and BOOM headlight on, I press the start button, and headlight off.. with no return. I checked the voltage at the black double connector and had 12v, but it seemingly wasn't getting through the start button to the bk/r wire - the wire that supplies power to the HL fuse.

What still confuses me though is if I disconnect the bk/r wire at the bullet connector near the coils, I DO have 12v on the male end from the start button. I don't get how voltage can travel through the start button to that point when that connector is disconnected, but can't when it IS connected. Does that make sense? Can someone explain this to me?     

Now my thoughts are how can I bypass the start button all together and wire the headlight so the start button has no control over it? This would obviously disable the start button, but I really don't mind kicking it. A baby could kick start this thing.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               

[Don R]

Or you could replace the right hand switch assembly.

[Frostyboy]

What still confuses me though is if I disconnect the bk/r wire at the bullet connector near the coils, I DO have 12v on the male end from the start button. I don't get how voltage can travel through the start button to that point when that connector is disconnected, but can't when it IS connected. Does that make sense? Can someone explain this to me?     

Put into simple terms, what you are seeing is this: Regardless of resistance in a circuit, if the circuit is open (ie: no load applied to the circuit) you will read battery voltage. It's not until a load is applied, (in this case the headlight) and current actually flows through the circuit, the voltage will drop across the resistance.
If you were to measure the resistance through the switch contacts in your start button (B to B/R), I think you'll find a high resistance. Use the ohmeter function of your meter, unplug both wires from the harness for this test. Anything above 0 ohms indicates a problem. I think there's a few posts here regarding refurbishing R/H controls.