Headlight not working; voltage at terminals, but seems to drop to zero with a bulb

[xwhatsit]

Headlight stopped working last night. How many times has Motu et al told us `NEVER ASSUME' -- yet, I found myself with half the wiring loom for the bike apart before I realised that despite looking perfect, a fuse had succumbed to vibration and the top cap was loose and not touching the fuse itself.

So, replaced the fuse, lights work with a bulb plugged into the socket, yay. Took the opportunity to put my mint SR400 rim/glass on (instead of the munted Bandit 400 item that was there -- there's a tip guys, SR400, maybe SR500 too, the headlight shell and rim are interchangeable with the Bandit 400). Noticed with the headlight rim on, headlight was a lot dimmer -- must be a short somewhere, something touching.

The wiring in the headlight shell is a bloody mess. Bits patched here and there, exposed connectors. A good portion of the responsibility is mine, but previous owners have had a hand in it as well.

I fixed connectors, broke a wire (directly going to the left indicator -- need to sort that). Now no headlight at all.

Interestingly, I'm getting a healthy 14 volts at the headlight terminals themselves. But when I put a bulb there -- nothing. Thinking that the connectors might be duff, I plugged in my multimeter (which is getting 14 volts quite nicely), then touched a test bulb to the two prongs of the multimeter. Nothing comes out from the bulb, and the multimeter goes down to zero volts. Is this expected? I suppose I'm wiring the bulb in parallel with the multimeter.

Resistance between the low beam terminal and the earth is not excessively high when I get a good connection on the multimeter.

But the thing I'm confused about most, is that if I have 14 volts at the terminals, why doesn't the bulb light? I've tried three different bulbs, which were all working about 10 minutes ago and conduct electricity between their terminals (I would think that means they're not blown). EDIT: and yes, the bulbs work fine using the momentary passing switch for high-beam.

I've got half an hour to fix this or I'm on the bus

So it's all very frustrating.

[MSTRS:Me]

Sounds to me like the problem is not the connectors etc...but is in the handlebar switch? If it is on the 'dead' side then it is affecting the flow to earth.

[xwhatsit]

I think earth is fine -- the bulb has earth all the time, and then either the normal switch or the passing switch gives it positive voltage. As the passing switch lights it up OK, I reckon earth is fine.

I've noticed that jiggling wires around also stops the parking bulb occsaionally, so I may just hop on the bus in a tick then replace the whole wiring loom tonight.

[MSTRS:Me]

Mr Honda did strange things with wiring over the years. And yours being an 'older' example of his dark art, do not be surprised if the pass flasher is on a different circuit.

[MSTRS:Me]

Just thinking about this...if the power wire has voltage at the bulb end, there must be two power-in wires (high and low beam) so the earth is on the far side of the circuit from the h-bar switch...
This suggests that it is the earth side that is the problem. Can you not just jerry-rig another earth off the bulb socket?

[xwhatsit]

Mr Honda did strange things with wiring over the years. And yours being an 'older' example of his dark art, do not be surprised if the pass flasher is on a different circuit.

It is, yep. Pass flasher takes voltage directly from the reg/rec and feeds it into the highbeam terminal. The other terminals come from the left-hand switchblock (high/low switch), then they go back to fuse box (which was causing me initial trouble), then go into the right-hand switchblock (lights on/park/off).

The problem is, where is the fault? As there is next to no resistance between the earth and low/highbeam terminals with the bulb out and the bike off -- meaning the connection is good -- and with the bike running and the lights switched on (but bulb out), I'm getting between 13.8 and 14.4 volts between earth and low or highbeam terminals. Switching the lights on/off, highbeam/lowbeam produces the appropriate voltages at the terminals (either 0 volts or 14 volts).

However, plug a bulb in, and despite there being voltage there, there's no light. I know the connections and bulb is good because it works with the passing switch.

[xwhatsit]

Just thinking about this...if the power wire has voltage at the bulb end, there must be two power-in wires (high and low beam) so the earth is on the far side of the circuit from the h-bar switch...
This suggests that it is the earth side that is the problem. Can you not just jerry-rig another earth off the bulb socket?

As far as my small brain can comprehend, I don't think that's quite right. I'm pretty sure the earth is fine, as the bulb works with the passing switch. The earth at the bulb socket doesn't switch at all, it's always connected, so passing switch and normal headlight modes use the same earth. The difference is in the source of positive voltage.

If one has voltage at a socket but a bulb will not light, does this mean somewhere along this twisted snaking collection of wires and sockets something cannot handle a load -- in my imagination, a loose bit of wire gets hot, curls up, breaking the connection -- take the load away, wire cools back down and touches again. Obviously my multimeter provides next to no load.

[MSTRS:Me]

...The earth at the bulb socket doesn't switch at all, it's always connected, so passing switch and normal headlight modes use the same earth...

If I was thinking 'fast' enough, I'd have got to that conclusion as well.

For what it's worth, I've about exhausted what I know about electrickery...
I do know that voltage is only the carrier wave for other stuff. What has happened to the Watts? Or Amps?

[CookMySock]

Interestingly, I'm getting a healthy 14 volts at the headlight terminals themselves. But when I put a bulb there -- nothing. Thinking that the connectors might be duff, I plugged in my multimeter (which is getting 14 volts quite nicely), then touched a test bulb to the two prongs of the multimeter. Nothing comes out from the bulb, and the multimeter goes down to zero volts. Is this expected? I suppose I'm wiring the bulb in parallel with the multimeter.

But the thing I'm confused about most, is that if I have 14 volts at the terminals, why doesn't the bulb light?

It does this because there is a high resistance in the + line somewhere. The multimeter is high impedance test device, and the test bulb is a low impedance device, so they give conflicting results.

Nevertheless, it does indicate that the wiring is solid, except for perhaps a corroded connector or fuse. Reconnect the headlamp, and follow the wiring with the tester or the multimeter until you find 12V, and theres your fault.

Another way, is to remove the battery completely, and ground the positive and negative terminals to the bikes frame, and then follow the wiring with the Ohm meter.

Good luck. Its all time and trial and error. Once you have tried the possible, begin on the impossible, and you will find something you hadn't thought possible.

Steve

[Katman]

For what it's worth, I've about exhausted what I know about electrickery...

Yes, we noticed.

[MSTRS:Me]

Yes, we noticed.

Thanks for pointing out what I've already made public.
What I noticed, and feel honour-bound to point out now, is that you made no attempt to be helpful to the OP.

[MSTRS:Me]

I've had a chat with an auto sparky mate, and he tells me that it will most likely be an earth problem in the high/low switch block. The on/off switch is sending power, but the hi/lo switch controls what happens to it. And yes, the flasher switch is on a different circuit altogether, so all that proves is that your bulb is ok.
He also says that using the volt meter is not going to be helpful, you need to be using a probe light to test the circuit.

[xwhatsit]

I've had a chat with an auto sparky mate, and he tells me that it will most likely be an earth problem in the high/low switch block. The on/off switch is sending power, but the hi/lo switch controls what happens to it. And yes, the flasher switch is on a different circuit altogether, so all that proves is that your bulb is ok.
He also says that using the volt meter is not going to be helpful, you need to be using a probe light to test the circuit.

Thanks for having a chat with your friend. I find this interesting -- I'm next to useless at all of this electrickery magic, but I don't think the switchblocks actually have an earth, I think they're just simple switches that either let voltage through or not. It's all positive voltage going through them, the high/low switch just chooses which wire to send it down, and the on/off switch stops it from getting to the high/low switch in the first place. But then again my grasp of all of this is tenuous at best.

Anyway, after catching the bus tonight, I've decided I'm not doing that again in a hurry! That's a whole week's petrol money blown on one trip to work. So I'm going to rig up a temporary wire directly from the main positive line to the low-beam terminal of the bulb and deal with that until Wednesday, when I can skip lectures and have the night off work. Then I think I'll rig up a test lamp, wire one end to the cylinder head fins and then probe around with the other end down the connector blocks until I find where the fault is lurking.

Spoke to girlfriend's father, who is an electrical engineer (but sometimes fiddles with house electrics as well, even though he's way more qualified than that), and through a ghastly manglage of Cantonese and English he thinks that I might be onto it; the multimeter is too light a load for this bad connection to surface. Suggested the same thing as your auto sparky mate, that I need a probe light or something, preferrably a wattage close to that of my headlight bulb.

Fun, fun, fun!

[xwhatsit]

Well I've run a wire from the magic spare positive terminal in the wiring loom (it's on the wiring diagram too -- black wire, terminates in a little round female connector -- is it there on purpose for emergencies? If so, thanks Honda!) to the lowbeam terminal and we have lights.

The dimming lights when connecting the headlight glass/rim to the shell I mentioned before seems to be a problem with the SR400 glass/rim. It would seem the SR400 rim conducts electricity to somewhere which causes a short and the lights to go dim. Original Bandit 400 item does not exhibit same behaviour, problem solved. Bandit 400 item also has much larger rubber shroud (can't get the parking light in, however) which may help, or may not.

Went for a wee pootle down the end of the farm and back. Quite nice riding around in the middle of the night wearing naught but a helmet, dressing gown and slippers. I added the boxers as a last minute afterthought, after all, there is the fuel tank and the wind which could cause issues for onlookers.

I've packed a fairly comprehensive toolkit for tomorrow just in case, so if anybody sees me around varsity or Parnell with oily hands and a bike spewing spaghetti wiring out of it's blank eye, feel free to come over and yell abuse.

[CookMySock]

Good work bud. You will be a pro at this before long.

If you suspect some part of an electrical circuit is not conducting properly, use your voltmeter to measure across the suspect part while it is under load, and it will show a few volts if there is an resistance there.

Steve