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

[notme]

Are you able to measure the resistance in ohms of the low beam (troublesome) wire, by connecting one multimeter probe to the bulb end, and then the other probe to various points along the low beam wire at connectors as far back towards the battery as you can?

Or did you find the problemo?

[xwhatsit]

Are you able to measure the resistance in ohms of the low beam (troublesome) wire, by connecting one multimeter probe to the bulb end, and then the other probe to various points along the low beam wire at connectors as far back towards the battery as you can?

Or did you find the problemo?

Not yet -- I have tomorrow free so I'm going to get stuck into it then. I thought about measuring the resistance just between the terminals on the bulb socket, but it's negligible (less than 1Kohm from memory, whatever it is it's on par with the parking light bulb).

[pete376403]

If this is a double filament bulb (ie hi & lo beams) then you're checking between one of the bulb contacts and earth, right? There shouldn't be any voltage between both of the contacts (unless the dip switch is stuffed)

[notme]

Not yet -- I have tomorrow free so I'm going to get stuck into it then. I thought about measuring the resistance just between the terminals on the bulb socket, but it's negligible (less than 1Kohm from memory, whatever it is it's on par with the parking light bulb).

It should be a simple test to check the obvious - does the low beam wire have significant resistance in it, which would allow voltage at the terminals that would then collapse with any significant load (i.e. multimeter is not significant, bulb is).

If you find more than say 1 Ohm along the low beam wire then you have a corroded or broken connector or wire somewhere along the way, so 'd suggest you leave one probe at the buld connection to low beam, and go along the wire as far as possible until you start reading significant resistance. If that point is after say some connector block, tidy up the connections or check for corrosion etc, or if it is after say the headlight switch, open her up and check the state of the contacts.

HTH

[CookMySock]

If you find more than say 1 Ohm along the low beam wire then you have a corroded or broken connector or wire somewhere along the way [...]

This is true, but very hard to measure down to the one ohm area without a quality test instrument. It is much easier and quicker to energise the circuit and prod along it with a test lamp until you find a discrepancy. The eye can measure varying testlamp brightness quite well.

HTH
Steve

[xwhatsit]

If this is a double filament bulb (ie hi & lo beams) then you're checking between one of the bulb contacts and earth, right? There shouldn't be any voltage between both of the contacts (unless the dip switch is stuffed)

Yup! Haven't quite lost my marbles yet!

This is true, but very hard to measure down to the one ohm area without a quality test instrument. It is much easier and quicker to energise the circuit and prod along it with a test lamp until you find a discrepancy. The eye can measure varying testlamp brightness quite well.

HTH
Steve

Yeah, that's one reason why I didn't bother checking under 1Kohm, the multimeter I'm using is a $10 piece of crap from DSE; hell, even touching the outside of female sockets is enough to change the resistance value markedly (the contacts within are quite clean and conduct well, but the blunt face of the connectors often have a little bit of corrosion).

[notme]

Yup! Haven't quite lost my marbles yet!

Yeah, that's one reason why I didn't bother checking under 1Kohm, the multimeter I'm using is a $10 piece of crap from DSE; hell, even touching the outside of female sockets is enough to change the resistance value markedly (the contacts within are quite clean and conduct well, but the blunt face of the connectors often have a little bit of corrosion).

Seriously? Even a $10 POS meter should be able to measure to 1 or 2 ohms...out of interest, connect the probes and measure the resistance of the probes and wires by shorting them together, the reading should be < 1 ohm and shouldn't jump about much.....then try a lightbulb - should be 10's of ohms. If the meter can't read the difference between these 2 things, chuck it, because if it really is that bad it will be useless for checking other things - e.g. your charging system (was that 12v or 2V?).

Back to the original problem....if you must use the test light, try the same trick as I suggested with the meter, i.e check the length of wire that feeds the low beam, but clip one side of the test light to the frame or battery negaive, and then probe back along the low beam wire at each connector or switch or relay or whatever until the light comes on. Where it lights is the last point where you have a good power feed to the low beam wiring, so if for e.g. it lights on one side of some connector but not the other, that connector is corroded or loose or broken.

[Ixion]

I got a couple of them DSE meters. Checked them against a good(ish) one (Fluke) they were reasonably accurate. measure down to 1 ohm OKish. Course, at that level contact is important, and corrosion on exposed wire ends and stuff. But I think the meters are OK. I didn't check high voltages, that is where I would expect to see inaccuracies.

[davereid]

Most cheap meters are perfectly adequate for fixing motorcycles. They will have a degree of accuracy at least 10x what you need.

As commented by others in the thread, the best tool for diagnosing a problem with lights and switchgear is a $10 test bulb.

Apply the rule of halves, and the rule of movement to your testing.

1) Find a good earth for the probe, and get a visual idea of the test lamps brightness on battery POS.

2) Go to the FAR end of the circuit under test - in this case your headlight. Turn the headlight on, and with your test lamp, probe each of the wires in turn.. HIGH / DIP / EARTH chaging the DIP switch as required.

- Bright test lamp = circuit OK
except if any glow on test lamp at all on EARTH - this means a faulty earth.
Dim test lamp ? Hmm.. are you measuring the right wire ? not looking at a feed via a lamp with a dead earth ? Are any of the lamps under test glowing weakly ? - dodgy earth. Could be a high-resistance wire or switch.. easy to find with...

3) Rule of halves/rule of movement.

Find the middle of the circuit, more or less. Exactly the same test as before, with the same answers. But you have now halved the amount of circuit to test.

Something dodgy ? rule of movement - switches fail first, then wires that have to move like steering head, then connectors, then (almost never) wires that don't move.

G'Luck !

[xwhatsit]

Well, Dave, looks like it was your `almost never' failing wires-that-don't-move wot failed. I think. I dunno. It's working now (for now?), so I won't touch it anymore!

This morning fiddled with some connectors, crimped new ones on, lights were working. Woot. Go to uni and back in the afternoon, on the way home, lights fail again. Shit.

I made a little test light (old indicator warning lamp, a metre or so of wire, and two fancy crimped-on connectors), earthed it to the same earth as the headlight, then traced it back. I finally got a light when I connected to the back of the bike at the fuse box, headlight fuse. So that means the wire between the fuse box and the high/low beam switch must be faulty, right? Probed and prodded and measured until I was 100% that was it.

So I spend half an hour separating that same wire from another loom, then installing it in place. Ran it up beside the existing wiring loom, used bits of drain tubing from old airboxes to shield the fragile little wire. Tested it -- lights work! Yay!

Go to ride down the road and back, and it's not working again. Just about in tears I've spent how many hours doing this and I thought it was sorted?

Do all the tests -- same wire. How can it be?

I realise that I was missing out one little wire in the middle. It's about 3cm long, simply goes from the fuse to the fusebox connector. Never moves, looks sturdy, fine. Swap fusebox with one from my spares collection, works first go. Stood up to 10km down the road and back. I hope that was it?

WTF?

I'm too tired to examine it right now, but I wonder what on earth could've failed in the fuse box. It seems to be an intermittent thing. Who knows.

I've still got that patched-on replacement wire in the loom. I'll leave that there because it's working now and I don't want to upset it

Thanks everybody for your help. I've bookmarked this thread for future reference.

[gijoe1313]

Huzzah! You have made the lekky gremlins run out of places to hide, so they had to jump ship .. probably to your cellphone or laptop now ...

You needed piccies!

[howdamnhard]

One thing I've learned with electricity is that even though the multimeter says the voltage is great doesn't mean if you try draw current it will work.Sounds like a bad switch contact.Eventhough voltage is great as soon as you apply a load the resistance is too great and result no light.Had it happened on my car headlights once,np brights.Turned out the metal contact on the switch needed tweaking to insure a better contact .Anyway thats my 2 cents,goodluck.

[Wired1]

Well I'm a bit late on this topic and it looks like you've found the solution, but in case it recurs here are some things that may help with the fault diagnosis.
Firstly, although bikes use a (generally) negative earth this is only used for the ignition and charging circuits. The horn on modern bikes is permanently supplied with positive and the horn switch grounds it to make it honk and the the neutral light is similarly connected to the positive and the neutral switch grounds it to make it glow but the lights all have both positive and negative wires run to them - ie lights do not rely on the frame earth to make them glow.
Secondly, if you can measure a no-load voltage with your meter of 12 or more volts at the headlight, and zero or bugger all volts with the light switched on then there is a high resistance on one side or the other of the headlight. The easiest way to find out which side is to run a lead from the battery negative to the negative side of the headlight (probably the black wire) and see if it goes. If it doesn't, repeat with the fly-lead running from the positive side of the battery to one or other of the other two pin on the headlight. If it is on the negative side then that's the easier one to find as it will be a loose or worn connector, poor fitting fuse or loose battery terminal.
I would put money on a high resistance in the actual switch if the bike is 20 years old or more, they get filled up with crap and the contacts lose their effectiveness.
You also mention the possibility of a fault in the headlight area when you put the headlight unit back in - assuming the bike hasn't been in a fire (been there done that) this might be something as simple as the insulation slipping back from a connector. Visually check them and also look for cracks in the cable insulation.

[xwhatsit]

The funny thing was, is that there was really low resistance; multimeter showed anything between 0 and 5 ohms, provided I got a good surface connection with the probes.

I sorted the headlight rim thing; turned out to be the metal barrel of the ignition (which I've mounted sticking out the back of the headlight) was touching the shell; the SR400 rim to Bandit 400 rim has conductivity or something, which the Bandit 400 rim to Bandit 400 shell does not.

[classic zed]

Its a fairly common problem on cars too, you will have a bad connection somewhere on the positive line, if both dip and main beam are affected the problem is either in the switch or before the switch. When you test the circuit "unloaded" and get a reading everything appears to work, as soon as you add a load i.e. a bulb the bad connection cannot flow enough current and so the voltage drops off.

Everyone must have met this problem.
Think of your car battery with a loose terminal, when you turn the key the dash lights come on, then you try and start it and you hear a "click" and everything dies. Wiggle the battery terminal and everytrhing works again. Its the same thing as any wiring connector