Headlight fading

[kheerema]

My headlight started to emit weak light on the low beam so I started riding with the high beam on.

Now the highbeam is weak, to the point where I can't safely ride at night. I'm not having any other charging issues and I've never monkeyed with any wiring.

Is this what happens when the headlight itself fails or is this a switch issue?

I've noticed the little lever between high and low beam is kind of loose, thats about it.

[crazydrummerdude]

If this happened in my truck, I'd say alternator.

If this happened on my motorcycle, I'd :ear .

[kheerema]

:scratch Listen? Listen for what?

[crazydrummerdude]

:scratch Listen? Listen for what?

Haha... an experts reply.

[bikerfish1100]

I'd be thinking voltage regulator, generator or battery, probably in that order. Not very likely to be the bulb or the switch, as those are pretty much yes/no functioning items- either they owrk, or they don't. If a switch (or bulb) is failing, it generally fails as works sometimes, does not work others; as opposed to always working poorly.

[sumran]

You will lose some brightness over the life of the bulb, but not very much. The bulb either works or it doesn't. It is likely a wiring problem. By the time a switch feels noticeably loose, it is usually shot. Since a complete failure would leave you in the dark, I would replace it. If that solves your problem, great. If not, look for loose or corroded connections. Don't forget the contact points of the fuse in the circuit.

Are all of your other electrical functions working properly?

[kheerema]

Everything else electrical is working fine. I replaced the battery when I bought the bike last February, so its not a weak battery. As a check, when I installed the battery I tested the voltage at the + terminal and it was good (greater then 12V) revving the engine a bit. I have a hard time thinking its something with the wiring because I haven't monkeyed with anything, and the bike is relatively "unmolested" but maybe something corroded over the course of this year.

I'll have to see if I can take apart the switch at the handlebar, its very loose and maybe thats the issue, although I would think it would either work or not work like you said, not kind of work.

Other then that I'll see if I can figure out how to check the regulator. :drink

[20774]

Is there a headlight relay in the headlight shell? Maybe this is failing and not latching or providing the necessary pass through current. I'm electrically challenged, so this might be a red herring...

Do you get substanitally more than 12v, like 13.5v or more at the battery when revving the bike? Slightly better than 12v is not enough to keep the battery up to snuff...

[bikerfish1100]

like Kurt said, your bike should be charging at (don't quote me on the exact #, this is an IIRC moment, with no manual in front of me) about 13.7v at 3K rpm or above. so "just above 12v" sounds like you might have a faulty VR.
I'm pretty positive that the headlight is not fused- it's considered a "got to have" item, so not much point looking there.

[kheerema]

Is there a headlight relay in the headlight shell? Maybe this is failing and not latching or providing the necessary pass through current. I'm electrically challenged, so this might be a red herring...

Do you get substanitally more than 12v, like 13.5v or more at the battery when revving the bike? Slightly better than 12v is not enough to keep the battery up to snuff...

The numbers on the voltmeter fluctuate quickly, but yes, I believe it was 13.5+ blipping the throttle. I'm not sure if there's a relay in there. Guess I should check!

[grossjohann]

Everything else electrical is working fine. I replaced the battery when I bought the bike last February, so its not a weak battery. As a check, when I installed the battery I tested the voltage at the + terminal and it was good (greater then 12V) revving the engine a bit. I have a hard time thinking its something with the wiring because I haven't monkeyed with anything, and the bike is relatively "unmolested" but maybe something corroded over the course of this year.

I'll have to see if I can take apart the switch at the handlebar, its very loose and maybe thats the issue, although I would think it would either work or not work like you said, not kind of work.

Other then that I'll see if I can figure out how to check the regulator. :drink

I'd put my money on the switch. Unless you have rigged one of those special relays to the bulb, all power should go through the switch. If the contacts are bad or weak, then this could be the source of the power loss.

If you want to test the switch theory, you could use a volt meter to measure the current before and after the switch. If the light is really dim, then you should see quite a drop...

[Boxerkuh]

You stated that you don't think it is the wiring.... well, the bike is a 75, so 32 years old and I have talked to enough electiricans that all have told me that wires do have a "shelf life". So, I would not rule out wiring.... I don't think it is the bulb, because it is working, I doubt that it is the battery, because it is fairly new. I doubt it is the alternator, because your battery would be suffering if it is weak. Since you have a "dimming" affect, I would check the voltage regulator and would check for loose connections. Take the switch apart and see if it is there. Good luck and keep us posted....

[20774]

The voltage regulator's job is to ensure that the rotor gets enough current to keep it energized so the alternator can produce the necessary charging current...or something like that. Seems to me that if the headlight is dimming, than every other circuit on the bike would also dim under your description. If the voltage regulator was not doing it's job, then the charge light would come on intermittently as well. I don't believe that was a symptom.

I think the problem is local to the headlight - switch, relay, wiring, bulb contacts, life of bulb... You could remove the bulb and try hooking it up directly to 12v with a good connection and see if the bulb works fine. I suppose you could also ohm out the bulb...and clean contacts at the same time.

[sumran]

It doesn't take much corrosion to create a significant increase in resistance in a 12 volt circuit. The environment we operate our motorcycles in is perfect for creating corrosion. You would not to do anything to cause it, it is just nature taking its course. If you get increased resistance you will dim the light. It is unlikely that the wire itself is bad. The connection points and insulation are the trouble spots.

If you have reduced voltage it will have the same effect. Reduced voltage at the point of origin is fairly easy to measure.

The switch is a mechanical device that is opening and closing contacts. If the plastic parts are worn to the point that the switch "feels" loose, there is not enough pressure on those contacts to make a good connection. The weak connection will cause arcing and speed up the demise of the switch. On a good day the switch will stop working. If you take the control assembly apart and bypass the switch with a jumper wire, your headlight will probably become bright. If so, you have positively identified your problem. I haven't taken this assembly apart, so I can't tell you the best way to bypass the switch.

A weak ground could cause your problem as well so check the quality of connection on the ground side of the circuit.

Bikerfish1100, you are much more experienced than me on these machines so you may be right about the fuse. But if that is the way they are designed, it a mistake on BMW's part. Any unfused circuit creates the risk of fire or smoking the whole wiring harness.

[ebeeby]

Don't assume the battery is OK. They short out. Cells go bad.

Re-test the battery voltage with engine off and then with engine idling and at 3000rpm.

[kheerema]

WoW it ended up being a really dumb thing (or I am just dumb). First of all, let me just say I'm new to BMW's and their funny controls and I'm also on a bunch of different bikes frequently!

The headlight bulb internals appeared to be A OK, but upon closer inspection it was cracked a little tiny bit around each of the 3 prongs. It looks like I must have hit a bump and it jarred it enough to crack the housing or something. I thought the headlight was becoming dull, because only the running bulb was working and I didn't realize these bikes had a little running bulb in there in the first place. I guess better something dumb then something expensive...

Now I just have to make sure I don't take the key out in the parking light position and leave the lights on during the day :(

[bikerfish1100]

IIRC, these bikes had ignition positions of "off" (everything off), "park" (ignition off, but parking bulbs in front and back on), and "on" (ignition on, but lights were then operated by the handlebar switch- consisting of the 3 positions of off, park and headlights). but i could be confusing with my later '78.

[RJM2096]

If you have a DC voltmeter you can measure the voltage drop across a component to see if it is getting the full voltage.

If you can somehow get to the connections measure the voltage across the bulb. If it says 8.5v and the battery voltage is 12.4v then you have an issue.

To find out where the issue is connect the VM to any two points in the circuit. With everthing connected and the switch on there should be no or little voltage drops. If you have a break or bad connection between any two points you will find the resistance and a higher voltage (almost 3.9 with this example) there.

This also works with DC starters, relays, etc.

The difficult part is being able to get to connections for the measurement.