Stupid? Simple Electrical Quandary - More power, less light! : Grassroots Motorsports forum: Grassroots Motorsports Magazine

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Jan. 31, 2012 3:39 p.m. aircooled SuperDork

I think this is a simple problem, but I find it so strange I thought I would post here to see what everyone thinks. The the problem is:

Motorcycle, 12v, with the headlight wired directly to the alternator (one wire to the headlight, the other to the voltage regulator).

Symptom: Light comes on when the engine starts. When the engine is revved, the light dims, then goes out! Does the same on high or low beam (low beam just goes through a resistor).

What I have measured: Wire that goes to the headlight puts out 5 ish volts at idle, ramp up to 8ish volts when revved. Battery voltage is steady (VR is good).

I still need to do some more investigation (e.g. direct wiring the headlight to the alt wire, bypassing the switch and wires, swap lights etc) but it seem absolutely bizarre that the light would dim as it receives more voltage. Also strange is that this is after swapping the engine and not changing anything, and it worked before. Of course I might have messed something up but it is really simple.

Any ideas? I am thinking ground, but I have no idea WHY it would do this even with a bad ground.
Jan. 31, 2012 4:00 p.m. HappyAndy HalfDork

What type of bike are we dealing with? Its not a dirt bike with no battery and a "lighting coil" is it?
Jan. 31, 2012 4:02 p.m. Dr. Hess SuperDork

You have this wired up all wrong. Your headlight should be in parallel to the rest of the electrical circuit. You have it in series. Should be: Alternator to regulator, regulator output to battery. Headlight connects on the output side of the regulator. After a switch. What you are seeing in the headlight is the voltage differential between the alternator and input of the regulator. Uh, "Results are highly unpredictable."
Jan. 31, 2012 4:04 p.m. ransom Dork

EDIT: I think some of what I've said below unnecessarily complicates or otherwise is obviated by Dr. Hess' comment.

Focusing on just the part where the light gets dimmer as voltage increases, it sounds like the light's getting 12V positive from the battery one one side, and the output of the alternator on the other.

That way you're getting low voltage at start, and as the engine revs, the voltage across the bulb drops.

When the alternator's putting out 12V, you have 0V across the bulb, thus no light.

I'm a little puzzled by the description of one wire to the VR and one to the headlight from the alternator... I would think that the one to the headlight would also go to the battery +, ignition swich, etc... And that the headlight would be wired to the +...

The way you describe it, it makes it sound like the headlight's the only thing on the output of the alternator, which I'm sure isn't what's going on, right?
Jan. 31, 2012 4:04 p.m. aircooled SuperDork

Its a 77 Honda XL125 with a Lifan 200cc 12v motor in it.

And yes, I believe it is using a "lighting coil" if that is the same as wiring the headlight directly to the alternator. The stock engine uses that system, so I did the same thing put put a 12v headlight in.

I think Hess might be onto something with the regulator thing.

It IS the way the stock system was setup though. I even checked the stock wiring diagram to confirm.
Jan. 31, 2012 4:10 p.m. HappyAndy HalfDork

After reading the first post more carefully, I think Dr Hess has it exactly right.
Jan. 31, 2012 4:10 p.m. aircooled SuperDork

Here is the wiring diagram for the bike (correct year), a magneto fired (not battery powered) ignition btw:

As you can see, the headlight is wired to the alternator through the switch. Yellow wire off alt, to white wire, to blue, to switch, to HL

Here it is as I have wired it:

Link to larger version: XL125_/Lifan_Convert_v6-1.png

As you can see (hard to see yellow wires off Alt), I pig tailed off one of the alt wires for the HL. A bit of a best guess for me, but it seemed to work, until now.
Jan. 31, 2012 4:17 p.m. wearymicrobe HalfDork

aircooled wrote:

Its a 77 Honda XL125 with a Lifan 200cc 12v motor in it.

Lifan motor, make triple sure that the voltages coming out of each wire is correct. I have had a few 150cc's wired with the wrong color wire for the 12v out.
Jan. 31, 2012 4:27 p.m. aircooled SuperDork

BTW - I didn't want to wire the HL after the regulator because I did not want the HL to draw from the battery when the bike was off (just trying to be as easy on the Batt as possible).
Jan. 31, 2012 4:37 p.m. HappyAndy HalfDork

aircooled wrote:

BTW - I didn't want to wire the HL after the regulator because I did not want the HL to draw from the battery when the bike was off (just trying to be as easy on the Batt as possible).

According to the schematic you posted, the head lights don't draw directly on the battery. Follow the yellow circuit. I wouldn't be surprised if the head lights worked with out a battery hooked up.
Jan. 31, 2012 4:52 p.m. aircooled SuperDork

Yes, that was the idea. I wanted the bike completely functional if the battery was dead like it was stock. The new engine has an electric start, but also has a kick start.

I love the ridiculously simple and rugged magneto setup. A year or two later they went to a battery power supplied ignition, so if the battery died, no start (unless kicking it would supply enough power to get through the dead battery and fire the ignition, but I doubt it).
Jan. 31, 2012 5:08 p.m. mad_machine SuperDork

I would start by taking voltage readings from both wires that feed the headlight. Something is oddly amiss
Jan. 31, 2012 8:22 p.m. aircooled SuperDork

OK, I figured out what it was. Yes it was pretty stupid:

I reversed the wires being connected to the voltage regulator. The HL shared yellow wire is supposed to go to the pink regulator wire, not the yellow. I think it was just dumb luck I got it right the first time.

Although I am pretty sure this setup is less then ideal. I think the headlight draw may be causing an over voltage situation at high RPM ( I tend to blow tail lights, the only light on all the time normally).

Anyone know any VR theory? I guess I need to look some things up.

BTW - this is not any kind of factory setup, I just came up with it myself.
Feb. 1, 2012 9:09 a.m. ditchdigger Dork

How are you liking the Lifan 200 upgrade? I am just about to buy one for my CL125.
Feb. 1, 2012 9:24 a.m. noddaz Reader

My 1st imprerssion was it sounds like you had the headlamp wired up as a idiot light...
Feb. 1, 2012 10:47 a.m. aircooled SuperDork

ditchdigger wrote:

How are you liking the Lifan 200 upgrade? I am just about to buy one for my CL125.

I like it. I got two of the "black" 200cc engines very cheap, a bit over $300 a piece (200cc silver engines go for about $550). Never really seen the black engines anywhere else again, supposed to be a bit racier (they do look nice at least). I see they now have a 250cc single. I might go for one of those if I needed another one (I wonder if the front sprocket is still for the smaller chain though)

The addition of a starter is great. Finding room for the larger 12v battery and the electronics is fun (I stuffed it above and below the airbox). Wiring is not that bad. Is your bike a magneto ignition? If it is, wiring the kill switch is the same as stock (ground it to kill), if not you will need to add a relay (cut the power to kill).

I did gear mine way up since I do almost all street riding. If you need some sources for front sprockets, let me know, I am pretty sure I have the options noted somewhere (mater of finding a cross match). The stock engines seem to be geared pretty low.

Reliability, in general, seems pretty good, but I suspect falls well short of the Honda engines. I just switched out the engine that had about 9000 miles on it. The issues I had with it were:

Gear indicator sensor (made of plastic) which is fitted to the side of the case near the sprocket broke in half for some reason and created a nice oil leak source.

Chrome was pretty cheap on the engine and wears rather easily (don't try and polish it). I think the black engine is the only one that has any amount of chrome anyway though.

It would slip down a gear sometimes if it wasn't solidly put into a gear later in its life.

I do have issues with blowing the tail light, but I am pretty sure that is because of how I wired it. I need to try something else. I probably need to wire the regulator sensing wire directly to the battery so it acts as a buffer(?).

The worst though was the starter clutch (disengages the starter when the bike starts). This started going away at some point and only gave me about a half turn of starter before slipping. I would have kept the engine in longer if this still worked right. Kicking it over was killing me, especially since it was getting harder to start.

I will eventually take the old engine apart to see what was going on, but I am not sure how I am going to source any parts for it. There seems to be almost no source. hooperimports.com seems to be the best source I have found, and they have very few parts (no piston kits, no gaskets etc.)

Let me know if you need any pics or suggestions.
Feb. 1, 2012 12:10 p.m. wlkelley3 Dork

aircooled wrote:

Yes, that was the idea. I wanted the bike completely functional if the battery was dead like it was stock. The new engine has an electric start, but also has a kick start.

I love the ridiculously simple and rugged magneto setup. A year or two later they went to a battery power supplied ignition, so if the battery died, no start (unless kicking it would supply enough power to get through the dead battery and fire the ignition, but I doubt it).

Figured that out the hard way on the XS650 I have. Has both kick and electric starter. Had a dead battery so I kicked it till my legs were tired, gave up and bought a new battery for it and kicked it and it started first try. Electric start worked also. I figured why kick when pushing a button would work and if it won't start then it probably wouldn't start by kicking since it has to have battery juice to start.