Need alternator help

[pscook]

Okay, it's not a GasGas, but the basic principles should apply.

I need help troubleshooting the charging system on a 1980 Honda XL185S. This engine is installed in a 1978 CB125S chassis. everything works great, except that the charging system won't keep up with the demand from the system. Allegedly the charging alternator puts out 108w at 5000 RPM. However, I can't validate this claim. What I do know is that with the headlight turned off, the battery will charge right up to 6.8 volts. When I turn on the headlight, the voltage plummets and will drop below 6 volts, stabilizing around 5.8 volts. The headlight is 35w, tail light is 3w, and the brake light is 10w. I believe the turn signals are 10w each (plus the thermal flasher), but let's not worry about the signal lights right now. Just the headlight, tail light, and intermittent brake light.

Here is what I know:

Configuration: Four wires come out of the alternator: One goes to the headlight and rectifier (Yellow), one goes direct to the rectifier (Pink), one goes to the CDI (Red/Black), and the last goes direct to the headlight switch and then ties into the to the rectifier (White, changing to White/Yellow in the loom).

Resistance between the Pink and Yellow (not running) is 7 ohms. When running, testing between the Pink and Yellow shows 7-8 VAC. When under load (battery connected), Pink and Yellow each provide 4.0-4.7 VAC (one is lower, I can't remember which one. important?).

The White wire (direct to headlight switch, then ties into rectifier through the yellow wire) provides 27-28 VAC, amperage unknown.

Red/Black works just fine, as the bike runs like a champ.

If I eliminate the charging system and just run the bike and headlight, the light is quite bright. Great idea, not an option. However, if I put a diode in line, I may be able to isolate the headlamp from the battery circuit. I just thought of that, will test. It would be a diode, right? A one-way circuit to prevent the headlight from drawing from the battery. Or just start cutting wires to isolate the headlamp circuit. Hm.

Anyway, what is going on? What should I test to prove that everything is working properly? Any suggestions to improve stator out put? This is warranty work, so I don't want to spend any more than I have to, if at all possible.

 

[Zman]

I am not familiar with that bike and its wiring schematic. It is possible that you have a bad connection (point of increased electrical resistance) that is providing a voltage drop across that bad connection. That voltage drop consumes part of the overall system voltage and reduces the voltage measured at the bulb and battery. The voltage across the bad connection increases with current flow. There are a few things I would check. 1. Is the timing plate screwed down tight. 2. Are all ground connections tight? 3. Is the battery good? Are any of the alternator coils burnt looking? Overheated coils can be wounded and have internal winding to winding shorts that will reduce your output voltage. 5. I have had wire strands break internally in the harness from flexing especially near the steering stem. This can add to increased resistance and a voltage drop. Some old bikes used the head light assembly as a junction box for much of the wiring. The wire insulation may still be intact with broken wires internally. You may be able to see the insulation darkened or bubbled. A good check is to bend or pull on the wires across the flexing point the broken ones will be apparent. I suppose another option to make it go away is to reduce your overall electrical load and convert some or all of the bulbs to LEDs.

 

[pscook]

Last night i "fixed" the problem. On the XL185 wiring harness, the alternator goes direct to the headlight switch, no further. On the CB125 harness, the alternator goes to the headlight, then back to the rectifier. I simply unplugged the jumper that goes back to the rectifier and went straight to the headlight. Instant correction of the charging problems. 7.2 vdc at the battery, 7.2 vac at the headlight. Battery happily charging.

Somehow, the CB125 feedback loop was interfering with the headlight leg, creating too much demand or because of a lack of diode between the circuits. Regardless, the headlight is on when the bike is running, and the battery charges at the same time.

If it were (still) my bike, I would tear it down and sort out the wiring so the battery was running the headlight when it wasn't running, but the alternator was powering during running, but it is 95% sorted so I'm satisfied. And the new owner of the bike was very pleased that I figured it out and fixed it.

Now I have extra XR185 and XL185 rotors. Anyone need paperweights?