Lighting Issue

[ahead92]

Hey guys, I'm new to the forum and motorcycles in general, was hoping someone could enlighten me on a few issues. I picked up a decent shape 1970 TC120 for $250 on craigslist. The guy said it didn't run but after some new gas, points adjustment, and a new plug it fired right up and runs beautifully. The bike runs, shifts, and handles great.

My question is on the lighting/charging system. When I turn the bike to the second on switch ("night mode") the head light will come on but is extremely dim. When I flip the dimmer it almost completely goes out. I would naturally point to the magneto/coil assembly but everything seems to be in great working order and hooked up correctly.

If anyone knows anything about the electrical systems on these bikes I would really appreciate it. I know the TC120 is a rare bird but as far as I can tell the electrical system and engine seems to be a correct copy (if not really close) to the popular KT120/B100P style bikes.

If the battery charging system still works I have considered running an DC voltage headlamp wired to the battery as well as the brake light, instead of the current AC voltage running from the magneto. The bike no longer has blinkers and I do not plan to put them back on.

Thanks in advance for the help! looking forward to being a part of this forum!


Andrew

[pearljam724]

Im not familiar with TC 120's. But, all bikes work the same. Start cleaning up electrical terminals involving the lights. Often, when a bulb is about to blow out. It will lose it's brightness slowly before hand because the filament is barely hanging on by a thread. The filament could be slightly oxidized also. Replace the bulb first. Clean and check that all grounds are secure. If you have a hard time locating a bulb at a decent price. An automotive headlight can be often modified to fit and work on your bike. I would try to locate the proper bulb first. Electrical connections on old bikes or cars have to be very clean of oxidation for them to work properly.

[alanr]

chances are you have a flat battery.check horn for sound.these bikes will start with not much juice in the battery.

[jabcb]

Am not familiar with the setup on your bike.
But if its similar to other Suzuki dirt bikes of that era, your bike has a 6V split-AC/DC electrical system with some components running on AC & some on DC.

Have not seen a wiring diagram for your bike, but its likely similar to one that listed under:
viewtopic.php?f=29&t=4743&sid=bf95a3895 ... 507e0ed90d

The headlight probably is something like 25W/25W 6V.
A 25W/25W isn't very bright when everything is working properly.
If low beam is a good bit dimmer then it could be due to the dimmer switch or an electrical connector.
So you should check/clean the parts in the low beam circuit.

The replacement headlight is likely hard to find & rather pricey.
Your bike likely uses the battery as the voltage regular.
Despite being dim, it doesn't take much for a mistake or an issue to burn out the headlight.
So some forum members add voltage regulators to protect the headlight.

The bike likely a half-wave rectifier.
Switching to a DC-powered headlight would require quite a bit of rewiring as you will need to switch to a full-wave rectifier.

[rngdng]

My Yamaha 100 has that type of system. The headlight is very dim until you rev it up. Also, you need to have a battery in there, as it functions like a voltage regulator.


Lane

[johnbradshow]

I had the same problem with my old bike and I upgraded the headlight to LED kit from Xtralights online. The headlight is now very bright all the time since the LED takes very little power from the battery compared to the power-hungry halogen bulbs, give it a try...