Vintage Dirt Bike Q & A

OK its simple as my tail light being out while my brake light still works. So i check the bulb and yes the tail light filament is burnt out, I replace the bulb and same thing was wrong just the bulb was fine. I cleaned all the contacts and but contact grees on everything and im extreamly confused.

All the other lights work, I found the battery was bad so i replaced that on top of making sure the entire wiring harness was intact..
any ideas?

I have the same problem. My tail-light blows out constantly. If you figure it out, please PM me. Thanks.

Once you replace the battery you may find the problem resolved.

The brake & tail light rely upon the battery as a voltage regulater.

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Big Block Cadillac Forum

I did replace the battery, because the old one was bad. My new numb isn't burnt out yet, the tail light filiment wont light but the beak light will. My blinkers blink fast and my headlight/ tail light isbright. I had this bike for 4 months with out a problem.

Oh! Right when I had the first blown tail light issue my neutral light and highbeam light went out. Does this raise any alarms for anyone?


If your tail light keeps blowing then you prob need a New battery.

These bikes have really simple electrical you just need a voltmeter and to start checking, its not fun but its not hard. The tail light has three wires one for ground one for break light and the other for the running light. If one is not working you need to start at the bulb and work your way back towards the engine makeing sure all switches and connections are clean and connected properly. The bike is old the wires are also old and going to be hard and they do crack and short out. The head light runs on a different coil than the tail light does. The battery is for signals and tail light and horn. If the signals blink well than the battery is good and if its staying charged the battery coil is also good. I would look into the wires, thats probably where your problem is. The headlights do burn out I get some to last a long time and others burn out right away even the new ones. I just assumed some of them have sat so long the sealed beam lost its seal and then burnt out super fast.

JM is also misstaken about the Rectifier the mt250 did come with one. The coil puts out an ac current thats is fed thru the rectifier into the battery. Its located on the airbox I had to relocate mine under the seat so I know they use them. It also uses a resistor on the hi beam for the headlight its a little green single wire tube on the headlight bracket they do get hot with the bike on just so you know.

The rectifier converts AC to DC. It does not control voltage. You need a regulator to control voltage output. The MT relies upon the battery to regulate the voltage to the taillight.

~JM~

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Ha I know nothing of electrics just that the coil put out ac and the rectifier changed it into dc. I have called them regulators for a long time and dont know the difference betwen the two or how they do what they do. I had to check the parts fiche to make sure it was a rectifier and not a regulator

I would measure both the DC and AC voltages across the battery terminals and report back with the results, which I'm guessing will indicate a rectifier failure (apparently common in the MT and MR models).

If a shorted rectifier is allowing a big AC voltage onto the DC bus (red, brown, brown/white, black, etc. wires), you can expect all sorts of things to fail.

WIring Diagram:

http://alp-sys.com/vintagedbforums/down ... &mode=view

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric