Vintage Dirt Bike Q & A

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 Post subject: Voltage too High
PostPosted: Sun May 07, 2017 6:57 pm 
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Joined: Tue Mar 28, 2017 10:38 pm
Posts: 14
I have a 1974 Mt 250.

The headlight is burned out. It keep melting them.

When it idles it is producing 7+ volts. If you rev the bike it produces 9v.

It has a Stanley resistor that is plugged into the light circuit.

I am just not sure what the problem is.

I am thinking of buying a new rectifier. But I do not think this will help much. Pretty sure it will not do a lot to regulate the voltage.


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 Post subject: Re: Voltage too High
PostPosted: Sun May 07, 2017 6:59 pm 
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Joined: Tue Mar 28, 2017 10:38 pm
Posts: 14
I should mention that I have tested the rectifier. It passes the test in the manual (continuity in one direction but not the other for each of the pairs of wires)

Also the Stanley resistor has a resistance of about 3 ohms.


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 Post subject: Re: Voltage too High
PostPosted: Sun May 07, 2017 7:44 pm 
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Joined: Sun Nov 30, 2008 12:10 pm
Posts: 995
Location: Connecticut, USA
If the Stanley resistor is in-circuit, it should get quite hot when the headlight's on. If you haven't already, I'd check the resistance (engine off, obviously) from the white headlight wire to frame ground rather than across the resistor, to make sure the resistor is actually connected to the harness & doing something.

To test the rectifier, check the DC voltage on the battery cables with the engine running & battery disconnected. 7-8VDC would be ideal.

Ray

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


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 Post subject: Re: Voltage too High
PostPosted: Tue May 09, 2017 2:22 pm 
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Joined: Tue Jan 27, 2004 8:51 pm
Posts: 136
Location: Gilbert, AZ
Isn't the rectifier for the charging circuit so you don't get any AC into the 6V battery?
I think the headlight works off a separate coil from the rest of the 6V system.
Mine had a broken wire in the resistor and poor connection at the single wire to harness.

Rick


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 Post subject: Re: Voltage too High
PostPosted: Tue May 09, 2017 9:32 pm 
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Joined: Tue Mar 28, 2017 10:38 pm
Posts: 14
I ended up just using a 12 v bulb and bulb holder from a Yamaha Moto 4.

The bulb seems bright enough and seems to work. Seems less likely to burn out a 12v bulb.

So problem solved... sort of.


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