1978 Yamaha DT125 (175) Electrical Issues?

[74Dusted]

SOLVED - See reply.

 

 

I think I've got a problem on my '78 DT175 (she started life as a 125, but wears a MX175 Top End). I'll run it by you guys though, to see what you think before I go hair-pulling.

Years ago I converted my DT to 12 Volts, using a SH522 Rectifier/Regulator Combo (pretty much the same thing as the SH570 that NEO used on his UK 12 Volt Conversion). Fast forward to this year, I wanted to check to make sure everything was tip-top on the lil DT before I took her in for Inspection (to be road legal).  She's only been on the road once in her life, the original owner put 50 miles on her driving it home in 1978.  Don't worry, she's got new everything (tires, brakes, cables, lights, battery, etc)

So I fire her up and throw a voltmeter on her to make sure she's charging (Was reading 11.2 Volts at the battery). Okay, a couple hours of wire chasing and testing, a wire got disconnected at the regulator.

Here's where the problem starts.  Turn signals flash, but put a heavy load on the bike (Neutral Indicator flashes dim/bright as the turn signals function).  If I turn the headlight on, it kills all the lights on the bike, battery voltage drops to 9 volts and the headlight looks like the end of a cigarette burning at an idle. Rev her up to 4k and the headlight looks like a Zippo Lighter.

So I go back and check all the wiring, try another regulator/rectifier (just in case). Same thing. I test the output of the Magneto, she's putting out 21 Volts on the Lighting Coil and 27 Volts on the Charging Coil (disconnected from the regulator/rectifier) at an idle.  I go back and check the voltage at the battery, with the engine not running and it's now at 11.8 volts (still below normal charge)

Is it possible, that the battery being low (got a battery charger on it right now) would draw so much power from the Magneto on the bike, that there would be no power left to run the Headlight at the same time?

Edited April 23, 2016 by 74Dusted
Solved

[74Dusted]

Okay, I figured this one out guys.

 

Years ago, when I was getting this bike ready for the road, Pennsylvania had a law that stated the Headlight had to function without the engine running... A Jumper wire had to be installed between the Parking Light (Tail Light) and Headlight, inside the Headlight Switch, so that the headlight would draw power from the battery, and work without the engine running.

 

After testing, hair pulling, swearing, and threatening the bike, I narrowed it down in a process of elimination that something in the headlight circuit was shorted and drawing down the rest of the system.  Then I remembered the Jumper Wire, removed it and everything on the bike is perfect again.

 

Also, Penndot has changed that law in Pennsylvania (so my bike no longer requires that jumper wire). Any bike manufactured before January 1st 1978 does not require the lights to function with the engine off (my DT was built in 1977)