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Old 06-18-2002, 04:08 PM
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Testing the Voltage Regulator

Testing the Voltage Regulator
by Robert Campbell and Sid Hansen

This is important for anyone that is having troubles with the charging system. I checked restances of the stator (phase to phase) and field coils of the alternator and they checked out at about .4 ohms and 3.7 ohms respectively, and were within specs.
  • My tachometer just wobbles at about 500 rpm no matter what the real rpm was while running.
  • Charging current was not to spec nor was the voltage across the battery (went down when the bike was running.)
  • The resistances at the volt regulator/rectifier were to spec according to Clymers.
Soo... that indicates a bad alternator RIGHT? ...WRONG!!! ($495 mistake.)

Analysis: If the voltage regulator does not "excite" the alternator, it will not generate.

Alternate checking procedure:
A current must be produced into the field coil for the stator to produce charging voltage. A bad regulator will not allow this. The result will be the misconception that the alternator is bad. To check this out:
  • Leave the volt meter hooked up across the battery to check charging voltage.
  • Unhook the field coil connector clip from behind the fuse panel.
  • Hook a ground to one wire of the field coil.
  • Jumper the positive side of the battery and get the other
    end of the jumper ready to provide current to the field coil at the clip.
  • Start the bike and read the voltage.
  • Touch the remaining wire of the field coil and observe the voltage response across the battery.
Also observe the tachometer. If the alternator is good and the regulator is bad, the tach will start working and the voltage will start rising.

  • Do not leave this 12 volt connection across the field coil or you may cause electrical problems like blown bulbs and such. I am not sure if it matters whether the direction of the current matters through the field coil.
  • The regulator is about $140 new.
  • Less expensive automotive regulators can be adapted for much less.

An even easier method, taught to me by a friendly mechanic in Cleveland after a night spent sleeping (or trying to) next to the I90.
  • Take one of the thinnest feeler gauges in your set, and hold it parallel to and about a 1/4" away from the case covering the coil.
  • Turn on the key.
If the coil is operating, the magnetic field will pull the feelly guage
towards the case. No pull, bad coil.

He also mentioned he had been working on Yams for fourteen years, and had never changed a field coil.

He suggested I buy a lottery ticket.

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