Testing alternator total output?

[Kingbee]

I have two Interstates equipped the same, electrical load speaking.  One of them will indicate over 13.5 volts, full load, at idle.  The 2nd one, without full load, will also indicate over 13.5 volts at idle, but as soon as load is applied, it will drop below 12v.  Even at 2000 RPM, volts never reach 12 with the same load as first bike. Turning off the load, and the voltage pops right back up.  I'm thinking one of the 3 phases is toast.  Is there a way of checking that before removing the alternator?  A local rebuilder will replace the offending diodes for $75, if that's the case, but can't test it in the bike.

[Kingbee]

Alright, how about this idea?  Removing the fuseable link from the alternator circuit and jumping those terminals with an ammeter of sufficient capacity, IE: 50 amps?

[Grandpot]

Kingbee:

That's a good way to check the output.  However, it still won't tell you if the problem is in the diodes or a phase in the stator.

If you let the bike sit with the ignition turned off for a couple of days and the battery gets discharged, the problem usually is with the diodes allowing the current to bleed through.

The only way to check if a phase is bad without an oscilloscope is to take the alternator apart and check continuity on each of the stator windings.

[Kingbee]

Thanks.  I think I'll just yank it & take it to the rebuilder.  I have one of the M.A.R.S. high output units on one of my bikes, but as he later explained to me, they have less output at idle speeds than standard output units, so I won't use one of those again.  Per Honda, Interstate alternators start porducing at lower RPMs than other valks - I've noticed that, and like it.

[Ricky-D]

It could simply be the rotor (armature) reacting to the extra rpm's.

The increased centrifugal force could be breaking a contact within the wiring.

The alternator guy told me that that is the greatest problem with the alternators.

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