Low alternator output question

[mrc]

Hi folks,<br /> I'm having some problems with 1970 85Hp Johnson Alternator. This is the most basic setup with a 9 amp stator under flywheel with Bridge Rectifier, no regulator. <br /><br />The original problem was no alternator output, and no tach reading. Replacing rectifier which was shorting to block due to flacked off insulation. <br /><br />Tach then worked but still could not keep battery charged. With ignition off, no draw on battery was measuable. With Rectifier disconnected, voltage across the stator was about 17Vac. With rectifier connected, and ammeter measuring output. At 1700 RPM in neutral the charging circuit was putting out about 2amps (Should be about 6-7) Barely enough to feed the shift solinoid let alone the amplifier. Battery was low enough on charge that it should sink that much current without a problem. Dash mounted Ampmeter never reads positive at any RPM with motor in gear.<br /><br />Resistance of Stator appears to be within spec although I am concerned about the accuracy of my Meter at such low resistance (~0.8 Ohms) Resistance to block from either stator wire is too<br />high to measure.<br /><br />I'm puzzled, Shorted coil on stator? Weakened <br />magnets on flywheel? <br /><br />What should my plan of attack be?<br /><br />Thanks in advance,<br />Michael Carney

 

[12Footer]

Re: Low alternator output question

This is a tuffy, but you gave a lot of info to werk with here.<br />Welcome to iboats,Michael. You say you have resistance too high to measure, as in a dead short. If it is like this with the rectifier disconnected, the stator may be shorted to ground,thus, it's not delivering anything.<br />I say may be...I don't have the manual for your engine. You may have a defective new regulator,but not that likely.<br />Weakened magnets would be a gradual degridation of operation that you would have noticed,i think.<br />Believe it or not, the 17volts is normal,if not a bit low.<br />Try testing voltage/amps charge with all the lights running to see if it may "kick" the circuit, giving the current someplace to go. Check battery cells for boiling (the 17-18volt charging system will do this normally once the battery is fully charged. And it is death to the battery.<br />Does the bridge rectifier show any reverse current on the ohmeter,10K scale out-of-circuit?<br /><br />I still highly suspect that stator.It just doesn't seem 'right' to have the leads test continuity to ground.

 

[mrc]

Re: Low alternator output question

I think you misunderstood "resistance too high". I was trying to say, that as far as I <br />could tell, there was no short to ground ie, resistance was reported as infinite.

 

[tesmel]

Re: Low alternator output question

I would have to theorize that since the bridge rectifier was shorted to ground this would in effect take the stator to ground while it is trying to deliver current thereby shorting the coils.<br />Measuring the resistance of the coils is not a fool proof way of testing the integrity of the stator because it will still measure some resistance with a few turns shorted.<br />I would firstly verify that the replacement rectifier is the right type and that it is connected properly, after which I would replace the stator.<br />Weak permanent magnet, highly improbable.<br />Terry

 

[wilde1j]

Re: Low alternator output question

You can check the stator w/ a good VOM by reading from either yellow lead to grd. Should read ~ 0.75 ohms +/- 0.2 ohm. An infinite reading means windings are open circuited (and bad).

 

[mrc]

Re: Low alternator output question

Terry, I agree with your accessment. Replacement rectifier is of correct type and <br />tests out okay. Guess it's time to pull the flywheel <br /><br />Jim, With the stator disconnected it should be isolated from ground, The .75 +/- .2 is <br />supposed to be the resistance across the stator leads (ie through all the coils). And <br />like I said, while I have (several) good VOM's, I really don't trust their readings for that<br />low of a resistance.<br /><br />Thanks guys,<br /><br />Mike

 

[mrc]

Re: Low alternator output question

Okay, replaced stator, even tried a different flywheel to no avail.<br /><br />When sitting in neutral the monkey balls (The shift diods) Get real hot?<br />Since that is an item that can pull down alternator current before the <br />rectifier gets it, that (or the associated shift solinods) are my major suspect now.

 

[mrc]

Re: Low alternator output question

It was the monkey balls, thanks to those who pondered the problem.<br /><br />Mike