Alternator W terminal and erratic rev readings

[Peter Morgan]

Rev counter is showing erratic readings, going from 0 to full and back and sometimes staying at the correct revs. But alternator output ok. Checked wiring harness from alternator W terminal to rev counter and all ok for continuity. Rev counter is new.
Anything I can check in the alternator, happy to take apart if required. Is it a fixable problem or a new alternator?

 

[Boater Sam]

s

Rev counter is showing erratic readings, going from 0 to full and back and sometimes staying at the correct revs. But alternator output ok. Checked wiring harness from alternator W terminal to rev counter and all ok for continuity. Rev counter is new.
Anything I can check in the alternator, happy to take apart if required. Is it a fixable problem or a new alternator?

Supply to tacho, + and - checked? Could be a loose connection there or inside the alternator. A duff diode in the alternator could do this but its unlikely if it it is charging the batteries properly. May be worth looking at the brushes if its easy to get at, like on an A127 alternator where they are on the regulator.

 

[grumpygit]

The "W" terminal is a stator connection prior to the diodes thus producing an AC voltage of up to 8 ACV. It could be likely to be getting a voltage spike due to a breakdown of insulation of the stator. Firstly I would check both AC and DC voltages at the alternator to be sure. If you have intermittent or incorrect DC voltage you will also have an intermittent or incorrect AC voltage.

 

[Alex_Blackwood]

The "W" terminal is a stator connection prior to the diodes thus producing an AC voltage of up to 8 ACV. It could be likely to be getting a voltage spike due to a breakdown of insulation of the stator. Firstly I would check both AC and DC voltages at the alternator to be sure. If you have intermittent or incorrect DC voltage you will also have an intermittent or incorrect AC voltage.

The tacho reads frequency not voltage . However an intermittent connection, or failure thereof, could cause a "Swing" It will of course have a max voltage operating limit.

 

[grumpygit]

Of course but I was trying to keep it in layman's terms, not everybody has an oscilloscope in the tool box where most do possess a voltmeter. Until we have some positive readings it's difficult to come to any conclusion.

 

[KLBH]

Check the fan belt tension, especially if the problem occurs mostly just after starting engine.

 

[peteK]

I had the same problem over the years ,cleaning the alternator brushes seem to cure it.

 

[LadyInBed]

On the grounds that electromechanical devices have a tendancy to failure, don't discount the possibility that it might be the tachometer that is faulty.

 

[snowbird30ds]

Do you have solar charging at all? I wonder if the extra power could cut the built in regulator so there's no power through the field coils, W terminal is before diodes and an ac pulse.