Re: In operative Tach = Voltage Regulator partially bad?
Tachometers read off the alternating pulses from the stator. Any of the leads from the stator charging portion can be used for the signal. The designated tach signal wire is merely attached to one of those. Tachs have rotary switches on the back that set it to different numbers of coils in the charging circuit, times two because there is a positive and negative pulse for each coil for each cycle. Tachs measure pulses.
Johnson/Evinrude has three wires coming from the stator to the regulator/rectifier. One is common, and the others lead to separate banks of charging coils. That way, if one of the coils fails, the other bank should still charge.
Alternators put out AC current. Graphed-out, it's a sine wave. Simple rectifiers take half of that sine wave and convert it to DC. Bridge rectifiers take the whole sine wave and convert it to DC. So bridge rectifiers charge at twice the rate of simple rectifiers. If one of the diodes in a bridge rectifier fails, it still acts as a simple rectifier.
Alternators only alternate if there is a complete circuit. When a rectifier fails, the circuit is broken. That's why Johnson/Evinrude uses two separate banks of charging coils and bridge rectifiers. Those allow back-ups should a connection be broken.
Once produced, the electricity from the alternator has to find a place to go. If it loses connection at the battery, it will find it's way out someplace else. The rectifier is the usual place. I really can't explain why they don't fuse them. Yamaha does, and they frequently save the rectifier. Anyhow, that's why regulator/rectifiers usually fail, loose or corroded battery connections. Connecting the battery backwards also blows diodes. Been there, done that. Loose connections usually take out single diodes, but can do that more than once eventually causing a complete bridge rectifier failure. Cross-polarizing takes out both sides of the bridge rectifier right away.
Mercury V-6s do it slightly differently. They have two completely separate charging circuits in the stator with completely separate wires leading from them, and two completely separate regulator/rectifiers. That also provides a backup if one connection fails. The added feature is that one regulator/rectifier can partially fail and charge at half the rate and still have one perfectly good regulator/rectifier charging at the full rate. So a Merc can charge 3/4 rate with a single failed connection and 1/2 or 1/4 with additional failures. Johnson/Evinrude only has the 1/2 option. But the smaller Mercs only have one bank of charging coils and a single voltage regulator. Same 1/2 rate if one diode fails, but no charging at all if one charging coil connection breaks.
