Re: Alternator Output
I usually recommend a Factory service manual, available from any Tohatsu/Nissan dealer.
As for the charging system and tach situation... I have to agree with psteurer here. Often a bad rectifier results in an inoperative tach. Keep in mind that you only have access to the AC inputs of the rectifier, and one output. So you can't actually ring out each diode of the bridge, since you can't isolate the diodes -- you can only electrically "see" pairs of diodes. Although you can then confirm a short, you can't confirm a single open diode. You would only see an open if a pair of diodes were both open.
Putting on my EE hat, I can offer this advice. I hope it's not too technical. Many just won't follow it, since they don't have the EE background:
1. Operating the charging system without any load on the rectifier (no battery in the circuit) can blow a diode due to high PIV, depending on the characteristics of the individual diodes. Newer rectifiers can run with no battery, but most really old ones were not designed to operate without a battery connected. Still best to have a battery in the circuit.
2. A properly-operating bridge rectifier will multiply the ac input by roughly the square root of 2 (about 1.414) (less characteristic losses of the P-N junction of the diode), since the negative pulse is inverted up to the positive side... so a 12.3v p-p input would result in a DC of suitable charge voltage in excess of 13-14vdc with the battery load connected. You need over 13.2v to push a charge into the battery. If you are only seeing 12.7, you are not charging enough. Of course, if you have a really low battery connected, that will drag down the DC output. Is there a battery connected, and if so, is it healthy and fully charged? Since you have a scope handy, is the rectifier output a series if curved pulses, or are there flat spots in it? Flat spots mean bad diodes.
3. An AC alternator will normally have a fairly constant voltage across normal RPM ranges, but of course increase in frequency as the RPM comes up, so your feed to the rectifier doesn't sound abnormal, from an engineering standpoint.
4. The tests in the service manual normally use ohm readings, as opposed to voltage. That way they can be tested without the motor running. So we don't have factory voltage specs to the input of he rectifier for you to compare.
I suspect that you have an open diode, and need a rectifier. The rectifiers tend to fail more often than the alternators. Take resistance readings, compare them with the specs in the Factory service manual, and that should remove any doubt.
