Here is a pic of the LM318 Appendix A diagram for Prestolite Alternators if anyone is interested. Field pole ( aux. ? ) is at 1:00.. Pos is at 8:00 and ground is at 6:00 in case you can't see it.


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Sean,

Don't rely on the alternator brackets to provide your ground connection. There is normally a ground stud on the alternator that you can use for the ground wire. It should go to wherever the battery ground strap connects to the block, usually on one of the starter mounting bolts so that the starter gets a good ground. You can use a yellow wire of the same size as the orange alternator output, or you could simply buy a ground strap (a light one will do) since it does not need to be insulated from the block.

The 12.75 volts is a little low, but could be just that the battery was low. You would have to check the final voltage at full charge to get a better reading. The 14 volt open output when idling is no surprise. You should avoid running the alternator without a battery attached, normally.

Many engines don't drive the alternator fast enough at idle to get full voltage. You should check the voltages at cruise to see what the battery is really getting.

I'd check that voltage reading with a good multimeter--15 volts is a bit too high and might harm your battery.

Jeff

Doc, I posted a procedure for checking the gauges that use a sensor. As it happens, neither the voltmeter or ammeter have a sensor, and so cannot be verified using the procedure.

Test a voltmeter by putting a good multimeter across its terminals. Verify that they read nearly the same.

The only way to test an ammeter is to put current through it. You can remove one wire and put another meter in series with it, but this is some trouble.

Sean, if you disconnect the battery from the alternator (I assume that is what "shut the battery off" means) then the voltage at the alternator output could be most anything -- it is designed to work with a battery. I would measure the actual voltage while charging the battery. Of course, that charging voltage is measured at the battery. The alternator puts out more, and there is voltage drop between the alternator hot terminal and the hot battery terminal, and also a drop between the battery negative and the alternator frame. You should measure both. Your battery needs from 13.8V to 14.25V (depending) to charge. Your alternator will have to put out more.

Your boat was built to use the alternator bracket and mounting as the return current path from the battery. That is standard practice in cars and so that is the way the engine was made. In hot, salty, wet Florida, relying on a steel-to-steel electrical connection is daft. You can't tell with an ohmmeter if the connection is good. At 50 Amps, a resistance of 0.01 Ohm, not measurable without special stuff, will cost you half a volt. Things are probably better in fresh water, but I would go with the wire. I used to make them up myself, but you can buy ground straps at Autozone that will do the trick, if you can find a guy who knows the stock and doesn't need make/model to get the part.

Or you can get one at http://www.delcity.net/store/Switch!to!Starter-Cables/p_1005.h_89497.a_1.t_1

I am not sure why your current reading fluctuates. I would get everything else working, and then take on that problem if it is still doing it, at cruise. 1500 rpm is still on the edge of working for most alternators.

I'll do that .. Hopefully we'll be able to read them.
Sean

Hey Karl, ever check your set up? Mine has been charging well, but I still am going to replace the ground strap and add a a lead from the ground pole on the alternator to the block..
Thanks,
Sean

Found out my stbd alternator is not producing voltage yesterday . If I understand correctly, the voltage regulator has an output that goes to the alternator to excite the field in the alternator so it that it puts out voltage required by the battery or load.

Didn't have time to troubleshoot. I have one of two batteries on that bank out of the boat so I don't know if that would cause a problem....I wouldn't think so.

I guess my first step will be to swap voltage regulators. Check grounding of alternator (need to add grounding straps as you guys recommend...more work...great!) Then pull alternator to get tested.

A buddy of mine switched the perko switch on that bank a few weeks ago (engine running) while I watched . Newbe mistake because there's a large warning sign next to the switch that says don't turn this switch while the engine is running !

I guess we damaged something...not sure what yet...Any comments

Woulda Coulda Shoulda used a tape easy out they work great in most instances

Norm,

Big s Live On

Jeff might try to make a forked tool to back out broken piece, just a thought.

Roger