Q for Bob M or others - diode in 3-wire, starter s

1) Wire the '2' terminal on the alternator to the 'BAT' terminal on the alternator with a 16 or heavier gauge wire.

2) Using 16 or heavier gauge, wire the '1' terminal on the alternator to the same side of the shut-off switch that holds the wire that runs to the '+' pole on the coil. Would it make sense to wire both a diode (Radio Shack PN: 1N200X - where X is 1-4) _and_ a warning light (bulb# 161, 194, 1813, 1815) in this line? My primary concern is to avoid running down the battery but having the warning light as well would be nice.

I was also wondering what the benefits would be to wire a solenoid in between the plus terminal of the battery and the starter. Currently I have 4/0 copper cable running from both the plus terminal of the battery and the starter to the start button. Putting a solenoid in would cut out a two or three feet of cable but, with that size cable already in place, would it make much difference?

Thanks for your time.

1 – As the others suggest below, for the #2 terminal wire you can use as small gage wire as is mechanically possible. (#2 is the voltage sense terminal for the internal regulator. It carries only a miniscule amount of current - something on the order of 1 milliampere.)

2 – Use either a warning light OR a diode, but not both! No need to worry about draining the battery when shut down thru the warning light – the system is specifically designed to work with the light. However if you are concerned about the warning light burning out and disabling the alternator while running, you can wire a 15 to 30 ohm, 2 watt (minimum) resistor in parallel with the lamp.

3 – You sure you’ve got 4/0 primary wire there and not AWG #4? 4/0 is HUGE - like ½” iron pipe. Anyway, with cable that heavy you certain won’t need to add a solenoid to reduce the cable length – the extra couple feet of cable is inconsequential!

Hope this helps...

Thanks for your quick reply. How do you view the benefits of an oil pressure switch vs. a diode in this application?

To your question my vote is for the diode. It's cheap (less than a buck) and correctly installed is pretty much bullet-proof. An oil pressure switch on the other hand will cost probably $20, needs to be plumbed in, is subject to leak, and if it sticks "on" you'll never know it until you discover the battery is dead some morning.

My PERSONAL preference however is for the warning light! Not only does it tell you the instant the alternator stops charging, it'll also warn you immediately if the fan belt breaks or the water pump siezes - a nice feature when people other than myself operate my tractor!

Thanks again for your time.

As for the diode - I think you are referring to a 1N400x diode. The 'x' is the PIV rating. A 1 means 100 volts... 2 means 200 volts... etc. The most common size you will find at Radio Shack is the 1N4003. This is plenty of diode.

As for the lamp: This works and is a good idea. Short of installing a voltmeter, the lamp is the single best way to determine if your alternator is charging. In reality, if you use the proper size lamp you would not even need the diode. The lamp will drop enough voltage when you shut the tractor down and your back feed will not be enough to fire the coil. Remember that a silicone diode (don't even think of putting a germanium diode here) drops .6 volts in the forward bias mode. This may or may not affect where your alternator starts charging when you also have a lamp in series with it.

As for the solenoid..... I wouldn't bother with it. Besides, 4/0 wire is plenty heavy. How do you even bend that stuff? You have more than enough of a hunk of cable between your battery and your starter - I wouldn't cobble it up with a solenoid.

If you don't like the idea of adding a switch to excite the alternator (and having to remember to turn it off), get a 2-terminal N.O. oil pressure switch that turns on at low pressure, and hook it in-line between a source of battery voltage, and the #1 alternator terminal.

When the engine is up and running, there is then voltage to the "excite" terminal, and when the engine stops and there's no oil pressure, the voltage to the "excite" terminal on the alternator shuts off.

BobM has a diagram posted showing the hookup of thoil pressure switch. The use of the "idiot lite" with the oil pressure switch is OPTIONAL. The system will work with or without it.

• BobM's diagram

If you don't like the idea of adding a switch to excite the alternator (and having to remember to turn it off), get a 2-terminal N.O. oil pressure switch that turns on at low pressure, and hook it in-line between a source of battery voltage, and the #1 alternator terminal.

When the engine is up and running, there is then voltage to the "excite" terminal, and when the engine stops and there's no oil pressure, the voltage to the "excite" terminal on the alternator shuts off.

BobM has a diagram posted showing the hookup of thoil pressure switch. The use of the "idiot lite" with the oil pressure switch is OPTIONAL. The system will work with or without it.

• BobM's diagram