Alternator troubles

I put a new Delco 10SI alternator on my H to replace another one that stopped charging. The new alt worked great for about 2 months and now when I pull the knob to charge the ammeter shows a slight discharge, around 5 amps. The battery voltage drops when I pull the charge knob and then whe I push it in again it rebounds. The PO rewired the tractor right before I got it, so I haven't worked on the wiring at all. Is this just an internal alternator problem or could I have a wiring problem that is killing them slowly? Any advice is very much appreciated.
Zach

Why are you pulling knobs? Wired correctly, the alternator charges automatically when you turn on the ignition switch and start the tractor.

The assumption that the Prior owner did it correctly is the issue.
A single heavy 10 Gauge wire runs from the alternator main post to the amp gauge terminal that does not go to the start switch. (load side of gauge)
A single wire goes from terminal #2 on the edge of the alternator to the alternator main output post (short wire).
The #1 post is connected to one terminal of a side marker light bulb (or a 3 amp diode with the band on the diode toward the alt, or a 10 ohm 10 watt resistor). The other light bulb lead goes to ignition voltage. (or if it has a magneto, battery voltage through a oil pressure switch that is on with pressure (NAPA) then to battery volts at the starter switch). It could have a manual switch to excite (yours may) but this is sure not the best option. Jim

(quoted from post at 07:50:33 06/19/10) I put a new Delco 10SI alternator on my H to replace another one that stopped charging. The new alt worked great for about 2 months and now when I pull the knob to charge the ammeter shows a slight discharge, around 5 amps. The battery voltage drops when I pull the charge knob and then whe I push it in again it rebounds. The PO rewired the tractor right before I got it, so I haven't worked on the wiring at all. Is this just an internal alternator problem or could I have a wiring problem that is killing them slowly? Any advice is very much appreciated.
Zach

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Since it was working before I'm going to assume that the wiring will do the job. The 5 amp draw indicates that the brushes and rotor are working correctly. (The rotor winding should have 2.8 ohms resistance and assuming a fully charged battery at 12.6 volts results in 4.5 amp draw.) At this point the regulator is doing what it is supposed to do. Until it reaches the upper voltage limit the regulator grounds the brush lead. If it were open you would not be getting the draw with the switch on.

With the engine off, check to make sure you have battery voltage at battery terminal and on terminal # 2 at all times. You should have voltage at terminal # 1 only when the charging switch is on. If that checks, out the problem is inside the alternator. A shorted stator and/or blown diodes are the most likely cause of your problem.

Thanks a lot for the explanation. It sounds like mine is all messed up. On mine the wire from the main post on the alt goes to the + terminal on the battery and both of the other two wires from the smaller terminals go to the amp gauge box. There is no resistor that I have seen. I'll take a couple of pictures. Is it safe to assume that my alternator is dead and I need a new one as well as rewiring the system? Thank you very much for all of your help.
Zach

If your tractor is wired as you posted then you are always going to show the 5 amp discharge even if the alternator is working. You would be charging the battery directly and the ammeter is going to show the total amps being used, not what is coming from the battery.

The following assumes you have the standard regulator and not the self energizing "one wire" regulator.

As long as you have battery voltage at the battery (output) terminal and at terminal # 2 all the time and some voltage at terminal # 1 with the engine running your wiring is good enough for the alternator to work.

The output wire running to the battery terminal meets the electrical requirements but I would rather see it attached to the ammeter on the same side as the lights and ignition wire. That way the ammeter will show the amperage going either to or from the battery. Connecting a small wire directly to the battery is not a good idea because of the corrosion that is likely to occur there.

On these old tractors without much electrical load it is fine to attach the voltage sense wire, the # 2 terminal, to the output terminal on the alternator. On equipment with more electrical loads it is better to run this wire closer to the battery. That way the regulator will compensate for the voltage lose in the system and raise the charging voltage to maintain the correct battery voltage.

Although some alternators will begin to charge without voltage applied to the # 1 terminal it usually doesn't work on tractors. The small amount of residual magnetism in the rotor requires higher RPMs than is available for it to begin charging.

The # 1 terminal wire needs to be switched off when not in use or the field coil in the rotor will draw 4.5 amps draining the battery. Because when the alternator is charging it provides charging voltage on this terminal, if the wire is connected to the ignition coil, it will provide voltage to the coil even though the ignition switch is off. This will keep the engine from stopping when the switch is in the off position.

On your tractor it sounds like you have a simple On/Off switch to accomplish this. It is completely usable like that but it requires some effort on your part to first turn it on and then later to shut it off so that the battery doesn't discharge through the alternator. Five other methods are commonly used, all of which function automatically, requiring no effort on the operators part.

1) A normally open oil pressure switch. Oil pressure closes the switch, when the pressure returns to zero the switch opens. The power to the switch must come from somewhere other than the ignition switch.

2) A resistor between the coil and the alternator. This drops the voltage in the wire enough that the coil will not provide enough spark to run the engine with the switch off. The resistor also drops the voltage going to the alternator but the only difference it would make is the alternator would have to spin at a little higher RPM to start charging. As long as you have 6 volts on the # 1 terminal the alternator should start to charge at idle.

3) An indicator light. This functions the same as the resistor but can give some indication that the alternator is working. An alternator that is charging some may turn out the light but still not work enough to keep the battery charged.

4) Diode. The simplest to install and provides full battery voltage to the alternator and no back feed to the coil.

5) Ignition switch with separate accessory and ignition terminals. Generally not found on farm equipment.