This old Ford 555D isn't charging it's battery. There are 3 small wires going to the alternator besides the main battery lug. I disconnected each of them to test for 12v with key on but I get nothing at any of them. I assume one of them should be a turn-on or exciter wire? I have no idea what the other two are for.
Yes, one should be excite, one could be a tachometer drive if it's not cable driven, that would be a "W" usually. The the other could be a voltage sense wire, but that should be 12V with the key on, possibly with the key off. Find the brand of alternator and find a wiring diagram that looks similar. Then trace those wires, if you have a battery warning, or alternator warning light, does it light with the key on but not running?
does the machine have an alternator light? If the bulb is burnt out you will not get 12v to the regulator to excite the field.
Thanks guys, I think it is a problem with the wiring harness. The tach isn't working and neither is the battery warning light. Both of those used to work but now they don't. The bulb for the battery warning light looks ok, but I didn't test it with my DMM. Next time I have the machine home I will look into the wiring and see if I can find any problems.
If you checked those wires to the alternator and you had no voltage on any of them disconnect then you have either a bad bulb, bad wire/connection or no power to the bulb. Do you have a wiring diagram? I can help you with testing if you can post a diagram.
Another possibility is a defective ignition switch. A VOM or test light with a probe would be useful.
I don't have a wiring diagram. I'll have to try to find one.
I think it is a chafed wire. The tach needle started bouncing around when I was using the machine today. Probably the wire wiggled just enough to make a momentary connection. Hopefully I'll have it back at the shop to work on it next week.
From the service manual.
Not exactly the machine's wiring diagram but it will have to do. The D+ terminal should have 12 volts on it when the key is in the run position. With the key on/engine off the charge indicator light should light up then go out once running. If you have 12 volts with key on at the D+ terminal then your issue is with the alternator, most likely the regulator. If you do not have 12 volts at the D+ terminal with the key on then check if your charge indicator light works. If the light does not light up with key on engine off check the bulb and see if you get power to the bulb. If you have power to the bulb and the bulb is actually good then your alternator, most likely regulator is bad. As you can see in the schematic the D+ is your field, W is the tach signal as it is connected to one phase of the stator and will have an A/C voltage signal you can check with an oscilloscope. B+ is the heavy ware bolted to the stud on the rear of the alternator and is connected to the battery. Just do those simple measurements I listed and you will have your diagnosis.
FWIW, Here is the schematic. The print quality in the service manual allows the image to be increased in size and retain good visual detail. Unfortunately it has to be downloaded to be able to read it.