To20 red idiot light on

I have a Ferguson to20 with the red idiot light. I replaced the wiring harness and now the red light is on all the time. How do I check to see if it
is the voltage regulator or the generator or something else? Wiring is my weak point. I have checked and rechecked to insure the new wiring
harness is hooked up correctly. I am getting 6 to 8 volts across the battery terminals when it is running. The voltage is sporadic and not
steady.

The red light means you are not charging your battery. Could be bad brushes in the generator or your voltage regulator is gone or possibly not polarized. I have seen voltage on my battery do about the same when running, it should have a steady voltage or increasing voltage when charging.

Try polarizing, especially if your battery went really low and you had to jump or recharge it. Generator brushes are easily obtained or bring the Benny to an auto electronics shop that works on them for about 30 bucks it will be rebuilt. Check the archives or wait for someone to post there is a way to check if the regulator is working, I can't remember how it is done.

I have a TEA20, just replaced the gear shift activated, start switch. I missed putting a wire terminal back under one of the start switch mounting bolts.
Thus, my red light stayed on while the tractor was running. Don't know what the TO 20 wiring is like....Alex

I don't know when the poms and strines are going to chime in, but I am with Tom here, I thought if the light is on, it means it is charging, hench the tractor is running good, and if it stalled rather than shut off with the key switch, then the driver would know to turn the key before leaving.... or so the legend goes...

One side of the light connects to the switched side of ignition switch. Other side of light connects to the Armature or Generator connection either at the generator or regulator. When switch is on it supplies voltage to one side of the light and other side grounds through generator causing the light to glow. When generator begins to charge the Armature terminal rises to charge voltage causing the light to go out. Light on means either incorrect wiring or system not charging.

I assume you have a 6 volt system. Wide variation (6 to 8 volts) of battery voltage suggests you are using a digital volt meter. Due to the nature of the digital meter it is common to see erratic voltage readings due to the "hash noise" from the generator and ignition system.

Assuming this is a Type A charging system.
1. Polarize generator. With engine off momentary short between Arm/Bat terminal on generator or regulator to hot battery connection or BAT terminal on regulator. Do not arc to the post threads on the generator as the arc can burn and damage the threads. If light now goes out it should be charging.

2. With engine running, short the field terminal to ground. If charge light goes out the regulator side of the voltage regulator is not working.

3. If light still glows, leave field grounded and jumper from Armature of generator to BAT connection on the regulator. If light now goes out you have a problem with the cut-out side of the regulator.

4. If light still stays on, you have bypassed the regulator thus indicating a problem with the generator or the wiring.

fergusonenthusiasts dot org resources has wiring diagrams and other good information