Help Troubleshooting JD 4020 Diesel headlight failure

The headlights on my 1965 JD 4020 Diesel w/24v starting system are not working. They worked briefly when I bought the tractor at a sale last March. However, upon replacing the batteries I noticed the lights no longer work. I was careful to replace the batteries as I found them with one side grounded positive and the other negative. I get voltage at the light switch on one of the headlamp terminals on a voltmeter but only very intermittently. I took off one of the lights and rigged it to one of my other tractors, sure enough it works so its not the lights themselves. Tractor otherwise starts just fine and batteries show charge of 13+ when running, but the generator light does not work. I am not much for electrical work so I would appreciate any advice on what obvious problem I am overlooking. Should I be thinking defective switch, grounding issue or something else entirely? Thanks,

Pete

The midpoint of the two batteries needs to be connected to chassis ground.

If you have an older model, there is a battery cable from the (+) terminal of the LH battery to the chassis, and a battery ground cable from the(-) terminal of the RH battery to chassis ground.

If it's been updated (for safety) or is above #90999, there's a common cable from the (+) terminal on the LH battery to the (-) terminal on the RH battery, and a slow-blow fuse in a jumper wire (or a length of fusible link wire) from that common connection to chassis ground.

If you have that system, with the crossover wire, that connection to the chassis MUST be present and intact for the lights, indicators and fuel gauge to function properly.

The (-) cable from the LH battery connects to the upper large terminal on the starter solenoid, from there a blue wire makes it's way through a circuit breaker (if new enough or updated) to the key switch and on to the light switch through some wiring harness connectors and a circuit breaker on the tip out steering pedestal panel... you should be "seeing" (-) 12 Volts (nominal) with respect to the chassis, on that blue wire both at the key switch and the light switch.

The (+) cable from the LH battery connects to the terminal on the starter "belly" facing the engine block, from there a brown wire makes it's way through a circuit breaker (if new enough or updated) to the key switch and on to the light switch through some wiring harness connectors and a circuit breaker on the tip out steering pedestal panel... you should be "seeing" (+) 12 Volts (nominal) with respect to the chassis, on that brown wire both at the key switch and the light switch.

If power is missing at the blue or brown wire at the light switch you'll need to trace it back through he breaker(s)on the tip-out panel, the wiring harness connector in that area, on through another harness connector under the rear of the hood/front of the steering pedestal cover, on down to the starter, though the pair of breakers there (if new enough or updated for safety.

So, check for the midpoint grounding of the batteries, and for the (+) 12 and (-) 12 Volts at the switch panel and let us know what you find!

Bob covered it fairly well. Be sure to check circuit breakers(key 31) that are mounted on access cowing under dash. Check for good continuity(ground) between light housing/fender & fender/axle housing

After you drive yourself crazy,,you will switch it to 12 volts and all will be good...so skip the "crazy" part and switch it now...

I traced the voltage from the solenoid through the key and circuit breaker and found it to be 12 volts at both the ends of the blue and brown wires at the switch. So I cleaned those terminals with some sandpaper and reconnected and the headlights turned on. However, after about 30 seconds I noticed a little smoke coming off the coil on the switch. What would cause this and is a replacement switch in order?

Smoking coil is a resistor that will smoke a little if it's been inoperable for very long period of time.