My nelson travling sprinkler wont move, just cicks

Generally the axle is frozen in the nylong bushings causing the gears to jump internally. Normally it can be patched by spraying oil into the axles where they go into the transmission and repeating until they spin freely in Nuetral. If all else fails, buy a new transmission for $10 from the Nelson website.

I* am certainly not an expert repair technition but I do own a Nelson traction sprinkler and did the following to mine.

I've not checked but I doubt seriously that the transmission is only ten dollars and I doubt if you can spray oil into the axles that it will last very long. The transmission is plastic and nylon and surely gets brittle over time so do not go nuts forcing it. The transmission is held to the casting with hexhead slotted screws. I did not even try to loosen them with a screwdriver as you will only strip the slots-use a socket wrench-they are surely stuck. With the bolts out the transmission is easy to remove. I could open the case about 1/4 inch before it would not go any further. I think you need to remove the part onto which the T shaped piece that the arms attach to. My sprinkler is quite old and I did not want to disassemble too much so with a clean dull old knife it was easy to apply marine waterproof grease to the gears and the axels. I then, reinstalled the transmission and hand turned the arms in both high and low gear to distribute the grease.
I works fine now with water and should last for another 20 years.