Turns out to be a bad low-speed circuit on the fan. High still works fine. Not sure why the low circuit is bad. I suspect that the whole fan assy could be easily repaired by someone with better electrical skills.

Great thread. We only recently had our first 70 degree day and my coolant boiled. I had a shop install a new coolant temp sensor and thermostat and radiator hoses while there and asked for electrical troubleshooting on fan not coming on. They said the fan has no low speed, need new one at ~$500. I r...