Fan not turning on even after Thermostatic Switch replaced

OK Folks, I'm in need of a Sanity Check here--my radiator fan quit working some time back. It wasn't a critical issue because I live so far out in the country that as soon as I ride a few blocks I turn onto the highway and I'm up to highway speed. It's only when I return that the engine temp starts to rise just as I'm backing into my parking spot.

I'm a trained Electronics Technician, so a pretty good trouble-shooter.

I grounded the lead that usually goes to the temp. switch, and the fan starts up immediately and runs normally. I removed the switch, checked for any kind of corrosion at the threads, cleaned the contact and ran the bike again--the fan still never comes on.

As recommended, I installed one of the drop-in replacements linked below, and tried it again. No fan. I haven't yet checked the new switch by putting it in boiling water to see if its resistance goes to zero, but I live at 2700' elevation, so water boils at slightly less than 212F, and the switch is rated at 210 on/195 off, so it might not close at the boiling point anyway if the tolerance is a bit off.

The silly fan has worked perfectly for a couple of years, so what besides the temp switch might be at fault? Yes, I know that the brand new switch might be defective too. Is there a known problem with galvanic corrosion where the brass switch screws into the aluminum?

Ideas?

   ebay.com ELECTRIC COOLING FAN GROUND-ING THERMO-STAT SWITCH RADIATOR TEMP-ERATURE SENSOR | eBay Find m

Did you check the relay? Page 207 in the manual if you have it.

I'm not an electronics technician but consider myself somewhat savvy with electrical circuits and simple switching so......
By grounding out the lead that attaches to the fan switch and the fan operates, you've tested the entire fan circuit . By grounding out that lead, you energized the relay and closed the power circuit to the fan and the fan runs.
That only leaves 2 possibilities.
Either the fan switch is defective or
Possibly the engine ground strap is corroded. Depending on "where " you grounded out the lead going to the fan switch would eliminate the engine ground strap. ( if you grounded out the lead to the engine case) If you grounded out the lead that normally attached to the fan switch to the body of the fan switch, that would eliminate your concern with corrosion between the fan switch and the engine.
At this point I would suspect the switch and go and test it for closing in a vessel of boiling water. 

I hear the relay under the seat click as the fan turns on.

The fan seems to go on regardless of where I ground the lead, but you make an excellent point: I'm not sure I ever tried grounding it to the body of the thermal switch itself!

Be right back...

I decided to look up the  EBay listing for fan switch  you referred to and I see that the thread size and temp opening/closing points are optional so that being said, which fan switch did you order?? and what fan switch did they send??

From pages 206-208 of the KZ1300 Service Manual
 

Looking at table H85 and H86, we see that the thermostat operating range is 172F- 203F
The rad fan should not be coming on during the operating range of the thermostat since the purpose of the rad fan is to drop the coolant temp when it exceeds the operating temp.
That being said, I know that some members have elected to install either  a manual over ride switch to turn on the fan when they "feel" that the coolant is too hot, or they have elected to change out the fan switch for a switch that comes comes on at a lower temp

Looking at the options available in the EBay listing, I would have chosen the "200F on- 185F off" option and accepted that there would be times when it came on a little early