Yamaha wires their two grips in "series" which means their 18 volt power source is fed into one grip and then the other grip before completing to negative ground. So each grip is going to be running off 50% of that voltage,i.e. 9 volts. If any of our Yamaha heated grips are used on a non-Yamaha snowmobile (Arctic Cat, Ski-Doo, Polaris) then they will not get as hot because the available voltage (12 volts divided by two = 6 volts per grip so output would be too little on high heat @ 12 watts per grip). You cannot run the Yamaha Hot Grips in "parallel" with a 12 volt source because then they would get so hot that they would melt. You CANNOT run them at 12 volts per grip. That would result in 48w per grip, far too much heat. With a Yamaha 18v power source, when properly wired in "series", each grip gets 9 volts. 9x9=81 then divided by 3 = 27 watts per grip. And that is a very -very hot hand warmer. You cannot leave them on high heat for very long, because your hands will be uncomfortable from so much wattage. Our Variable Heat Controller is designed to run on 12 volts, therefore you cannot use our Variable Heat Controller with the Yamaha grips.  All Yamaha heated grips are approximately 3.0 ohms resistance whereas all other snowmobile grips (except model 123) are approximately 2.4 ohms.  The Ohm's Law formula is:  Volts squared divided by ohms equals watts.