Agree check the voltage stabilizer first. It is a small metal box usually attached to one of the tach mounting thumb screws. If you can't get a good 10v reading, it may be pouched, or one of the original bi-metallic type that can't be accurately measured with a digital voltmeter. For under $20 replacement rules out an issue. I got a replacement from Moss Motors, and found their write-up helpful. The stabilizer supplies the temperature and fuel gauges in the Plus 2.
http://www.mossmotors.com/graphics/prod ... 31-555.pdfI followed advice here and did the same as John by adding resistance at the sender. Just bought a blister pack of about 100 resisters from Radio Shack for about $10. Trial & error of a few values brought the gauge in range pretty quickly. Recall that putting resistors in parallel with one another alters the total resistance, so you can dial in the value pretty accurately.
http://www.sengpielaudio.com/calculator-paralresist.htmhttp://www.digikey.ca/ca/en/mkt/calcula ... stors.htmlThe small resistors I used needed a couple in parallel to keep from overheating. Covered the soldered resistors with heat shrink. Only going from memory, but I think the total required resistance value was about 20 to 40 ohms. Figure the added resistance just makes up for a bit of spring loss in the gauge movement.
As I have a Clivey Boy thermostat housing with a blanking cap, I was able to get an accurate coolant temperature using a glass candy thermometer (from the grocery baking section) immersed right next to the sender.
HTH
Stu