exhaust temp gauge - jap. manual HELP!!!

[idiot goes sideways]

i have a omorimeter exhaust temp gauge (buyed it used)

http://www.omorimeter.co.jp/english.html
(this one has a warning lamp mine`s got a check lamp)

and a jap. maunual so no info how to install.

i installed the sensor like it was show`n in the manual (the only thing i could figured out) and got three cables from the gauge red, black and white

i installed:
red to + (15)
black to - (31)
white to dashboard light

first try:

dashboard light was working
i started the engine and at same time i got 400C
with every electric i turned on (light, brakelighte, etc.) the temp. was increasing about 200C ore more (depents on how much electric i used)
so i checked the ground (black cable) and was thinking i foung a bad groung
by the way the red check lamp was light all the time i turned the dashbords light on

second try:

started the engine temp comes slowly 200C to 300C and so on, tryed lightes and other electric stuff on nothing changed on the temp.
i fought that was it, BUT after some time under driving i was looking at the gauge and it showes 1200C or even more, after that i turned the lights of and the temp was falling about 700-800C and yes the red check lamp was on the whole time
i dont know how the check lamp should be working so i ignored it at first

what could it be??
maybe still bad ground??
why is the check lamp always on???

HELP!!!!!!!!!!!!!!!!