Zonestar P802M Heatbed sensor gives to high values!!!

I was trying to print with ABS in my zonestar.. But total failiure. Prints does not stick to bed.. they fall apart in the layers. Set extruder to 270 C and heatbed to 120. Still did not work so investigated.
In repetier and on lcd screen the temp show 120 C but with measuring the plate it is just around 85C.I took heatbed apart and insulated the bottom and no help. The MK2 heater have good contact.
But somehow something is wrong. Does anybody have any idea what the problem are?
Help me please.. I am tearing my hair and crying rivers
And on top of it I am a newbie on 3d printers. But i am inclined

Think it is solved!!! I Think it is the same problem many i3 owners have... After carefully examining the bed.. i saw that from factory the had wrapped tape and put om the mk.. sp on each end there was 1-2 inch tape.. making it not in Contact with glass. mounted it all.. looked great.. but did not work.took a flashlamp andlooked o side of bed with lamp on opposite side... then i could see a small glimmer of light.. a tiny tiny gap between heater and glass made it loose 35c betweeen recorded and true temp. now abs works great!! i just use some smaöll clips and put arfound. I will probbaly get som really weak springs that will gently keep heat and glass i Contact