Wiring Diagram for Electrolux Oven SK 7104 Type K40U?

I have bought a almost brand new Oven from Electrolux ,Model SK 7104, from a private person . But that person has been changing the inner wiring so very strange so that you cant control the heat from 3 of the 4 heating-plates anymore. Its either FULL power or not at all. Now i tried since days to find in the internet or on the Electrolux Side in the web the wiring diagram to control that and see if its correctly done. But unfortunately I cant find anything except how and where to buy parts to exchange. I am an electrician and certainly want to do the repair myself, but have no idea without the wiring diagram how to. ( I do not mean how to connect it to 400 or 230 Volts. I know how to do that and done it correctly.) Still I cant control 3 of the 4 heating plates. And i rarely can believe that it is a damage of 3 Thermostats at the same time. Would you please have the kindness and help me with this ? Thank you in advance for your efforts to consider so.