credit rating drop

[brixia76]

Hi. Can someone explane me how rating works in this mod? I read the wiki but i can't found the solution. i have my rating dropping down despite regular payment. Asked for a loan.. rating drop down. I did early payment and rating incresed for a while but after buying gears (cash payment) it drop down again. I can't move my rating over 600 points. Maybe i'm doing something wrong but i dont' konw what.. in the wiki the guide talks about small increments with regular payment but i can't see my rating get over 600... so i will never obtain the service truck.
Thank you in advance for your reply