A code -51 will come up on start up if there is an accessory like a scale or something running straight off of the battery power. It can draw the power out of the capacitor before your line contactor has a chance to pull in. You have to have a direct short brom battery positive to the control card, powering up the card only so the display is working and it thinks you are requsting a startup. You need to check wires and connections for a direct short to the battery. I can get you some wire numbers to look at if you want. I agree with checking battery voltage to the frame with the battery in the truck and then out of the truck.