on these trucks it is a single digit display
it can flash just an F or if there is another issue it may be an F & 2 or 3 or 4 etc. Now if it flashes numbers like 1, 2,... F then its a low battery issue. There is a difference, F and then a number or just an F indicates a component failure. scrolling numbers followed by an F indicates a low battery issue.

A flashing E would indicate no seat switch input.

If it is just flashing an F with no other numbers this indicates a logic board failure.

You may want to double check your connections, wiring around the board. Since you were working on one of the contactors you may have inadvertantly pulled a wire loose or connected one up incorrectly? Also since you cleaned those contacts hopefully you didnt file them off? maybe you filed them too much and now have no contact? If this was the line contactor try manually pushing it in when you turn on the truck and see if that makes a difference. By default when the truck is activated the line contactor should pull in, then the steer contactor and a few seconds later the steer contactor will drop out. Of course if there is a logic circuit problem it may not pass the self test on start up.

Does the truck have the decal under the hood that shows a diagram of components and how they are laid out along with a self test diagram of instructions on how to run the self test? Most of these trucks had that unless at some point it came off or was removed?
There is also a page or 2 in the front of the troubleshooting manual specifically directed towards battery care and troubleshooting frame grounds etc, you should re-check the battery and make sure it reads no less than 33.1v for a 36v battery or 44.2v for a 48v battery. So a per cell check and each cell should read around 1.95v to 2.12v. If any cells read below 1.95v the battery should be charged or repaired.

hope this helps