Was an aircraft mechanic in the USAF. Had mechanical knowledge and had been around trucks and farm equipment since I was a kid. I knew or could figure out how to do a lot of things, but most importantly, I had a good idea of what jobs were beyond my ability and/or tool box.
I had a general idea of how vehicles work in general. I would use what I had to attempt to figure it out. If I was unsuccessful I would ask some of my old timer friends who've been trucking for 30 or more years. They'd usually take the time out of their day to help troubleshoot the problem and even guide/assist on fixing the problem. Old timers have not only helped me with mechanical problems, but in every aspect of trucking they have formed me into the driver I am today.
I became an O/O as a 27 year old woman(in 2001) with 2 years of driving experience only because of the great instruction I received and sought out from the previous generation of truckers that were more than willing to teach everything they knew if you were willing to listen. I give full credit to the rest of the O/Os passing down their knowledge. I knew nothing about trucking when I started but because of them I've been trucking 17 years now and had my Authority for 12. I still don't work on my own equipment but manage my money well enough to do what needs to be done. I have a ten year old truck with 1,300,000 miles that hasn't been rebuilt yet!