It Only Gets More Complicated
Dec 12, 2017
My friend and customer, Tim, came in the other day asking for advice on a do-it-yourself project. Last fall his combine's clean grain slow shaft speed warning kept coming on. He checked to ensure the shaft was actually turning at the proper speed, replaced the magnetic sensors, checked the wiring, and did all the traditional things he normally does to diagnose problems with electrical systems. The results of his testing didn't make sense and were leading him in circles.
Long story, short, the problem was in the combine's moisture sensor. Because I've dealt with similar problems before, I was aware that the clean grain elevator sensor is "secretly" wired through the moisture sensor, even though that connection isn't shown on the wiring schematic for the actual clean grain elevator sensing system.
The take-home from this is two-fold:
First, never assume that any wiring schematic for a modern machine is the COMPLETE schematic. A schematic for a modern combine would cover an 8' by 12' wall if it was all on one sheet of paper. In order to fit the diagrams for individual systems and circuits onto the pages of tech books and the screens of computers, engineers sometimes take short cuts that sometimes accidentally omit interactions between systems and circuits on separate pages.
Second, if you try diagnosing a modern machine using traditional do-it-yourself strategies of tracing wires, jumping wires and testing voltages, you may not know about hidden connections or circuits that follow unexpected paths. Dealership mechanics have learned to never fully trust a wiring schematic, and never assume a particular circuit follows a logical path from Point A to Point B. It MAY, but with modern computerized systems, it probably DOESN'T.