What are the devices in those path(s) that change the data?
This post is part of a series describing the generic, high-level troubleshooting process.
What is Troubleshooting?
Simplified Troubleshooting Overview
Step 1. Understand the correct inputs and outputs of the “black box.”
Step 2. Determine which output(s) are incorrect.
Step 3. Figure out which input(s) affect that output(s).
Step 4. Verify that those inputs are as expected.
Step 5. Analyze the path(s) from input(s) to output(s).
Step 6. Check all connections.
Step 8. Eliminate the devices that could not cause this output.
Step 9. Test the remaining devices.
Step 10. Repair/Replace faulty device(s).
Step 11. Fully test the system.
Step 7. What are the devices in those path(s) that change the data?
What are all of the items between input and output that have the ability to change the data? What are the “black dots” referenced in Step 3? Are there relays, PLCs, ICs, circuit boards, processors, etc.?
Make note each item, what it is, where it is in the path (what is before and after it), and any visible defects. If the item appears broken, go ahead and skip to Step 9 and test or replace that item. If it fixes everything, great! If not, come back here and pick up where you left off.
And Don’t forget that connections (connectors and splices) can change the data too. See Step 6.
I recently worked on a system in an automobile that hadn’t worked in years. When I got to Step 6, I found a large connector on the back of the fuse box that had the automotive style lever to lock the connector in place, and it just didn’t look right. Upon closer inspection, I found that the lever wasn’t properly engaged on the tabs. The bottom of the connector was seated, but the top was not. For the system that I was troubleshooting, the contacts were just barely touching. Without a load, you would read 12V, when you turned the system on, it would drop to 0V. I removed the connector and reinstalled it properly, and like magic, the system worked perfectly. My client was expecting the repair to take all day, I had it working in 2 or 3 hours after walking in the door.