Showerthought: A loose connector in a Cessna 172 taught me to trust my meter over the manual

I was troubleshooting a nav radio in a hangar in Boise, and the book said to check voltage at pin 4. My Fluke showed zero, but the pin looked fine. I finally wiggled the whole D-sub backshell and the reading jumped. Ever had a manual point you to the wrong spot entirely?

3 comments

3 Comments

taylor.jesse2mo ago

Had that happen with a bad ground strap on a Cherokee. The manual said to test at the bus bar, but the real problem was a corroded connection under the insulation. My meter found the voltage drop the book never mentioned.

linda6582mo ago

Check for voltage drop across every connection, not just the ends. Found a bad crimp under the heat shrink on my truck doing that.

the_ben1mo ago

That's exactly backwards though. Manuals aren't supposed to point you to every single possible bad connection, they're supposed to give you a solid starting point. If you wiggle every wire in the plane every time something's off, you'll never develop real diagnostic skills. I've seen guys spend hours jiggling harnesses when the actual fault was a cracked solder joint they could have found in five minutes with an oscilloscope. The manual gave you a spec to check at pin 4 for a reason - because that's where the voltage should be present if everything before it is good. Your meter was telling you the truth, but you had to work backwards from there. That's not the manual being wrong, that's you learning how to actually use process of elimination.