Got hit with a weird voltage drop on a King KX 155 install last Tuesday

I was wiring up a replacement nav/com in a Cessna 172 at the hangar near BFI, and the unit kept cycling off after 10 seconds. Turned out the ground strap on the engine mount was corroded, causing a bad ground loop that backfed into the avionics bus. Has anyone else seen a simple ground strap cause that kind of intermittent failure?

2 comments

2 Comments

stone.thomas24d ago

oh man, that ground strap issue is a classic sneaky gremlin. i've had almost the exact same thing happen on a King KX 155 install in a 172 out of KPAE. spent a whole afternoon chasing a voltage drop that would only show up after the engine was running for a bit, and it turned out the main ground strap on the firewall was corroded right where it bolted to the engine mount. it's wild how a little bit of corrosion on one strap can make the whole avionics bus act like it's got a brain tumor. that intermittent cycling you saw is a dead giveaway of a bad ground loop, and it's so easy to overlook because everything else tests fine when the engine's off. honestly it's one of those things that makes you want to check every single ground strap on every install now, even if they look clean. really appreciate you pointing that out because it's one of those "it's always the simple stuff" moments that saves everyone a headache.

angela_dixon24d ago

Did you ever find that the corrosion was worse on the engine side of the bolt than the firewall side? I started cleaning both mating surfaces with a wire wheel and adding a dab of dielectric grease after getting burned by that same issue on a Piper Archer.