The day in Phoenix that made me question multiplex testing forever

I was working on a CRJ900 out in Phoenix last August, the heat was brutal, like 115 degrees on the ramp. We had this intermittent comm fault that kept popping up, and I spent 3 hours chasing it with the multiplex box because every pin check came back fine. Turned out the issue was a bad ground strap hidden behind a panel that only showed up after the plane sat in the sun for an hour. Now I'm torn between sticking with the official multiplex test procedures or just doing a full visual and continuity check on every ground point first thing. Some guys swear by the box because it catches hidden shorts, but that day cost the airline a delay and I felt like an idiot. What do you do when the factory test passes but the problem is still there?

3 comments

3 Comments

milesbarnes11d ago

Just pull the ground strap first every time now, learned that one the hard way too.

nancyramirez11d ago

Next time I'm just gonna name my multimeter "Ground Strap" so I remember to check it first. Saved me from chasing phantom ghosts more times than I care to admit.

lucashenderson11d ago

Oh man, the multiplex box is like that one friend who swears they know the best shortcut but ends up getting you lost for an hour. I've definitely been that guy staring at a perfectly good pin check while the real gremlin is laughing at me from behind a panel. My trick now is a little ritual: before I even plug in the box, I take 5 minutes and do a wiggle test on every ground strap I can reach (which is great exercise in 115 degree heat, let me tell you). That bad ground strap sounds exactly like the kind of sneaky thing that makes you question your whole career choice. The factory test passes, the box says everything's fine, but the airplane is basically telling you "nice try, buddy." These days I trust my hands and eyes way more than a box that didn't have to stand on a scorching Phoenix ramp.