Recurring Mode C Problem

[ggroves]

Arrrgh...it's back.

I'm taking the bird back to the avionics shop for the fourth time to fix an intermittent, infrequently recurring Mode C problem. Tonight, ATC reported that we were showing 10,300, when level at 3,300...7,000 high. This indicates a bad pin/wire somewhere between the transponder and encoder.

Last time they straightened a bent pin, and cleaned the contacts. The problem disappears for about a year, then it returns. Sometimes, simply pulling it from the rack, and re-installing fixes the problem. However, tonight it would not clear. Hopefully it will stay "broken" long enough to find a permanent fix.

Thankfully, we were 20 NM outside of the ADIZ.

I'm half tempted to have them pull the old gear, and install a Garmin GTX330 Mode S....I just hate to have to write the $4,500 check.

Greg
182RG

 

[Eamon]

Just remember that changing the transponder will not fix the problem. The encoder usually has the harness attached to it.

Buy a new encoder & harness. It should be $300 - $400 with labor.


Good Luck

Eamon

 

[Don Jones]

Just remember that changing the transponder will not fix the problem. The encoder usually has the harness attached to it. Eamon

Mine is just the opposite. the harness is hardwired to the tray and connects to the encoder with a connector. Either way the encoder is likely to be the culprit, although one bad connection in those 15 or so wires will be a pain to find.
Good luck,
Don

 

[AirBaker]

Well, if you keep tabs on what altitudes are being reported when, they should be able to look up which lines are on for those altitudes.

 

[gismo]

Well, if you keep tabs on what altitudes are being reported when, they should be able to look up which lines are on for those altitudes.

Actually that doesn't help as much as you'd think since the encoder is generating pressure altitude and ATC tells you what "indicated" altitude they see for your plane. That said the difference between your altimeter and ATC's readout will indicate a likely candidate for a missing bit if you have several examples at different indicated altitudes.

 

[ggroves]

It's back in the shop this week for a new harness. He suspects that the shop who installed the 430 may have created the problem when they tapped into the harness. The encoder is new, and the transponder tests fine. He could jiggle the harness and watch the altitude vary on the test box. Hopefully, all will test fine on Friday.

Greg
182RG