Troubleshooting Intermittant Mode C

[Larry Biever]

Hi,

I'm having issues with the mode C on my 180. What is happening is that mode A is working fine, but altitude is working intermittently. I took the plane in for its annual, which included the static pitot check. Everything was working fine prior to the annual, and the static pitot check was completed with no issues. Upon flying home the controller called me and said that the altitude was intermittent. We have tried a new encoder, new antenna, and tried a loaner transponder with the new encoder. The wiring harness between the transponder and the encoder was also checked and is wired properly. We have been troubleshooting this over the last month, but the problem is escaping us so far. For example today we installed a new antenna because the mode c was found to cut out if we tapped beside the antenna. We tested the system thoroughly on the ground and it was working perfect. As soon as I was airborne, the altitude was cutting in and out. Any ideas on what could be happening, and what to try next would be very welcome. My transponder is a Narco AT 150.

Thanks
Larry

[JIMICS2452]

Did you replace the antenna cable and connectors?

[Larry Biever]

Hi Jim,
Not yet. Mark was thinking this would be next.

[JIMICS2452]

Always fix the cheap stuff first, especially when "shot gun" trouble shooting!

[Larry Biever]

Thursday we will be checking the pin connections and the antenna cable/connections. Here's to hoping we find something obvious. Finding something intermittent that only fails while flying and shows working on the ground is slow and painful.

[Larry Biever]

I should add that the shop I'm working with is being great. Mark is loaning me things to try so we can eliminate items, one by one. This would be brutal if I had to keep buying each piece, then find out it wasn't the problem.

[Quint Van Deman]

Sorry to dig up an old thread, but I am facing this same challenge, though I've not yet begun the troubleshooting process. My first question would be - is there a way to isolate down the problem between the altimeter, the encoder, the transponder, and the antenna? Checking the output of each incremental component would seemingly accomplish this, but I must admit I'm on square one in terms of understanding how/if the various outputs can be checked. If anyone has any thoughts on where to start in order to avoid shotgun-swap-tronics I'd appreciate it.

Thanks,

-Quint

[Pat Elliott]

The most often culprit and most often over looked component is the Antenna cable, including the pigtail from the tray. The coax is often original and 40+ years old. It will absorb moisture over time and become a pretty good attenuator.

The plane will check fine on the ground, because the test equipment is close to the airplane and They're checking for signal function, not for power out. the Transponder will check out great on the bench because their test rig has a new good coax.

If the coax is over ten years old, replace it all.

FCC lic'd Cellular radio tech for 25 years.

[Ron B Keil]

Find a Radio Shop that can test the encoder and the transponder together as a unit on the bench. This can usually be done in an hour or so. They should be able to duplicate your failure conditions. ie temp, vibration, etc. The bench testing should also validate pulse widths, timing and frequencies from the Transponder during both Mode A and Mode C interrogations. If the parts work together on the bench, then it has to be plane wiring. ie. Ground, Power, Antenna Coax, or data wiring to the encoder(one data wire with a bad connection can cause no altitude to be transmitted). The encoder wiring can be tested using a box with switches that simulate an encoder. (These are easy to fabricate, and most Radio shops should have one).