ELECTRICAL/AVIONICS QUESTION

Hi All

My electrical knowledge is VERY basic so I have a question for all of you electrical avionics gurus.
I have a GARMIN 335 ADS-B which has been functioning perfectly since installation 11/2018. Recently I started receiving messages indicating "pressure altitude failure" which according to the troubleshooting guide suggests a loss of signal from the altitude encoder. The GARMIN receives pressure altitude info via the RS232 serial output wire from the harness of a DYNON D-100 which has a built-in altitude encoder. The pressure altitude will function for a few seconds and then drop out and then return and so on. I realize there are many sites that could be the culprit. My first thought was to ascertain that I was receiving a continuous signal from the serial RS232 out wire from the DYNON D-100.
I can easily access this wire where it connects to the GARMIN 335 RS232 serial input wire on the ADS-B harness. Is there a simple way to attach a meter of some sort to the RS232 output wire from the DYNON and check the signal?

Sorry for the long post.

Peter K
RV9A 800 hrs.

The 232 output can be looked at with a scope.

Multimeter

Hi Peter,

There is no simple way to check the signal per se. It's a bunch of square waves. What you can do is check the continuity of the signal wire between the two devices. You will have to determine the pin number at each end of the cable and use a multimeter which had been set to continuity. Place your probes at the appropriate pins at each end of the cable and hopefully read zero on your meter with associated buzzing or beeping. The continuity setting normally has a diode symbol and a sound wave symbol. If you have an open circuit, your multimeter will probably display OL.

Your symptoms certainly suggest a loose connector or broken wire. A broken wire can be intermittent so be sure to wiggle the cable and connectors as you test it.

I hope that helps.

Cheers, Sean

Try disconnecting the connectors on each end, and then plugging back in. Sometimes it's the simple things.

A loose wire also seems likely.

As mentioned, intermittent problems like yours are likely to be a bad crimp or failing wire. These can be quite difficult to diagnose with meters for an amateur, due to the intermittent nature. I would simply replace the one wire that is causing the issue. It is somewhat unlikely that the wire itself is the issue, so if there is enough slack, just pull the two D-sub pins off and crimp on new ones. You will need a D-sub extraction tool, wire stripper and a crimper. You can get these from Stein Air or Amazon. If you have a friend that built a plane, they likely have these.

It is also possible that the Dsub pin was not fully seated into the connector and therefore creating a poor connection. This should be obvious when you pull the cover off of the connector and examine the backside of the connector.

Larry