EGT fluctuation on D-180

[ncq6c1eq4u]

400 hours on RV-12 912ULS Flightdek 180...

One of my EGT indicators shows >2000 at startup, fluctuates for a minute, then settles down to normal operation for most of flight, then acts up on short final. Variations from 0 to mid-range until shut-down. Then 2000 again.

This is the second time this has happened and I worry that it could be a portent of future problems. I re-seated all of the EGT connectors FWF. What does this sound like and where should I start the trouble-shooting?

TIA, Jim

 

[planecurious]

The background below may not apply to an RV-12 with its prefabricated wiring harnesses, but the solution is likely applicable.

The most troublesome part of thermocouple CHTs* and EGTs on Dynon EMS units is the connection between the harness (stiff brown wire) and the sensor wires. Dynon uses "faston" connectors, which work, most of the time, but are trouble-prone. The usual problem is that use of a typical builder's crimp tool cannot apply an adequate crimp onto the "faston" connector that the installer applies to the stiff thermocouple wire on the harness. Such a connection usually works after the installation, but over time, heat, and vibration, the (crimp) connection can become intermittent, showing wildly varying CHT and EGT readings.

To correct such an issue with a thermocouple, it's not really sufficient to just "check" or "re-seat" connections. When a thermocouple starts to act intermittent, the connection usually has to be completely re-terminated.

In my opinion, these connectors are a very good replacement for the "faston" connectors that Dynon provides. The idea is to strip back the insulation from the respective wires with sufficient length to overlap inside the connector, and then insert the stripped wires on opposite ends. Then, tighten the Allen screw and that compression provides a very, very solid connection between the two thermocouple wires.

* RV-12s and other aircraft that use Rotax engines do not have thermocouple CHTs - Rotax pre-installs a "thermistor", not thermocouple CHT sensors on their engines.

 

[ncq6c1eq4u]

Well, it's good to hear that the problem is likely in the connection, albeit not the connectors themselves. Your suggestion would have been my next attempt to solve. I found on other RV-12's that the small wires from the Dynon really need to be doubled over before crimping the connector to the wire. Sometimes even a gentle tug will pull the wire out of the connector.

I'm guessing that the heat generated from the engine starting up expands something enough to make the connection solid for a time. Maybe power reduction does the reverse.

I will wiggle the connectors with battery power on to see if I can verify a loose connection. Then, if I can extract enough wire to strip and recrimp....

 

[Robski]

Sometimes even a gentle tug will pull the wire out of the connector.

Sounds like the builder needed a better crimper, ideally an AMP one (hugely expensive), but at least something like this:
http://www.harborfreight.com/ratcheting-crimping-tool-97420.html
and definitely not something like this:
http://www.harborfreight.com/8-inch-four-way-crimping-tool-92410.html

For aircraft wiring a decent terminal crimper is a must. The fact it saves a HUUUUUGE amount of time as well as doing the job right is a bonus!

I'm no great expert, only being on my third aircraft rewire, but when you get a decent crimper you'll wonder why you didn't get one years ago!  :-?