Weird sound coming from Ardupilot Mega IMU shield/Oil Pan

I soldered on the headers to connect the Ardupilot Mega to the Oil Pan IMU shield.  (I'm still waiting for the 3x8 right angle headers...those aren't on yet)  When I hook it up to my computer via USB, there is a loud, sort-of clicking sound.  You can get the general idea from the video.  If I had to pinpoint the sound, it sounds like it is coming from the Relay.  Any ideas?

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Thanks, Jani !


It turns out it wasn't anything I did wrong and the problem was actually with the ArduPilot Mega.

Using a multimeter on the relay switch of the IMU, there was AC passing through that was causing the relay to rapidly turn off and on; hence the clicking noise. As seen in the highlighted part of the board schematic attached, a pin on the relay is connected with the header pin located directly underneath the 'ua' of 'Dual 3.3V Reg'.

If you match that up with the ArduPilot Mega, it is pin PL2. Again using the multimeter, there was a short between PL2 and PL3 of the ArduPilot Mega. Tracing these, they go into the ATMEGA 1280 chip.

If you look at the schematic online, they correspond to pins 37 and 38; pins that are right next to each other.

Even looking underneath a magnifying glass, I couldn't see anything in the between the soldering joints holding on the ATMEGA 1280, but I took the smallest needle I could find, and ran it through the spacing. Then, measured the resistance between PL2 and PL3 again and voila, no short! I put the boards back together, hooked it up via mini usb and no more clicking!

As a side note, I'm not sure who here made the schematic for the IMU, but I believe the relay was drawn incorrectly to how it actually is on the board. I think it should look like this:

I also uploaded the changed file.
Nice one, Tony! Excellent detective work. We haven't had any other reports of similar manufacturing problems, but if we do we now know what to tell them. Many thanks...

I'll bring the schematic change to Jordi's attention.
Just ignore it, when i designed the library i swap the pins order and i batch some boards year ago and didn't work, to make it work i just swap the signals on the schematics to save time...

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