Sender Isolation

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When I went to calibrate my fuel senders through the Blue Mountain Avionics Engine Pod, I did not get any reading. Continuity tests showed that the connection was good and the senders were grounded. So I contacted BMA and was told that the common ground was not good for the sensitive Engine Pod. I would have to isolate the senders from ground and use a dedicated return wire.

So, off came the tanks and I removed the senders. Fortunately, my senders were installed with the rubber gaskets instead of the permanent Pro-seal method used by some. I took the opportunity to order and install new gaskets. However, the gaskets don't isolate the sender because the screws through the attach plate to the nutplates in the tank ground it to the aircraft structure. Therefore, it was necessary to install shoulder washers under each screw.

I could not find shoulder washers to match the thickness of the sender plate so I ordered some with extra length. I drilled the plate to accommodate the washers and then trimmed them to length with an X-acto knife. I also replaced the screws with longer ones to make up for the thickness of the shoulder and, in one position, the terminal ring. The terminal ring was sized to fit the washer and went under it against the plate. That way, it grounded only on the plate, not the screw.

Once the plates were reinstalled, I pressure-tested the tanks again, reinstalled them, and routed the return wires to the Engine Pod. This time the fuel sender calibration went smoothly. Here are some pictures of the installation:

Grounding Terminal and Shoulder Washer

Here is one of the grounding terminals installed on a shoulder washer. The plate has already been drilled for the washer and the washer will now be installed and trimmed to length.

Sender Ground Wire

This is the ground wire installed under the shoulder washer and installed on a sender.

Trimming Shoulder Washers

This shows the shoulder washers installed on a sender and being trimmed to length. I used a screw to keep the washer from deforming as I trimmed it. The washer with the screw in it has been trimmed and the excess is just sitting on the end of the screw.

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