A diagnosis tip for a 12v system. The sender in the transmission (and all senders) varies from no pressure to high pressure. Different systems work differently however the extreme is disconnected or grounded.
You could turn on the electrics and disconnect the sender. The gauge should state zero. If the gauge has a reading it isn't the sender.
If the gauge always reads zero, disconnect the wire to the sender and ground it. If nothing happens, again it isn't the sender.
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