Meter reads zero with key off. Around 60 with key on, & 100 with engine running.
That almost sounds like the sender. If the wire was shorted to ground it would be pegged all of the time.
From the Autometer website. I think most gauges are the same.
My SSE oil pressure and water temperature gauge are reading too high, and getting there too quickly.
Sometimes with this symptom, it will read pressure merely by turning the power on, or read temperature before the engine has even been started. This would be most often, the loose or missing nut on the ground terminal, on the rear of the gauge. The next most common is using poor or insufficient ground for the gauge. After that, check for an incorrect, or faulty sender.
SSE pressure and temperature have no reading.
Assuming the gauge power and ground are proven good, attach a jumper wire from the end of the sender wire (at the sender), and the other end to ground. The gauge should deflect to full-scale reading.
If it does, then check to make sure the sender is threaded into something grounded. For example, threading into an adapter, teed into a rubber hose requires the adapter to be grounded.