Working on a 73 1.7.
Original center console 3 gauges.

Just got around to installing a "taco plate" and sending unit.
Power off - needle is below the lowest temp mark.
Power on - it is exactly at the lowest / start of range on dial.
After some run time (warm weather almost an hour ) the gauge needle moves about 1/4 of an inch.
This seems to be not working correctly.
To test the gauge can I ground the sender wire and what effect should that have on the gauge movement ?
Thinking it should peg the gauge into the hot / far right sweep.
If it does would the culprit be a wrong sender ?
Ordered this sender from AA and it was meant to be the right, one but now thinking it is wrong .

Your help or thoughts ...

Different senders give different readings. The 200C sender was what came stock. You can calibrate your gauge to a known point on the scale by submersing the sender in boiling water (212 degrees). Gauge and sender (with the taco plate) out of the car, hook up everything to the car battery. Positive (+) to the gauge, ground the gauge at the taco plate to negative (-) and the G terminal to the sender. Touch the red zone with temp. Gauge 914.641.118.20 (1973) and 200C sender is 300 degrees.

See attached.

When you ground the sending wire at the sender, the gauge needle should go full hot. If it does that, and you still feel you're getting a low/bad reading, it is probably the sender.

If you don't get full hot, try shunting the ground and signal wires on the back of the gauge. If the gauge goes full hot, your problem is almost definitely with the sender. If not, the problem may be with the gauge.