I know it varies, I have two identical senders, one measures 244 ohms, the other measures 196 ohms, cold, no coolant. The 244 ohm unit registers about 1/4 on the temp gauge, the 196 ohm unit registers about half way on the gauge. Engine not running, hasn't been run for a month and no coolant in the engine, just key on.
Aftermarket shows the same sender for my 74 and an 89 model year.
The gauge, cluster and wiring all came from a 93 Ram Charger 5.2 magnum. That used a different sender with a different connector. It's taller and the connector would interfere with my fuel line to the carb. I don't have one to measure its resistance. So I'm trying to figure out what the senders resistance should be at X temperature and if the early senders ohm readings are different from the later magnum engines.
It looks to me like around 300 ohms would register cold on the gauge and as the engine warmed up the resistance would go down and the temp gauge would go up.
If that isn't correct then I have another problem like a ground issue or something.