OK I edited your post for ease of read..hope you don't mind..
OK, let's address each issue as listed then deal with what remains..
- OK, sounds like you have the rating for the output side of the drop..If you note it s exactly half of the 12 volt math..
Current = 12 volts divided by 1.5 ohms = 8 amps
- The reason it didn't reduce voltage (I assume) Is because there was no load on the resistor.. If you put 12 volts on a 1.5 ohm resistor, and measure to ground , It will read 12 volts..If you then add a 6 volt lamp..It will read 6 volts..at the resistor between It and the lamp to ground.
If you "Unload" a lamp, by removing the GROUND wire to it..then put a meter between the lamp ground, AND actually GROUND , the lamp will not light BUT it would appear the GROUND wire is 12 volt positive..Yet if you GROUND the junction of meter and ground, it will go to 0 volts and the lamp will light without a "Shorting" Effect..This is because the lamp filament is consuming the current..doing useful work (lighting the lamp) ..
- Not a Pontiac Guy at all..but wanted to be sure a vehicle of that vintage wasn't "Born" as a positive ground..
- The sender wire SHOULD be separate..To test for open / Short condition..Get your DVOM, Set it for OHMS , R X 1 , Calibrated 000, run a LONG jumper from the disconnected sender end to one probe on your meter..Remove the wire from the back of your meter..hook your other probe to that wire..The reading should be 000, If not replace the wire..THEN if good, measure that wire (either end) to ground...It should read infinite..If not, replace the wire..If so It's good..
Senders, Are Gauge/Circuit Specific..and Visa~Versa..you Can't just toss a sender at the gauge and expect it to operate accurately...If at all..You need to determine the resistance of the sender circuit on the gauge (On yours, that vintage, I'm guessing 0 to 30 ohms..) And MATCH the sender to the gauge resistance..
A temperature Sender can not read "AIR" It must be immersed in the flow of the media in which it is being tested..(I.E. in the water or stream)
To accurately test, you must boil a pot of water, (212 degrees) and place the sender fully wired AND a lab or other accurate thermometer in the water..
As the water Temps decline , Note the readings on both..If you have a portable hot plate..you can heat it back up, then note the differences as it rises..
Make a list as you do, and you will find the accuracy and linearity of the gauge..and know at a certain temp, it's 5 degrees off for example..
The sender IS probably OK, Just wrong value for the system to be accurate..
To test for Value, Go to radio shack, get a collection of 1 watt resistors..in the following values..30 ohms, 60 ohms, 90 ohms, 120 ohms, 150 ohms, 180 ohms, 210 ohms, and 240 ohms..
Next , monitoring your gauge, place one resistor in line on the sender wire, to a known good hard ground..as the meter climbs , note the value..keep "Upgrading" the resistors until "It Just Barely Pegs" the gauge..note that value..it is the resistance Value you need to get for your sender..
Say that value is 30 ohms..then remove YOUR sender and go to the parts house, and get a value sender that will be 0 to 30 ohms..AND have the same thread pattern as yours AND the same depth..and that will fix that.
BE ADVISED..Some senders work in reverse..It may be that it needs a 30 to 0 to be right..The test still applies, except for the meter drive will be backward, AND when you go to get a sender..be sure it's 30 to 0 not 0 to 30..
If your sender has Less resistance than is required , you can add a resistor on the sender line to bring it within spec, (I.E. a 0 to 30 sender needs to be a 0 to 60 sender..add a 1 watt, 30 ohm resistor) The reverse will apply if the sender is too high, but you need to do a little Math to figure the value, and make that value resistor, go between the sender line and Ground..
The best thing is GET the RIGHT sender..ton's of them in different values out there..