fuel guage pegged. wrong pump/sender?

my 89s fuel guage is pegged at 3 Oclock. ive disconnected the leads from the pump and to the guage side i used a jumper from the ground and the guage wire (center of pin connector). turned the key to on and the guage went back to empty like it should have. but when i plug the connection back up, the guage pegs out again. im thinking the PO installed the wrong year pump/sender which i believe changed in 91 and newer. any way of testing my theory? to me it seems like the wire for the sender at the pump side connection is the outside of the three available. and it should be the center wire. but the pump is working fine when i run it. does the pump and sender lead put out the same voltage? can they be reversed? i looked up new pump prices and saw they are about $129. ouch just to get the guage working again. any ideas? and yes i did search and all the threads didnt have a conclusion to the problem.

1. 12 volt to power the pump.

2. 12 volt to power the gauge, but the wire at the sender is the gauge's ground for the variable resistor--the sender.

3. Possible you have the fuel pump ground connected to the fuel gauge. For your 89 XJ the higher the resistance the higher the needle reads. Zero ohms = empty; 44 ohms = 1/2; 88 ohms = full. A complete ground would peg the gauge.

great info thank you. i will check with an ohm meter as soon as i can.

update: i checked the wiring with an ohm reader and got confused. just didnt seem correct. so i removed the pump/sender from the tank to trace which wire went to what, and wouldnt you know it? the wire to the sending unit was off its clip. it was a loose fit but i think it was knocked off by the PO when he installed it. i installed a better connector and hooked it up. then put the ohm reader on the pin connector and moved the float up and down. got 0 - 90 ohms. so i do have the correct pump. re-installed it all, turned the key, and i have 1/4 of a tank. yippee. im relieved i dont have to purchase a new pump/sender.

Congrats!