help with temp sender/gauge

pulsed voltage on the plus side is normal. That gauge does not work on 12 volts, but rather on about 5 volts. The way Chrysler does this is thru a dash-gauge voltage regulator. You are seeing it in action. It is just a bi-matalic strip that heats up when battery voltage goes thru it. Then it breaks the circuit and cools. This is then repeated endlessly.The result of all this switching is that the gauge sees an average voltage of around 5 volts. The gauge is slow to respond, so normally you don't see this fluctuating.
If your gauge pegs when full ground is applied but does nothing with the sender in circuit, then as others have said. Either the sender is bad, wrong,not connected properly,or not grounding properly. Use no sealer on this baby. It's brass; it doesn't usually take much torque to seal it.