I can think of a few possibilities here, but they are just guesses at best......you're 3.4L might have had an idiot light switch and not a sender....or your 3.4L wiring called for 2 wires, instead of one, which would incorporate the ground wire in it's loop.....instead of using the motor as a ground. Like I said though, I'm just guessing. Hard to do anything else without being there and seeing it.

If you ran the 3.4L sender to your 3.0L gauge though you probably did the same thing I did and burned out your gauge. So don't be too disapointed if the gauge still doesn't work when you get the new sender hooked up.