Sending Unit/Fuel Gauge Conundrum

[1979bettywhite]

Yes, I have a lot to learn in this area. Here is my issue:

78 Cherokee with a 1990 360. Aftermarket/separate temp, volt, and oil pressure gauge. Fuel gauge on the dash is still operable-ish. Poly tank installed at some point.

When I fill her up to the brim with gas, upon the first crank gauge reads anywhere from 1/2-3/4 full. Starts to move down slowly from there with each additional shut down and crank. It will usually get to "E" and I expect I still have half a tank or so.

I have checked the ground at the frame and that seems to be good.

So my question is, what process should I undertake in figuring out what might be the issue? I know on my CJ, the fuel gauge power is transferred over from the power to the temp gauge. But in the Cherokee's case, the original temp gauge is not hooked up. But with the gauge still working, I assume it is still receiving power. Just would like to have a working gauge if possible, instead of playing the guessing game on actual fuel left.

Thanks.

[tgreese]

The '78 TSM will tell you what the ohm values are for the factory sender. No '78 TSM here, but it should be 73/23/10 for E/half/F. Using your multimeter, test the sender to see if it is reading correctly for the amount of gas. This is described in the TSM. Follow the instructions for testing.

The FSJ uses the same interuptor-style voltage regulator as the CJ, except that the VR is in the temp gauge instead of the fuel gauge. If there is no power to the temperature gauge, the fuel gauge should not work. If you've removed the temperature gauge completely, you can substitute a 7805 chip to provide the 5V regulated power that the gauge requires. There are plenty of old posts about this.

Also note that the VR will not cycle unless the gauge is grounded. The cluster normally grounds through the dash. Thus if you test the cluster without the ground to the dash, the VR will not cycle and full voltage will go to the gauges all the time. This will burn up the gauges. Using the 7805 avoids this, since unlike the Jeep VR, it will go open if the ground is not connected.