fuel cell sending unit

[psychorodder]

I have a 65 truck with a fuel cell the gauage which is autometer reads full all the time I checked the fuel gauage have power and ground to it ran new sending wire to it someone said I need to run ground from gauage to sending unit anyone had this problem.

[kwkenuf]

Is your tank sending unit connected to a good chassis or body ground?

[psychorodder]

Yes it has a good ground I was told the resistance is critical and was told to ground from gauage back to sender I do not know if this would make a difference or not the guage is new do not know if sending unit is or not

[kwkenuf]

You might want to do a search of previous threads, this has been discussed at length recently. Basically the sending unit operates the gauge by varying the resistance to ground making a complete circuit. If you have your gauge hooked directly to ground you have essentially bypassed the sending unit. Your gauge has only two terminals. (not counting lighting) One gets 12V switched power (+) the other gets hooked to the sending unit (-). The sending unit also has 2 connections (1 normally a stud arrangement, the other a blade terminal). The stud terminal is connected to the gauge as described above. The blade terminal is connected directly to a nearby good chassis or body ground. That should fix it, unless you have a bad sending unit or gauge...

[psychorodder]

Here is what I have guage 3 post not including light black to ground red to 12 volt pink has s and goes to s on sending unit and ground on sending unit to frame

[kwkenuf]

Originally Posted by psychorodder

Here is what I have guage 3 post not including light black to ground red to 12 volt pink has s and goes to s on sending unit and ground on sending unit to frame

Tough to read without punctuation, but I'll assume that the ground connection on the gauge (the 3rd terminal) is to ground the case so the one wire light connection wil operate. Since your problem appears to be a shorted connection to ground indicating a full tank all the time, here is what you can do to isolate the problem. Verify that both your gauge and your sending unit are not shorted to ground. Using a meter and with wires disconnected, Check between the "S" terminal and the gauge housing (should be an open circuit) and between the "S" terminal on the sending unit and the tank body (should be a high resistance circuit depending on tank level). If either of these tests indicate direct continuity to ground, check the terminal itself to see that the stud is isolated from the mounting surface with either isolation washers or a bushing of some kind preventing direct connection to ground. If both these tests check out good, just for grins disconnect the ground wire connection to the guage and see if the gauge works properly...... Also if you haven't already, test the wire between sending unit and gauge by disconnecting from both ends and testing for continuity between that wire and chassis ground. It could possibly be chaffing or pinched somewhere. There is not much more to it than that.. I suspect either a cross connection on your wiring or a defective sending unit. Hope this helps.