Fuel Gauge.

[Spool]

Hey Guys.

Have a 1973 Chevy C20 for about 2 years now and the fuel gauge has never worked; just sits at 3/4 full.

2 years ago a mechanic tells me he tried to replace the fuse but no good and he thinks that it may be the fuel sensor.
Fair enough. Until I happened to go behind the gauge and speedometer today and found that the fuel gauge has 3 connections that are not connected to anything.

Does anybody know if I could trouble shoot this before trouble-shooting the fuel sensor?


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Thank you.

[hatzie]

Those clips connect the studs on the fuel gauge to the printed circuit.

[Spool]

Thanks Hatzie.

So if my Fuel Gauge isn't working.
Do you recommend a certain process of elimination before dropping the tank to check the fuel sensor?

[ronmauer]

One of your moderators has written a pretty good article on testing gauges on this site.
http://forum.73-87chevytrucks.com/smforum/index.php?topic=25339.0
Since you already have the cluster out follow the printed circuit board and use an ohm meter and test the gauge and connections. I believe you should have some continuity between any 2 of the 3 terminals. If you have the cluster connected you should be able to disconnect the wire at the tank and the gauge should go way past full and ground the wire and the gauge should  go to empty.
 

[hatzie]

Rich's writeup is quite good.

Pull the fuel gauge out of the bucket exposing the gauge connectors.

With the cluster bucket plugged in and sitting in the dash...

Bottom terminal should be ground  (GND)
LH terminal should be ignition power (IGN)  toward the Speedometer.
RH terminal is the sender (SNDR) toward the outside of the cluster bucket.

Set your meter to roughly 20 volts DC range. 
With the ignition on the meter should read battery voltage from GND to IGN terminal.

Set your meter on the 100 ohm or more resistance.
Measure from GND to SNDR.  Full will be 90ohms. Empty will be 0ohms.

You can test the gauge with a 12v battery or DC bench supply, test leads, and some kind of resistance that you have control over...  a resistance decade box or 100ohm linear Potentiometer or some 1/2W resistors... @ 30ohms, 45ohms, 75ohms, and 90ohms.

Apply power to the IGN terminal and Ground to the GND terminal.  The gauge will float to around the 3:00 position past Full.
Short the SNDR to the GN terminal.  The gauge should float to E.
Installing @ 90ohm resistor between GND and SNDR will set the needle close to Full.  The closer your resistance is to 90ohms the closer the needle should be to full.
If you use a decent Linear Potentiometer you can use a Digital Multimeter to set the wiper pretty close to the table below and plug it between the GND and SNDR terminal.  The two potentiometer gauge tester in Rich's writeup works for most resistance gauges.

I've run a bench test on 20 or more fuel gauges from this generation truck with a DC power supply and a General Radio 1432 decade resistance box.  They are all pretty close to the following averages.

F=90 ohms
7/8=76 ohms
3/4=64 ohms
5/8=53 ohms
1/2=45 ohms
3/8=37 ohms
1/4=29 ohms
1/8=18 ohms
0=0 ohms