Fuse value for fuel gauge?

[jrberriospr]

Hi

I'm restoring a CJ5, but it does not have the fuel circuit fuse. This circuit feeds the gauge of fuel and temperature but I do not know what is the value of the fuse that I must install?

 

[GRR]

Hi



I'm restoring a CJ5, but it does not have the fuel circuit fuse. This circuit feeds the gauge of fuel and temperature but I do not know what is the value of the fuse that I must install?

The fuse listed for the gauges is a 10 amp fuse.




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[gmillican]

How about for the 12 volt ignition hot wire going to the Fuel Gauge? What value is that supposed to be?

 

[TSB8C]

The fuel gauge has three connections. The red switched 12V source comes from the fuse block through a fuse. Then you have the pink wire from the in-tank sending unit that provides a variable resistance to ground via a separate ground wire mounted to the frame at the rear near the tank. Then there is a third connection that is a jumper over to the temp gauge. This supplies about 5-6 volts to the temp gauge that then has a purple wire with a stripe going to the temp sensor on the engine (through the firewall junction block) which provides the variable resistance to ground through its mounting. A common issue that can cause problems with the fuel gauge is related to the factory splice in the wiring of that 12V switch hot feed. There is a splice that branches off the 12V source to also feed the brake warning indicator. and the 4WD or "Trac" indicator. These factory splices are notorious for getting rusty and corroded and causing loose and intermittent connections or increased resistance in the circuit. On the fuse block pic below, fuse #8 feeds the gauges and is shown to be a 7.5A fuse. Some diagrams also show a 10A (as "GRR" noted), and some show a 3A, depending on the year. I've seen issues with the 3A systems intermittently blowing that fuse. It's too borderline. I suggest you just pop a 10A fuse in there and don't worry about it, unless you are having wiring/splice issues to deal with.

 

[gmillican]

This is all great info above. I really appreciate it.

One issue I've been dealing with is that the Fuel Gauges from OMX ADA seem to be grounded from the S post on the Fuel Gauge to the Back of the body of the Fuel Gauge itself. That doesn't seem right to me. Won't that just cause current to go thru the case/Speedometer/Dash and short the circuit instead of going all the way to the back to seek its ground thru the Sending Unit?

 

[TSB8C]

You are correct and that is definitely wrong. The "S" terminal is the connection for the sender unit in the tank. ("I" is for ignition source). It should not be grounded at all. It gets a variable resistance to ground based on the level of the float in the sender unit. If it is grounded all the time, it will just peg the gauge and never read the actual fuel level.

Now if you're reading that "S" terminal with an ohm meter (no wires attached) to the case of the gauge and seeing what appears to be a short to ground, then you are reading through the coil of the gauge itself, which while not zero ohms, is still low. That would be normal to see a very small amount of resistance between the fuel gauge "S" terminal and ground. But if you are saying that the Omix gauge actually has some type of grounded connection between the "S" terminal and the case of the gauge cluster, then that would be a problem.

Another possibility - are these some newer style aftermarket gauges with only two terminals on the fuel gauge. I've seen some of these that don't have the 3rd terminal on the fuel gauge for the strap that goes to the temp gauge. These newer style fuel gauges only have the "S" and the "I" terminals, and if not used along with the newer style temp gauge as a matched pair, they won't work right. With these newer style, the two "I" terminals should be connected. Here are the correct readings in ohms with no wiring at all on the stock style gauge:

• S to Ground 68-72 ohms
• S to I 19-21 ohms
• S to A 19-21 ohms
• I to A Zero
• I to Ground 49-51 ohms
• A to Ground 49-51 ohms

So to shorten all of this, if you have the original style 3-terminal fuel gauge, hook it all up normal but without the sender unit wire connected to the "S" terminal (leave the "S" terminal open). With the ignition on, you should read +12V on the "I" terminal and on the "A" (the strap going to the temp gauge). The temp gauge should be working and obviously the fuel gauge should read completely empty since the "S" (sender unit) is not connected. If you don't get these readings, you have a wiring problem with the ignition wiring (red wires through the splice I mentioned earlier and connecting to the "I" terminal) and/or your gauge is bad. If the readings are correct and as long as you know the sender unit in the tank is good, connecting that to the "S" terminal should make it all work. And just for reference, your sender unit should read:

• 73 ohms empty
• 23 ohms half tank
• 10 ohms full

If you have a few resistors laying around of values close to these, you can disconnect the sender unit and use some clip leads to connect different resistors to the "S" terminal and ground to test the gauge performance.