69 CJ5 Fuel Gauge Question

I finally decided to tackle some of the wiring issues. I've traced most of the wires in the entire Jeep and I'm in the process of making a wiring diagram, both as a learning tool (to see which wire goes where and try to figure out the flow)and also so that when something goes wrong, I'll have the diagram to refer to. One problem I've had since I've owned the Jeep is that the fuel gauge has never worked. Here is how it is wired:

1 wire is power coming in from a dash-mounted toggle switch for the light bulb. It is jumpered to all 4 gauges.

1 wire is connected to a tang on the back of the light bulb and goes to a good ground. Jumpered to all 4 gauges.

1 wire comes in from the sending unit.

1 wire runs from the back of the gauge to a good ground.

1 wire runs from the back of the gauge and goes through a 15 amp fuse in the switched fuse panel that is labeled "IG COIL METER" and then on to a stud on the 12V side of the ballast resistor on the firewall. This seems sort of like a roundabout way to power the gauge, but it seems to me it should work whenever the key is in the "run" position.

What's really confusing me is that there is only one stud on the top of the sending unit. That's where the wire is connected that leads to the gauge. When the key is on, there is power to that wire, verified by the test light. Here's the thing--when I had the test light hooked onto a good ground on the body and I probed that wire, the gauge came on and read what I think was a pretty accurate 3/4 tank. If I hold the test light on the top of the stud on the sender, the gauge works. The stud is tall enough to attach a ground wire and nut it down. This is what I want to do--is it OK? I'm still learning about electrical and it seems a little odd to me that the power and the ground should come off the same connection. Which way is the current flowing? Logically, from the sender to the gauge, right? That's the gauge's function--to read the ouput from the sender...so is it that the sender needs "power in" from the ground to be energized? I've always thought of a current going "into" whatever is being powered and then "out" through the ground. Can someone enlighten me and advise me if I can attach the ground as described without activating the ejector seat (underseat tank). The gauge hasn't worked for 2 years. I'd love to get it working, but at this point I also really wanna know exactly how it all works. Thanks and sorry for the long post!

 

I hope someone answers this one for you, I have the very same question.

 

I'm confused about your statement of "4 gauges", it makes me think you are dealing with aftermarket gauges.
Stock original Jeep gauges for a '69 will be only a fuel gauge and a temperature gauge.

Post up the diagram you have made, it will help us to figure out what you have.

Don

 

I should have mentioned they are aftermarket gauges. Also, I misspoke in the original post about the routing of the wires. I was trying to chase them myself and missed something. My son helped me this afternoon and here is how the wiring goes:



The third wire on the ballast resistor is the one that goes thru the fuse panel and then back thru the firewall to the brake light switch on the pedal. These are the 3 wires connected to the ballast resistor:



I guess whoever wired this was using the stud on the ballast resistor as an easy place to access power when the key is on. These are the gauges:



And this is the sending unit. I attached another wire on there, thinking it would be OK, but haven't grounded it to the body yet. Negative battery cable is unhooked...



Thanks for any help and advice.

Fuel gauge is really on "E" as you can see in photo, not like shown in diagram. And the wire that powers the bulbs jumpers to each gauge and comes from the toggle switch on the right in the photo of the gauges.

 

Your test light is telling you that power is getting to the top of the sender and that you need to make a good ground to the tank. Do not ground the wire going to that stud on the sender, as it will burn out the gauge. The ring holding the sender usually has a tab on it to connect a wire to run to ground.

 

OK. Thanks Walt, I will ground the tank one way or another.

 

Went out to see how to attach a wire to that ring. Turned the key on first and the gauge worked! Maybe a bad connection that I tightened up while screwing around with it. The sender is grounded through the metal tank being fastened to the body I guess??

 

Yes. You can add a wire jumper from the sender to the body, and avoid any problems relying on the tank grounding to the body.

 

I'm confused....are you saying that you have power at the post on the sending unit? If you do something is wired wrong. You should not have power at a gas tank sending unit! The fuel gauge should have power to the back of it from an ignition(keyed) source and it gets its ground through the sending unit. The sending unit is a resistor. As the fuel level rises from empty the resistance is decreased and the ground signal becomes greater causing the gauge to rise towards the full mark. Adversely, as the fuel level drops resistance increases and the gauge drops to empty. Twelve volt DC systems are easily understood if you think of every device as work. The power is attracted to the device when the device is grounded. As the power flows toward ground it is captured in the device and converted to "work". In the case of the fuel gauge it works to keep the needle at the right position. In your headlight it is forced to cause a filiment to glow. In your heater motor the power is forced to spin the motor and so on. If the power makes it all the way to ground without being used as work a fuse will blow or a wire will melt which actually is considered "work" or welding in some cases. SOOOO, back to your gauge, it should have 12 volts on one post on the back of it and a wire from the sender on the other post. If your gas tank is poorly grounded you may need to run a wire from the body to one of the screws that hold the sending unit in the tank just to make sure you have a good source for the sending unit to work. We just installed a plastic tank in our '62 and had to do this since plastic in not a very good conductor...duuuh.

 

Thanks for all your help. I'm gonna have to add that ground wire. Just got back from a ride and the needle was flickering all over the place. There's only one stud on the sender so I'll have to attach the wire to one of the tabs on the ring like Walt said. The tabs are too wide to just slide a female connector on. Maybe I can file the sides a little or else drill a small hole, run the wire through and solder it.

 

Fixed it! Narrowed down one of those tabs on the ring with tin snips, then sanded it clean.



Connected the ground wire good & tight...



and it works!



I just wanna say that for those of us who never had someone to pass down their knowledge and experience about working on old vehicles, it means a lot to be able to come on here and get help from complete strangers. You guys are the best! LT4NOW, hope this benefited you too!

 

Yep there is always a lot of help from the guy's here. Glad it was as simple as a wire. Not all are this simple tho.