Dropping the voltage for fuel sending unit?

I have the tank out an plan to reline it with the "RED"stuff from auto store.An ill replace the sending unit. question is where does it get its power? I understand the fuel guage is not 12 volt..My cluster is on the work bench with no wires hooked up. How can I check to see if its all working ok before installing everything..as said there are no wires hooked up any where?

 

On standard early CJ, sender power comes FROM the the fuel gauge, which incorporates an internal voltage regulator.

You didn't specify model year, but maybe this will help. http://farm4.static.flickr.com/3431/3966212004_392c903cfe_o.jpg
Battery power comes to terminal #1 (via ignition switch). Tank unit is fed from #3.

 

The sender is a variable resistor. The fuel gauge detects the sender's resistance by comparing the voltage drop from a fixed resistance at the gauge to the voltage drop through the sender. This circuit is called a voltage divider, which you can Google if you want to learn more.

If you have a multimeter, you can check the sender with the resistance scale. Your FSM will have the spec for resistance at full and empty. The sender grounds through the tank, so you must have a good ground from the sender flange to the Jeep body.

 

I wouldn't test it out of the jeep. The grounding in this system is complex, and a mistake can take out your sender before you can say "what is that burning smell?"

It is fair to assume that the new fuel sender works, and the gauges work as well if they are new, so put it together and then check it out.

For what it is worth here is the way I set up the gauges / grounds.
Always put a separate ground wire on the fuel sender housing at the tank.
Always make double sure that the fuel gauge itself is well grounded to the speedo housing. This can be tricky, paint is not your friend here.
Always ensure that the speedo housing has bare metal - to -bare metal connection when installing in dashboard. Spray paint after making the connection if desired.
Always put a separate wire grounding the dash to the frame or body tub.
Always put a separate wire grounding the body to the frame.
Always run a ground strap from engine to frame

Corroded grounds in the fuel circuit are the primary reason that the sender burns out. It is also easy to make a dodgy ground when using rubber hoses or insulators when installing the drive train. A certain and sure ground on the components is one's best insurance.

Chasing ground problems is never very much fun, so I always go overboard on making sure that grounding won't be a problem.

 

Sender resistance should be 10 ohms at "full" and 73 ohms at "empty."

 

Dam lots of great advice!!!!TKS. A LOT!!!Its a 71 v6....I am planning on doing my own wiring.So ill have more questions as time goes on...

 

I always bench test the complete wiring and equip in the dash while out of the jeep when doing resto or harness repair. If you have a bad ground somewhere it only makes that component inop. The component will have an incomplete circuit and will not draw any current so you will not fry the component. I presently have the dash from my step-daughters Tux that I am doing this very same thing to. And yes I build my own wiring harnesses complete. Have fun

 

Ill send off an get a new tank sending unit..when I get it all I have to do is show the back of the gauge 12 volts. ground the sender unit an cluster an move the sender by hand an it should work, if the gauge is ok? right?

 

On my newly acquired 68, there's one of each gauge. Neither work, but I'm hoping after reading this thread, that will change. Great information here.

 

Since maybe 1 year, I try to connect my gauge. Odometer with gauge is new, the sender unit was new. I tried to fix this at the pro shop, unsuccessful and my new gauge became out of use (wires teared on the rheostat inside the tank) so I bought a new one.
It is not connected yet, I plan to make tests soon.

I'll use the old/new sender to make tests out of the tank.

How can I test the gauge on the dash? If a use a 5 volts battery, or some batteries? just to see the needle move?

All what is explained by you Jeepers pro will be carefully attentively studied.

This is the only problem I have to solve, in 2 years I have repaired or changed a lot of parts, and now I have the pleasure to drive a nice comfortable Jeep.

Thanks to all.

 

Ok so I can bench test the new sending unit by showing #1 ON THE Diagram you sent an hooking the sending unit to the stud marked , an grounding the cluster ..all should move when i move the arm on the sending unit. right?,,,,,,,

 

I can't tell you how good that advice is - I just burned my regulator out the other night and my gauge was in the Jeep.

 

There is one ground in the system that is often missed as the gauge has to be removed from the cluster to see it. After gauge removal you will see two tabs bent over holding the ground plate. It's located in the center of the gauge between the three studs. You need to clean this conductor plate (fine emery/sand paper) and clean the inside of the speedo housing where this ground point makes contact. Without this ground, the regulator contacts will stay closed and shoot a steady 12V to the fuel indicator winding and burn it.

P.S. I always leave the sender wire off on the initial test and I use a test light between the sender wire and ground. When I turn on the key, the test light will flash off then on and continue to do this. This test tells me the voltage regulator is pulsing properly and I can see the fuel indicator move up scale. The indicator movement upscale will depend on the internal resistance of the type of bulb being used in your test light. Manufacturers use different bulbs. My test light brings it to about 1/4 on the gauge. Yours may vary.

 

thanks all,,,lots of great info..
what a great sight...