I've been working out some wiring issues on my truck and one of them is the fuel gauge. It never has worked. I put a new sending unit in it and ran the wires. I've got a hot wire from the ignition switch to the reducer on the gauge and a wire coming off of the other terminal to the sending unit. The sending unit is grounded. I thought the gauge was bad but I used a jump wire on the sending unit and the needle on the gauge moved. I am stumped at this point. Any advice?

 

It sounds like you have converted to 12 V ? If you removed the wire from gauge at sending unit and grounded it, did the gauge deflect to "full" or just move. If so, what voltage to ground to you have at the sender wire (back to gauge) . The problem may be matching the sender with the gauge range.

This is a tutorial, sorry for the length:

Turn on the Ignition Key.

Keep an eye on the Fuel Gauge Needle.

If Needle does not read full

Found a Problem there?

Repair It

Needle Higher then Cold Range

Gauge Reads empty

 

this is fuel gauge not temp.. so matching senders to the gauge is always fun if either are not stock..

sounds like one of these is not stock.. cause using ground and the gauge worked..

Sam

 

Hi guys,

Hmm, Tom, looks like a "Larry Help" left over. LOL We are talking about the fuel gauge, not the temp gauge, but your test method is exactly correct anyway.

Please allow me to interpret.

Newbldr,

First this has to be an OEM 6 volt sending unit made for the stock gauge or an aftermarket stock replacement sender made for the truck.

Hook your system up as it would normally be configured. Then ensure you have at least 7 gallons of gas in the tank (that's kind of a biggie to have gas in the tank if you want the gauge to register. It will stay on empty with 3 gallons in it and the voltage reducer will make a two needle width error low, if the generator isn't turning and you are using straight battery voltage).

Turn the ignition switch off.

Hook a jumper wire with an alligator clip onto the top of the sending unit stud. Set the other end on the seat or someplace not grounded.

The gauge needle should be parked below empty.

Turn on the ignition switch and note what the gauge does. It should migrate up to 1/4 full. Give it 30 seconds - it takes longer with reducers.

If it doesn't move, take the other end of the wire with the loose alligator clip and touch it to ground somewhere away from the gas tank. It should migrate up to full.

If this happens, your sender is bad.

If you want to test it, remove the jumper wire and turn off the ignition. Remove the sender from the tank. Leave the wire from the gauge installed on the sender.

Ground the sender top plate using the jumper wire.

Turn the ignition on and move the float arm all the way up. If the needle moves to full you need to adjust the gauge. If it doesn't move, the sender is bad (new or not).

Good luck!

PS Larry's temp gauge is all fixed by the way. It was an oversized hole for the wire mounting screw on the overtemp sender. There was a loose connection and no continuety to the variable sender (and thus to ground)

 

Testing all guages should be a "generic" test; I suppose the rider , insert fuel where temperature could have been added to the tutorial for clarity; but being that we are FTE'ers that would be unnecessary, yes ?

I guess after being an industrial maintenance electrician most of my working days I got off track.

 

I have tested the sending unit with the truck running and just using the battery voltage. I put 5 gallons of gas in the tank and whatever was in there before. I'll put some more gas in it to see if it registers. If not, maybe I odered a 12v sending unit by mistake? Is there any way to tell?

 

I agree Tom. These gauges are really pretty simple and all the same if you use the right senders. We know that but I guess other folks get intimidated because it's electrical and tend to make it more complex than necessary.

There's no "instant" way to tell if the sender is 12 or 6 volt. You have to take meter readings of resistances etc. Even if it was 12 volt and working you'd get some readings.

This isn't that complex really. If you use the jumper wire and touch the top of the sender (where the wire is attached) and the other end to ground, and (ignition on)the needle moves up to full, that indicates the gauge is good, power is being correctly supplied to it and the wiring circuit is complete back to the sender. The only other part is the sender itself. Are you sure the sending unit is adequately grounded, and that the tank is adequately grounded? (or for that matter is your cab adequately grounded?) Try connecting that jumper wire onto one of the sender mounting screws, then to a known good ground and check to see if the needle moves.

If not, the sender is bad. And don't think that because it's "new" it can't be bad. About 30-50% of all these "new" aftermarket electrical parts are bad coming off the Chinese or Mexican shelf.

 

I did jump the sender and the gauge needle moved to full. I jumped to from the terminal going to the gauge to the ground terminal on the sending unit. It seems like the tank would be grounded good since the sending unit is grounded to it. I used one of the mounting srews for the sending unit for the ground.

 

Newbldr

Since you get needle movement at your gauge when you ground the sender wire it sounds like your gauge is working. This only leaves:
(1) whether it is compatible with your sender and (2) whether sender is it any good.

If you have a multimeter, just for giggles why don't you measure the amount of resistance that your sender puts out, when empty and full. If you had a variable resistance source, you could adjust the resistance level on the sender wire and see how your gauge responds. Or maybe you know the range of the sender now. If the resistance doesnot change with different fuel levels then sender is obviously not working. If the resistance varies then you may have to put a resistor in series with the sender, to scale the sender to the gauge.

I am at a similar point with my guages, I need to determine what range of resistance my meters need for zero and full deflection, then build my sender circuit to correspond.





Tm

 

What year truck is this? If it is BonusBuilt with tank-in-cab, the tank sits in rubber mounts and the only possible grounding is the bolts that hold it in and the fuel line. If it was painted, could be poorly grounded.

 

Don't confuse "being physically connected to" with "electrically grounded" One is a physical connection, the other allows an electrical flow. Do you have "continuety" from the sending unit ground tab (or mounitng screw) to a KNOWN electrical ground - like the frame? Or, do you show any voltage flow from the sender stud to the sender mounting plate?

If you touched the gauge wire to one of the sender mounting screws and the gauge needle moved, then the ground is ok, and your sender is bad or the wrong one - just that simple.

Exactly!

But what sender did you install? Question again, was it a 6 volt replacement sender made for your truck?

 

Sorry I haven't checked in lately. I've been working on my wife's cars.

I still haven't got the gauge to work. I made sure I had power at the gauge/regulator and everything seemed good there so I used a piece of wire and jumped the sending unit. The needle went to full so I'm assuming it's the sending unit. I ordered it about 1 1/2 years ago and just now wired it up to the gauge. I remember debating whether I needed a 12v or 6v because I was running the 6v system a the time with plans for changing it over and I think I may have ordered the 12v. I just ordered another sending unit yesterday so I'm crossing my fingers, hoping this works.

 

Well, I finally got around to replacing the sending unit and I now have a working fuel gauge. Thanks everyone for the help......again.

I'm leaving to drive the truck to Shrevesport, LA in a week and a half so it's time to work on the brake.

Thanks again. - Brian

 

Super! Thanks for the follow up!

 

Hi everyone who posted on this about the sending unit. I had the same problem and found that the thin wire that connects the terminal to the spool inside the sending unit broke. It broke due to the terminal turning when we tightened down the nut that held the wire onto the terminal. I was able to open up the sending unit and solder the wire back on but it took a long time to figure this out. It only takes a slight turn of the terminal in order to break the wire. Just wanted to put my 2 cents in, thanks.