Testing my fuel gauge?

[Ripcity]

So my 66 fuel gauge does not work. I took it to a shop to do some other work and I had them check my sending unit. They said my sending unit is good and they even tested the wire going to the gauge and it has the correct reading so my gauge is bad. I purchased two used gauges and when I install them neither work. I filled my tank with gas. So I take the gauges to my bench and when I put 12 volts to one side and ground the other the needle pegs on both. Does that mean they are good? Or do I have all bad gauges. What kind of reading should I get out of the wire hooking to the gauge when tank is full. Maybe my two used gauges are bad? They said the sending unit is good and the reading they got from the wire is good. I am doing this by just connectiong the fuel gauge to the two wires and turning on my key. Do I need to reassemble everything and reinstall the cluster back into the dash for it to work?

Thanks!

 

[Viperwolf1]

The instrument voltage regulator drops the voltage going to the gauges to about 5V. I say about 5V because it is not a precision electronic device. It is electro-mechanical. It turns on and off in order to average about 5V out. This is the most voltage you should apply to the gauges for any extended period of time with any reasonable expectation that they will survive. The fuel sender is a variable resistor. When the float is at the top of the tank the resistance is about 10 ohms. At the bottom the resistance is about 73 ohms. You can measure this with an ohmmeter to see if it falls into that range of values. There is also a ground wire at the sender that needs a good solid connection to ground.

 

[broncnaz]

The guages will work if not mounted to the cluster. So thats not the issue. Since your guages move when hooked to full 12 volts that indicates that they should be working. They dont actually see a full12 volts when hooked up as the voltage regulator reduces the voltage a little. But I'm not sure how much.
I would say change your sending unit out they are usually the culprit. While the shop may have said its was good did they remove it from the tank and test it? If not they only tested it at one fuel level and it might have been giving a reading at that level but no other levels. Another thing to check is the ground wire at the sendig unit connector if not grounded good it wont work.

 

[Steve83]

FYI
The instrument cluster voltage regulator is a misnomer. It's actually designed to be a power regulator so the gauge readings don't change when the supplied voltage does. But it doesn't do that by affecting the voltage in any way. When it's on, it applies battery or alternator voltage (11-15) to the gauges; when it's off, there's 0V.

 

[oleguy74]

fuel gauge

will this help?

 

[whmoorejr]

in most senders, the resistance coil is exposed to the elements. When parked, bench test, results might be good. When on the road, it might be a different story.

Mine did the same thing. Wire = Good. Ohms = Good. Fuel level always on empty. Just swapped out the sending unit and for the first time since I've owned it... I know how much gas I have.

I agree with broncnaz, swap out the sending unit and see what happens.

 

[70_Steve]

Also, the one thing a lot of people miss is the ground for the sending unit. The sending unit has a two-wire connector. One side comes from the gauge, and the other side is a short pigtail that attached to the frame. If the frame is not grounded, the gauge will not function.

Make sure you have a good frame ground back to the engine block or battery negative...

 

[Jeepster]

I would have to say the gauge is good. (bench test says so) Look to the IVR and see volts from that (analog meter is best)Volts/no volts, repete.......I would assume its also good if the other gauges work but I would test the gas gauge leads anyway.,
If good check ground at the sender and cluster. use a jumper wire to good ground. lastly remeter the sender at the sender and at the gauge. Its possible the float is bad.

 

[Ripcity]

When I put an Ohm meter on the sending unit pig tails I get 74 ohms and I just filled the tank. I used the actual ground wire when I tested it to make sure it works. It is mounted solid. So my sending unit reads 74 ohms and the tank is so full it spurted out a little cause I wanted to make sure it was indeed full. Is that the correct reading I shoud get at full from the sending unit? If I test the wires going to the gauge should I get the same reading? Thanks you guys are great!

 

[bronko69er]

No, full should read 10 ohms. 75 at empty. Bad sender or float fell off.

 

[Viperwolf1]

No, full should read 10 ohms. 75 at empty. Bad sender or float fell off.

I agree.

 

[Jeepster]

I say bad float. If a bad sender it would not necessarily read anything/resistance. Seeing its reading correct for a empty tank, the arm is sitting on the bottom. IMO.

 

[bronko69er]

I have 2 bad senders sitting on my bench that seem to behave that way. You wont know for sure till you pull it out and bench test it.

 

[WyleCoyote]

I have 2 bad senders sitting on my bench that seem to behave that way. You wont know for sure till you pull it out and bench test it.

Dude, was the one I gave you bad also?

 

[bronko69er]

Dude, was the one I gave you bad also?

Yeah. I can make it read correctly, but only if I put an awkward twisting load on the float lever (one that will never orrur in the tank).

 

[WyleCoyote]

Bummer, sorry about that!

 

[bronko69er]

Bummer, sorry about that!

No prob, it was free, I cant complain.