New Fuel Sender, Now inaccurate Fuel Gauge Reading

[450SL]

I just installed a new fuel level sending unit from Ricambi in my '75 Spider, and now my fuel gauge doesn't display properly.
First, the low level light comes on at the proper time. I know I don't have the wires reversed for the low-level and gauge.
However, the fuel gauge reads about two needle widths from full when the float is at the bottom, and of course reads FULL when the float is at the top. As a double check, I get a FULL reading when I ground the wire to the gauge.
It appears I have a mismatch of resistance between the sender and the gauge.
The sender I ordered indicates 1970-1980.
Does anyone have any info in perhaps installing an inline resistor to correct the problem?
Do I now need to put in a new compatible fuel gauge?
The sender doesn't appear to be faulty, just a mismatch.
Maybe I'm wrong, but looking for thoughts/ideas/fixes.

[vandor]

Why don't you contact the vendor (me ? It could be a faulty unit, mind you it would be the first one I've ever seen. Somewhere I have notes on what the resistance values should be, but if I can't find them I can measure a sender tomorrow at work.
All the gauges were the same, so that would not be the problem.

[spider2081]

Maybe somebody can confirm this:
I believe if you disconnect the fuel gauge wire at the sender in the trunk the gauge should pep beyond full.
When you ground the gauge wire from the sender the gauge should read empty.
Not sure why you changed the sender in the first place but maybe your fuel tank is not making a good ground connection to the car body. Try adding a ground wire under one of the nuts holding the sender in the tank and connecting it to a known good ground and see what happens.
I realize the fact the fuel warning light works implies there is a ground, however the light can tolerate a poor ground and still light

[TX82FIAT]

If you were in a border town with Mexico I'd tell you to take the wrapped bricks of weed out of the fuel tank and all will go back to normal with your fuel gauge readings. Or you could contact the vendor or run diagnostics.

[vandor]

>I believe if you disconnect the fuel gauge wire at the sender in the trunk the gauge should pep beyond full.
>When you ground the gauge wire from the sender the gauge should read empty.

Just the opposite.

[131]

Why was the sender replaced? Hook up the old one and see what it does.

[spider2081]

Thanks Vandor
I have a Fiat Electrical Diagnostic Manual for 80-81 and in its troubleshooting it states disconnecting the sender causes pointer to go maximum. After your replay that I was incorrect I remembered I had a defective sender in the basement. I opened it and fully agree with you a disconnected sender should cause the gauge to read less than 0 and shorting the sender wire should make the gauge peg beyond full.
I tried to measure the resistance of the winding in the defective sender. I'm getting 3 Ohms full and 380 ohms empty. Not sure if this is correct.

[vandor]

That is the range I remember, very little resistance when full and around 500 when empty.

[TeeCee]

Couldn't a broken wire cause it to read wrong?

[Bennie]

I thought I would add to the mystery. I installed a new fuel sender, that I tested, and the gauge still shows empty. The wires involved have continuity, the idiot light comes on when I ground it, nothing changes when I disconnect the sender at the gauge or ground it, and no reading of any sort when testing the yellow black across to the pink.
Have I found the elusive broken gauge? I live on the west coast of Canada and have to order the parts from Ricambi in Texas. The prices are not bad but shipping is yikes, so I want to make sure before ordering. Anything I might have missed? Last 74 Spider standing in these parts.

[aj81spider]

I'm not an expert here, so defer to others who give better advice. However, when I look at the '74 wiring diagram it looks like there are two wires coming off the sender and going to the fuel gauge. One appears to ground the light and the other appears to apply the variable resistance to ground that drives the needle. Before I replaced the gauge I would check the wires on the back of the gauge for resistance to ground. It's possible that one of the connections (the needle) is bad, while the other (the light) is good.

It's also possible that I'm misreading the schematic. However, it's easy enough to make a quick check on the wires. On my schematic it looks like the blue/yellow wire drives the needle and the gray/red drives the bulb. Power appears to come in on the white.

[18Fiatsandcounting]

A.J., I think you've got it right. Here's how I see it:

The grey/red wire on the sender unit provides the ground for the "low fuel light". The other side of this bulb is always positive (with ignition on), usually through a yellow/black wire.

The other wire on the sender unit (different colors depending on model year) provides the variable resistance to ground to deflect the fuel gauge needle. The positive to the meter is usually through a yellow/black wire, often the same one that powers the low fuel light.

I'd first check that your yellow/black wire has a solid +12v on it with the ignition on. Next, if you manually ground the grey/red wire, the light should come on. If you ground the other wire, the gauge should read "Full".

Note that your gas tank must be grounded for all this to work. Make sure it has a good ground to the body.

-Bryan