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Kirkham Motorsports

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  #1 (permalink)  
Old 06-23-2019, 02:39 PM
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Cobra Make, Engine: ERA slabside #3014; 331 CID SBF
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Default Open circuit at fuel sender

The fuel gauge in ERA 3014 (a slabside with SW gages) has remained off the bottom of the scale, even though I now have 9 gallons onboard.

I disconnected the sender connection behind the wheel well access plate and connected a 210 ohm resistor across the white and black terminals of the chassis harness. This moved the needle up to a reading a little above empty, as expected. I believe this exonerates the wiring at the gage and saves me a foray behind the dash.

When I check the resistance across the corresponding wires (white and black) of the sender harness, I see an open circuit. I'd expect this to be around 100 ohms given the fuel state.

I've heard that grounding of the sender can be an issue due to sealant, but since my car has a dedicated black wire in the sender harness I don't believe I'm reliant on a mechanical ground where the sender mounts to the tank. The black wire in the chassis harness is definitely zero ohms to ground.

Any ideas? I'm mainly interested in knowing whether there's any further debugging I can do with the tank in place. At the moment, this isn't a high enough priority to warrant removing the tank for further investigation.

Bill
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Old 06-23-2019, 08:42 PM
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Bill -you can read about my battle with my fuel gauge here.

Need help with gas gauge

Good luck!
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Old 06-24-2019, 07:26 AM
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What happens when you ground the (harness) white wire where it connects at the tank connector? Ignition on.
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Old 06-24-2019, 06:49 PM
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Quote:
Originally Posted by strictlypersonl View Post
What happens when you ground the (harness) white wire where it connects at the tank connector? Ignition on.
Gage moves up to top of scale beyond Full, as expected.
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Old 06-25-2019, 07:32 AM
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Default ERA Fuel Sender

Bill:

Here are photos of the top of the tank and fuel sender from ERA812. I suspect the sender in your car is the same. There is no ground wire on the sender, the ground is established through the mounting screws to the tank. This is where the ground on my sender failed due to the sealant on the screw threads.

Since you are getting an open circuit between the white wire and ground, this indicates your sender is not grounded properly.
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Old 06-25-2019, 08:30 AM
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Chris -

Thanks for the picture - very helpful.

In my case there's more than just the white wire coming from the sender. If it were only one wire, then I could see how effective grounding of the sender to the tank would be essential to get a meaningful resistance reading at the gauge.

But my tank harness has three wires, including a black that is presumably a dedicated ground connected to the sender body. So an open circuit across the black and white wires in the sender harness suggests to me that one or both of them is not connected properly at the tank end (or that there's an open in the sender itself).

Since I don't have the tank out of the car, so far I'm just looking at the plug end of the tank harness; I'll try to see if there's a way to see the other end by taking out the trunk bulkhead panel.

Bill
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Old 06-25-2019, 08:47 AM
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Bill, are you familiar with this drawing of two versus three wire Smiths unit? It would be pretty hard to wire up the two-wire units without a pic as the connections are not intuitive.


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Old 06-25-2019, 09:37 AM
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Thanks, Patrick.

The plug end of my tank harness is a 4-pole connector with 3 positions populated: B, W, and P. Since my car is a 289 with SW gauges, I'm presuming but have not yet verified that:

1. I have a 2-wire sender
2. The hot P wire is unused and insulated from ground at the tank end of the tank harness; and
3. W and B are connected to the sender signal and sender body respectively

But I haven't secured access to verify this yet.

If the connection of B to the sender body is missing or bad, then I'd see myself as susceptible to the same result that Chris saw when trying to get a single white signal wire to work with bad mechanical grounding from sender body to tank.

Bill
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Old 08-27-2019, 08:11 PM
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Reviving a thread from a couple months ago because I have the answer.

And best of all, I didn't have to drain and pull the tank to diagnose or fix!

The problem turned out to be a misconnected tank harness. I couldn't see this until I drilled out the rivets holding in the flange that accepts the trunk bulkhead screws. With the flange out of the way, it's possible to peer in and see the connections from the tank harness to the tank.

Since I have the SW gauge, only the white and black (resistance signal) wires are needed, and the pink (+12V) wire is not used. On my car, the black wire in the tank harness was correctly connected to tank ground. But the white wire was cut off at the sender end, and the *pink* wire was connected to the sender output.

When connected to the chassis harness, this means that the sender output was connected to +12V, the sender chassis was grounded, and the white wire - where the SW gauge expects its signal - was open.

The correct way to fix this would be to pull the tank, disconnect the pink wire from the sender, and attach the white wire. The *easy* way is to move the white wire in the chassis connector into the pink position. This connects the white chassis wire to the sender output via the pink sender harness wire, and everything works.

I also moved the pink chassis wire into the unused position on the connector - I didn't want to swap it into the white spot because the sender end of the white wire is simply cut off, and I don't have access to insulate it properly. Putting pink into an unused position ensures there aren't any shorts/sparks next to my gas tank when the pink wire is powered with +12V.

If I ever take the tank out I'll swap everything around so the colors make sense.

Bill
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