Need guidance on why my fuel gauge got scorched

[MA1920]

The electrical issues started when mice made a nest behind the instrument panel, described here Ford 1920 alternator output

Things were working fine until earlier this year when the temp and fuel gauge stopped working. The needles were on C and E respectively. A couple weeks ago the fuel gauge needle "disappeared'', it was pegged to the left as far as it could go. While removing panels to find the source of a leak Advice needed on leak (I'll have an update on the leak tomorrow) I figured it would be a good time to work out the electrical bug.

My electrical troubleshooting skills aren't the greatest, but I checked continuity from the affected sensors to the back of the gauges to make sure no wires were broken. Everything was ok. I took the fuel gauge out and the back of it was burned or scorched, not severely but enough to be noticed. There was a wire in the gauge that was broken. The meter housing was also lightly scorched. I took a few pictures to clarify.

The middle picture in the second row shows the broken wire and the copper winding that is black. The front view of the gauge shows the needle pegged past empty. The other pics show different views of the scorching.

I don't want to buy a gauge until I resolve the underlying problem. As I stated my electrical troubleshooting skills are not very good, so I'm asking for any advice or guidance on how to determine what caused the gauge to get fried. All help much appreciated. Thank you.

Bob

 

[RickB]

Check the instrument cluster ground path.

 

[MA1920]

Rick, I checked the continuity of the grounds, they all were good. While tracing wires, I noticed the fusible link was being held together by 2 strands of copper. As I mentioned I struggle with my electrical diagnostic skills so I have a few questions.

Would the fusible link have an effect on the gauges in question?

What does the fusible link protect?

It doesn't appear to be chewed. What would cause it to "blow"?

The last remaining strands of copper broke when I was removing it.


Thank you

 

[RickB]

Yes.
Everything electrical except the starter motor.
High amperage draw and /or a short circuit.

 

[RoyKing]

Without being able to see the circuit we have use logic. Obviously too much current passed through the gauge, which to me means the rats shorted the lead to the tank gauge sender. So you need to measure resistance sender from empty to full like 80-230 ohms. Replace the gauge and the fuse link.

 

[MA1920]

I ordered a new fuel gauge and fusible link. I want to make sure there aren't any other issues going on that could affect the new parts. After work my son and I pulled the float and checked the resistance as it moved. We had one probe on the positive lead to the sensor and the other to a good ground. As the float was moved the resistance would increase for an instant then drop back to 0.11, which is where it started. As I'm typing, I think we might have been on too high a scale. Were we checking the sensor/float correctly? We'll recheck it with the meter on a lower scale. As always, I appreciate the help.

Bob

 

[RoyKing]

The sender should show smooth change like 240-33 or 10-180 ohms

 

[MA1920]

The sender should show smooth change like 240-33 or 10-180 ohms

We tested it again today and there was hardly any variation over the range of motion. Is there any other way to verify the condition of the sender? Is it just coincidence that the fuel gauge got fried at (I assume) the same time? Am I at the point where I have to open up the wire looms to look for damaged wires?

Thank you.

 

[RoyKing]

The sender is shot, it could have caused the problem. If the resistance is very low it could have caused the failure.

 

[MA1920]

The sender is shot, it could have caused the problem. If the resistance is very low it could have caused the failure.

The reading throughout the entire range was just above 0. 0.07-0.11 is what the meter read. As I stated in post #1, the temp gauge wasn't working either. We disconnected the lead from the temp sensor, put the positive meter probe on the lead to the gauge and the other probe to a good ground. From what I read that should have made the temp gauge move towards H. Nothing happened. This was before I discovered the broken fusible link. When I order a fuel sender, do I need a temp gauge and sensor too? Only the fuel gauge, with the scorching, had evidence of electrical overload. Is there any other method to test the temp gauge?

Thank you gentlemen,

Bob