fuel Gauge not working

[JD1973]

Hey I have a crazy problem, with the truck not running but key in the run position the fuel gauge reads correctly. With the engine running the fuel Gauge moves way past the full mark to almost point straight down.

I'm sure there is a grounding issue some where, But I'm not sure where to start looking.

anyone here have ant ideas on this.

Truck is a 1973 chevy C10 step side.

 

[chengny]

Here are the wiring dwgs we talked about:

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[JD1973]

thank you