Ok, here's how to test power & ground for fuel & tach:
1. with key ON, test for power at harness-side connector brown/black wire
2. with key ON, test for power at gauge-side connector brown wire
3. measure resistance between chassis-ground and harness-side connector black/yellow wire
4. measure resistance between chassis-ground and gauge-side connector black/yellow wire
unplug ECU connector
5. measure resistance between ECU-connector black wire and harness-side connector black wire
6. measure resistance between ECU-connector black wire and guage-side connector black wire
7. measure resistance between ECU-connector black wire and chassis-ground
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