I pulled mine and cleaned it awhile back. The reserve light circuit is tied to the float. When the float reaches the bottem of it's travel, the light circuit is completed and it comes on. I checked the OHM reading on all the lines and they were in spec, gauge still off a little. I just adjusted the needle to point to exactly empty when the flat was at the bottem of it's travel. Works reasonably well, old car's fuel gauges are seldom precisly accurate. Light comes on every 450 miles or so, I put in 16 gallons, and repeat.

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K