Ron,
I'm afraid it's not going to work. The wire is the sender has a higher resistance than ordinary wire. The span can be anywhere from 20 or 30 ohms to over 175 ohms.

The short length of 12 AWG you used will have a resistance of less than 1 ohm.

For more info, check out http://auto.howstuffworks.com/fuel-gauge.htm

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120