Ground the single wire and the gauge should go full scale - don't leave it grounded long.

Here's the 89 manual #'s:
Fuel gauge resistance value (ohms)
terminals 1 (power) & 2 (gauge) 55
terminals 2 & 3 (gnd) 165
Fuel gauge unit resistance value (ohms)
Float point ôFö: 1-5
Float point "Eö: 103-117


Not responsible for advice not taken...