[T2] Gas gage weirdness

[c.dreike]

Started testing my gas gage and sender. The sender worked when I 
installed it. Currently the tank is very low and the sender reads about 
80 ohms. According to The Samba this is about correct. The senders run 
from about 3-5 ohms at full to about 70-80 ohms at empty.

When connected, both of the gauges I have read full and a little past. 
The battery voltage is about 11.98. The voltage across the gauges are 
6.57 on the original gauge and 7.98 on the reproduction gauge.

On the test bench connected to a bench supply, the original gauge reads 
full at about 6.5V and the reproduction gauge reads full at about 7.2V. 
As I reduce the voltage to the gauge, the reading goes down. At 0 volts 
the gauges read empty.

Per the test specs in Bentley, the gauges should read full at about 12V, 
battery voltage. Both my gauges peg when attached to full battery voltage.

I have double checked the wiring against the schematics and all looks 
correct. The circuit looks simple enough. +12V on one side of the gauge. 
The other side of the gauge connects to the sender (5-80 ohms) and the 
other side of the sender connects to ground.

I'm an electrical engineer with lots of experience with simple stuff 
like this and this stuff makes no sense to me. I must be missing 
something. (Kinda like mistaking the wire stripe colors on the turn signal)
Anyone have ideas on what I should be looking for or doing differently? 
I'm stumped. I'm about to build an inverting operational amplifier 
circuit that would drive the gauge so that it would see 0 volts at empty 
and about 7V at full.

TIA
Chris
71 Sunroof
64DD Kamper Kit