Water temp not working

Here's the details:

'86 Allison with original Mercury gauges. All work except the water temp. Powerhead had no sending unit so I drilled starboard head at top and installed new sending unit from ProMarine. Hooked up light brown wire to gauge, but no reading at all on gauge, even when running the motor pretty hard. Needle on gauge never moves from pegged out at the bottom. Disconnected wire from sender and grounded it, with the ignition on, to test gauge. Gauge pegged out at the topside, then dropped back when wire was removed from ground. Have tried heating the top of the head with a heat gun to see if the gauge will register. No dice.

Could it be that the ohm rating for these old gauges are somehow different from that of the new sending unit? Also, could it be that I have such superior cooling in the powerhead that the gauge just won't read that low? Even though I have tremendous water pressure at speed, I can't imagine the powerhead running that cool. According to trim position, water pressure is as high as 18-20 psi, with a Sportmaster lower.

Thanks for any and all help.

Run a dedicated wire to the gauge, if that don't work you got the wrong sending unit for the gauge.

How does one go about matching a sending unit to a gauge?

Update: Finally determined that most late made sending units will not work with gauges made '96 or earlier. Found that info on AutoMeter's website. So, just finished installing a complete set of new Autometer ProComp Marine gauges in the Allison, less the GPS speedometer, which is backordered. Evidently the ohm range of the older gauges doesn't match up with modern sending units.