Temp Gauge /Sensor troubleshooting help needed for mercury ob 135hp

[gilligan12]

Issue Temp gauge is pegged at 220 when engine is stone cold.

So far:
Gauge is good: disconnected sensor wire (tan) from gauge at it drops to zero, ground sensor terminal and gauge pegs (12v adn GND good)
Replaced water pump impeller and seals as well as thermostats.
Water is pumping through engine
Sensor wire continuity is good (0 ohms from engine to gauge)
Resistance across both sensors when engine is off (cold) .3 to .9 ohm (I'll run engine and see if the resistance changes sometime this week)..

As soon as I reconnect sensor wire and turn key, gauge pegs
Am I going down the correct path ??
Any suggestions appreciated.

[oldman570]

Sounds as tho the gauge and senors are not compatable, as the sensors are grounded and they should not be when the engine is cold, for the gauge to work correctly. You can go to the gauge manufacture web site and see just what senders that the gauge needs. Your on the correct path. Check to be sure if the gauge is for water temp or head temp as they are diffrent.
Oldman570

[gilligan12]

Appreciate your response.
These appear to be the original parts (gauges and sensor) on the boat (2000 Searay 180) so Im not sure how they would not be compatible. All of the gauges mounted on the instrument panel are the same color from the back...none appear to be newer. The wire (Tan) that I disconnected form the port side block at the sensor was the wire that connected to the gauge. I verified with continuity test