Water Temperature gauge problem

[Little Harbor]

Using an infrared thermometer pointed to engine block right next to sender i read 171 degrees. What would be a good estimate for the difference between the block temperature and the adjacent coolant?
My water temperature gauge is reading high and seems to be dependent on the input voltage. If I provide an independent battery (12.15 volts) with the battery ground tied in to the engine block ground the gauge reads 185. If i use the ships electrical system with alternator running (14.46 volts) I read 200 degrees . This happens with 3 different gauges. If the gauges are used on the generator system they all read correctly. This all happened after a 3 month boatyard haul out. Also when the voltage is read from sender post to ground post on the instrument I read 4.68 volts. When I take thee same voltage reading on my generator water temperature gauge I get the same reading, but the gauge is reading 280 degrees.
Any ideas?

 

[alldodge]

The gauge reading being different based on input voltage sounds about right. Thermostat housing IR 177 degrees
12.15V = 185
14.46V = 200

If the gauges are used on the generator system they all read correctly. This all happened after a 3 month boatyard haul out.

When I take thee same voltage reading on my generator water temperature gauge I get the same reading, but the gauge is reading 280 degrees.

I would look for a bad or corroded ground

Is the Gen actually overheating to 280 degrees?

 

[Little Harbor]

Sorry bad typing. The generator reading is 180 degrees. My intent was to show that the same gauge, with a gauge sender terminal to gauge ground reading of 4.68 volts shows 180 on the generator and 200 on the propulsion engine. Therefore the variable would appear to be the input voltage to the I terminal of the gauge. in one case 12.15 volts in the other 14.46 volts. These gauges are supposed to account for that voltage difference.
My question about the IR reading of 171 on the block is that if the coolant versus block temperature difference is normally not that great, than the engine is definitely not overheating and it must be the gauge.

 

[alldodge]

This helps but guess I'm still not fully understanding everything.

Using one sender and having it installed on the Gen the gauge reads 180 with 4.68 volts

Move the sender over to the drive motor the same gauge and sender reads 200 with 4.68 volts

Now if your just moving the gauge an not the sender, then the sender is a variable along with wire length and grounding

As I'm sure you know, voltage is applied to the gauge which the sense lead goes to the sender which is a variable resister to ground. Varying the resistance will increase the amount of current passing thru. If this is a north American sender the resistance should be 240 to 33 ohms. Other resistance which will cause variations is wire and grounds. The sender it self may have some variations.

Don't think your question has anything to do with one sender supplying two gauges (local and helm)?

 

[Little Harbor]

I am not changing the sensors. All 3 gauges read the same on the Generator and all three read the same on the engine, so I believe the gauges are working. The engine sensor provides the correct resistance readings at 75 degrees and at operating temperature so I am assuming sensors are correct. Both sensors also provide the same voltage measurement (4.68) across the gauge sender terminal and gauge ground even though they are reading different temperatures (Generator - 180, engine 200).
The voltage input into the engine gauge is 14.46 due to effect of the alternator. I took a stand alone 12v battery reading 12.15 volts and wired it to the temp gauge and removed the ships positive input to the gauge. I tied the ground to the standalone battery to the engine block ground so the sender would work. the gauge then read 185 degrees - 15 degrees less than when ships voltage was used. I am thinking it has something to do with the alternator voltage. Maybe not a pure DC voltage? Seems like when the engines alternator is not in play the gauge reads correctly.

 

[alldodge]

Could be noise from the ALT. but just doesn't sound right unless its closer to AC signal, but that usually blows other things. Using a stand alone bat you might also be getting a better ground