City: Clearwater, FL
Vessel Name: Seas the Bay
Vessel Model: 1981 42' Hardin Europa
Join Date: Aug 2016
I don't know what you have, but assuming you have the ubiquitous setup with a temperature sender with one terminal (the engine block being the ground), a single wire going from there to the temp gauge, and a 12V supply to the temp gauge...
Those senders usually have really high resistance when cool and much lower resistance when hot.
You can try disconnecting the wire at the sender and measuring from the stud to a nearby really good DC negative ground. It should measure a large few to a small several hundred ohms at room temperature. Then you can try running the boat for a while and, fully warmed up, it should read a few dozen ohms. If this doesn't work -- the sender is bad.
You can also disconnect the "S" wire at the gauge (making the resistance nearly infinite) and the gauge should read as close to zero as it goes and stay steady there. Then, you can short across the two leads of the gauge with a screwdriver or alligator clip or wire. The gauge should jump to and hold a nearly topped out reading. You probably don't want to keep it shorted forever, but doing it long enough to test will be fine. If this doesn't work, the gauge is bad.
Lastly, you can test between the "S" wire and a good DC ground. You should see a resistance reading very close to the one that you saw measuring in the engine room. If not, your wiring may be bad. The normal failure case would be to see a significantly higher resistance, indicating a bad wire or connection, e.g. corrosion at a terminal or splice connector or wiring harness connector. If you see less resistance, the sender wire might be chaffed to a ground. That is also a possible failure case.
And, finally, you should check between the other terminal on the meter and a good ground. In this case, you want to measure voltage. You should see a good, steady, 12+V. It should be the same as you see if you measure for your other gauges, etc.
Be careful to keep in mind when you are checking voltage versus resistance. You'll need to set your meter correctly. Setting the meter for ohms when there is a voltage could blow it.
As other suggest, I think what you'll find will that the gauge isn't getting enough voltage and is, therefore, mis-measuring the temperature.
Typically, these are "one wire" circuits. Most commonly, +12V is supplied by one wire to one side of the gauge. It then goes through the gauge and comes out via one wire that takes it to the temp sender on the engine. It then goes through the temp sender on the engine to engine. The engine acts and the negative/ground/current return path.
I forget whether temperature gauges are typically ammeters (measuring the difference in current flow as the resistance of the sender changes. Or, volt meters, measuring the resistance of the sender by measuring the voltage across a known-value resistor, wherein the resistor and sender act as a voltage divider. Either is possible. I'd have to measure the resistance across a gauge or do some math to figure out which would be likely to work better.
But, in any case, since the "jump" is happening when you turn the key, it seems related to the +12V supply. Given that, running a new wire to the sender isn't likely going to be helpful. It is more likely that the +12V wire is the one of concern.
What would tell you that the wire to the sender is bad is if you get a different resistance (ohm scale) reading between the sender (when disconnected from the wire) and a //good// neg-ground in the engine room and a significantly different reading between the other end of the sender's wire (when connected to the sender) and a really //good// neg-ground on the other end by the gauge. Much higher resistance would point to a bad connection. Much lower resistance would point to chaffing to ground.
If that wire is bad, if the resistance is high, I would try to clean terminal/stud screw, find the connections and terminators on it, cut them back a bit, and recrimp them. Especially if they look crufty. If it were very low, I'd look at likely chafing to neg-ground points, e.g. around metal things and other terminals, look for chaffing into the metal of the wire, and insulate, as needed. If that failed, sure, I'd run a new wire.
Same for the +12V wire. If you determine it is the problem, you might be able to solve it by cleaning connections and/or cutting a bit off an end and putting on a new connector/terminator.
Most often when wiring goes bad, it is at the end with connections, not in the middle, which is sealed up much better. Chaffing is the exception.