Re: Temp gauge problem
The sending unit is basically a thermistor, that is a device that changes its resistance depending on the temperature. A change in resistance changes the voltage that the gauge will read. The gauge is merely reading a voltage between 0 and 12 volts (depending on the gauge) and displaying it with a needle.
There are three posibilities on where your problem is located:
1. At the sending unit
2. At the gauge
3. In between
To test, you need to have a multimeter (you can measure voltage, resistance and current), you can buy one at Walmart for under $10. Also, you might need a long jumper cable that you can make yourself with #12 wire also from Walmart. Run the following five tests:
1. Test the sending unit: Use the voltmeter and measure the voltage between the sending unit and the engine block. You should be getting a positive voltage. If you get a negative or no voltage, your sending unit is bad.
2. Test the ground on the sending unit: If test 1 was OK, use the voltmeter and measure the voltage between the sending unit and the negative battery. Ideally, you should get the same reading as in test 1. If you get a different reading, your ground connection from the battery and the engine might be dirty or corroded and might need to be cleaned.
3. Test your gauge: After performing tests 1 and 2, you know your sending unit is OK. Now, read the voltage between the two terminals at the gauge. Ideally you should get a very close reading to the readings you got in tests 1 and 2. If you do, and still you get negative temperature, then your gauge has gone wrong. If you get a negative voltage that means your gauge is OK and is just translating the negative voltage into the negative reading.
4. Test the ground on the gauge: If your gauge is OK then you should be getting a negative voltage on step 3. Now, measure the voltage between the positive terminal on the gauge and the ground on the battery (using the long jumper cable). If you get the same voltage as in step 3 (a negative voltage) your ground is OK. If you get a positive voltage, then you have a ground problem.
Also, you might want to measure voltage between the ground at the gauge and the engine block. The voltage should be zero. If there is a different reading, then one or both of the grounds are not OK, and I would repeat tests 2 and 4 to find which one.
5. If your sending unit and your gauge are OK, then the electrical connections between both of them should be inspected. A wire might have a crack and it might be touching other parts of the boat providing thus producing voltage drops. I would measure the resistance in the cable that go between the sending unit and the gauge. Resistance should be close to zero. Other than that, then there is something wrong with that cable.
Post your findings please, and good luck.
