inaccurate temp gauge -strange ground reading

[buzkill]

I have a 1990 3.9 Mercruiser that the temp gauge barely reads above 120. Grounded the sending wire and gauge moves accordingly. Thought maybe problem with thermostat or temp sending unit. Both seem to test okay. I checked the tan sending wire at the temp sensor. I was getting about 9.5V. Shouldn't I see 12V going to the sensor? I Have 12V at the ign terminal on the gauge. I checked the ground at my gauges and it shows perfect continuity, however when I turn the ignition switch to run the ground shows a reading of 25 ohms. Is this normal?

 

[MrBigStuff]

Re: inaccurate temp gauge -strange ground reading

I don't know about your specific configuration but the way I'm familiar with them working is the battery voltage is applied across a resistor divider. The temperature sender is a variable resistor. Its resistance changes with temperature. The sender and a fixed resistor are connected in series across the battery supply. The gauge is connected to the junction of the two resistors. As the resistance of the sender changes, so does the voltage across it. The gauge reads the voltage and responds accordingly.

 

[Gone]

Re: inaccurate temp gauge -strange ground reading

Your ground isn't exactly ground is why you are reading 25 ohms with the ignition on. The ohm meter uses its own internal battery. The residual voltage at the ground upsets the ohmmeter circuit giving an erroneous reading. You don't want to measure the resistance/continuity of any circuit that is powered up.<br />I could get very detailed but I wouldn't want to bore you with too much info.

 

[Silvertip]

Re: inaccurate temp gauge -strange ground reading

Disconnect the wire from the temp sensor. With the engine cold, measure the resistance of the sensor (terminal to ground) with engine cold and warmed up. Here are some "relative" readings:<br /><br />Cylinder Head Temp Sensor:<br />60 degrees = 1195 Ohms<br />140 degrees = 192 Ohms<br />220 degrees = 46 Ohms<br /><br />Water Temp Sensor:<br />100 degrees = 450 Ohms<br />175 degrees = 99 Ohms<br />250 degrees = 29.6 Ohms<br />Note that the type of sensor and gauge you have will affect the sensor resistance. These are for reference and may vary somewhat from whatever type you have.

 

[buzkill]

Re: inaccurate temp gauge -strange ground reading

Thanks everyone for making this picture a whole lot clearer. I'll look at those readings UPinsmoke mentioned.

 

[Gone]

Re: inaccurate temp gauge -strange ground reading

Note that Upinsmoke's numbers for both type sensors have the resistance decreasing as the temp goes up. This is referred to as a negative temperature coefficient probe. The higher the temp, the lower the resistance. If you are reading a lower number in temperature than you should be, then there is probably an increase in resistance(or corrosion) in your wiring. Be also aware that the failure mode for these type probes is to read erratically lower and lower and lower until they open. Erratic is the key word here.