How does the stock temp gauge work?

[ScotY]

I want to buy a fan controller that says it can tap into the stock water temp sending unit provided it is "variable voltage 0-5v." 

I measured the voltage at the the stock Samurai sending unit on the thermostat housing (completely warm engine) and did not see what I was expecting.  The voltage fluctuates all over the place, from roughly 2 volts to as high as 6 volts...it is continually changing up and down.  I pulled the terminal off the sending unit and measured again and basically got the same results.  I measured the resistance of the sending unit (warm) and it was roughly 50 ohms or so. 

I don't understand how the gauge and sending unit are wired together.  Can someone explain so that I can determine if the fan controller is going to work?  My simplistic brain makes me want to think there's 12v and you have a gauge and sending unit all in line, going back to ground.  Does the gauge act like a resistor?  Is that why the voltage on the wire at the engine is about 1/2?  Why is the voltage I'm measuring fluctuating so much?  I'm assuming something isn't right but my gauge seems to work perfectly.

[Kamster]

Can't say 100% but resistance is a function of temperature.  Usually higher temp means higher ohms.  I would think that the resistance changes as the temperature of the coolant changes and that affects the gauge.

Just thinking out loud here, so basic electrical math V=I*R (volts equals current times resistance, in a DC circuit).  I would think the voltage is constant at 12 volts or so in your system.  So if you buy into the fact that resistance is directly proportional to temperature, then as the resistance increases, the current would have to decrease to maintain the constant voltage.

Without performing an actual experiment, this is what I believe is going on.

[ScotY]

Hey Kamster,

Thanks for your thoughts.  Electrical stuff always confuses me!  I hear what you're saying and I still assume the voltage you'd see at the sending unit should be somewhat stable, assuming the temperature is stable.  Does that sound right?

Any chance you (or anyone else with a free couple of minutes) try and measure the voltage at the sending unit?

Thanks, Scot

[Kamster]

If I get time I sure will.

[HardLuk]

ScotY will you take pics of which sending unit you are measuring from.  Or, just basically of what it is you are measuring.

[ScotY]

I'll see if I can find a pic. It's the sending unit on the front side of the thermostat housing, points towards the grill. It's actually on a 16v motor but it's the original Samurai temp sender.

[ScotY]

Here's a pic...

[Kamster]

Here is what i copied from FSM for the Sammy.

21-5. WATER TEMPERATURE METER
AND GAUGE
The water temperature meter is located in the
combination meter and its gauge unit on the
inlet manifold.
The gauge unit shows different resistance values
depending on the coolant temperature. This
causes a current f lowing through the temperature
meter coil to change, controlling the meter
pointer. That is, when the coolant temperature
is raised, the gauge unit resistance is decreased
with more current flowing through the meter
coil, raising the meter pointer upward from the
“C” position.
INSPECTION
[Water temperature meter]
Disconnect Y/W (Yellow/White) lead wire
going to gauge unit installed to intake manifold.
Use a bulb (12V 3.4W) in position to ground
above wires as illustrated.
Turn main switch ON, Confirm that the bulb
is lighted and meter pointer fluctuates several
seconds thereafter.
If meter is faulty, replace it.