How to test Temp. Sending Unit

[211]

So yeah, how can I test my temperature sending unit (radiator)?

The fan never comes on, however the motor never really gets warm either. I can drive it all over and the engine compartment is always relatively cool.

 

Priced sending unit last week and they're like $30.00 .

So I'd like to test the one I have to make sure it's toast before I dump $$ into this thing.

[fj401968]

So yeah, how can I test my temperature sending unit (radiator)?

The fan never comes on, however the motor never really gets warm either. I can drive it all over and the engine compartment is always relatively cool.

 

Priced sending unit last week and they're like $30.00 .

So I'd like to test the one I have to make sure it's toast before I dump $$ into this thing.

 

With a volt\ohm meter, test the impedance while cool and while hot and see if the reading is different.

 

BTW, in the winter, my radiator fan doesn't come on either.

 

As an aside, I liked this idea for testing gauges when troubleshooting sending unit vs. gauge issues.

 

 

This tip is from Mark Humble:

 

<quote>

One tool you can make to help diagnose gauge problems is to buy two resistors, one 50 ohm and one 80 ohm, 1/4 watt. Connect one lead from each resistor together and attach a 12" black wire to this connection and solder them together. Then solder a red wire to the 50 ohm resistors other lead. Solder a blue wire to the 80 ohm resistor and attach alligator clips to the ends of each wire. To keep the resistors from getting damaged, put them in a short peice of PVC pipe and fill with silicon.

 

Now you have a gauge tester that can be used on almost anything. To use, connect the black lead to ground and the red to the temp sensor wire and turn on the ignition. The gauge should about 1/3 up the scale. (Note:The Red Wire Is For Temp Only) If no reading, repeat the test at the gauge. This is done to check the wire between the gauge and the sending unit. If still no reading, use a volt meter or test light to check for power to the gauge, if there is power, but no reading, the gauge is bad. The blue wire is used to check pressure and level doing the same as above.

<end quote>

 

Tracy

[idosubaru]

if it never gets hot there is no need for the fan to come on, the temp will never get hot enough to trigger the sensor.

[211]

if it never gets hot there is no need for the fan to come on, the temp will never get hot enough to trigger the sensor.

 

Well okay, when I say "never" I mean usually. If I get stuck in traffic or at a drive thru teller it gets warm. And last week it almost got into the Red! I think that was from a sticky T-stat though. Nonetheless, the fan never came on and that concerns me :-\ .

 

I'll ohm check it this weekend.

 

Problem is, last time I ohm'd a sending unit (an old EA81), it was open at cold (duhh) but as I heated it up the impedance increased linearly. So my question is, at what point (ohm-wise) is it supposed to bias the fan on? The second the sensor hits 0.1 ohms (because electrically, that would "close" the circuit)? Or is it something higher like 700ohms; or is the fan speed variable with respect to this linearity?