Temprature Gauge - Team Camaro Tech
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post #1 of 7 (permalink) Old Jan 12th, 03, 12:33 PM Thread Starter
 
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Whenever I first start the engine under any conditions the temprature needle pegs out and stays there unitl I tap the gauge a couple of times and then it comes back domn to normal readings. What would cause this and how do I fix it to operate normally!
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post #2 of 7 (permalink) Old Jan 12th, 03, 01:46 PM
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Bad ground. Your starting is pulling current through a portion of the ground circuit and raising it's voltage making the guage misread. Do you have all the bonds from engine to firewall, firewall to frame, and fender to battery?

-dnult

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post #3 of 7 (permalink) Old Jan 13th, 03, 06:05 AM Thread Starter
 
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Would this also cause the gauge to read incorrect. It is reading about 235 before the fan kicks in or is the gauge fiving me a false reading? I have only had the car for three days and it is my first and I am learning. I am geting a book on Wed but the book I know doesn't over everything. <BLOCKQUOTE>quote:</font><HR>Originally posted by dnult:
Bad ground. Your starting is pulling current through a portion of the ground circuit and raising it's voltage making the guage misread. Do you have all the bonds from engine to firewall, firewall to frame, and fender to battery?

-dnult

<HR></BLOCKQUOTE>

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post #4 of 7 (permalink) Old Jan 13th, 03, 03:32 PM
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Guage temperatures can vary considerably depending on where the sensor is located and the configuration of your cooling system. Although, your reading seems high. It is very possible for a bad ground or other electrical system problem to cause a misread.

About the only way to know if the guage is correct is to remove the sensor and boil it in water with an ohm meter attached. Keep the water stirred and record several temperature / resistance readings. You can then insert a variable resistor where the sensor would go in the engine bay and see if the guage matches your experimental readings.

Temperature sending units are non-linear. As a result, the guage readings can be accurate around the 200 mark and way off at the other ends of the scale. You'll probably notice that the graduations on the meter face aren't linearly spaced like a pressure guage is. The uneven spacing is supposed to compensate for non-linearity in the sensor to some degree.

Some guages, like the autolite guages, have a calibration adjustment in the back of the guage. Mine are accessed by removing the light bulb. I use a small screwdriver with a length of heat srink tubing around the shaft. I don't know if the adjustment is hot or not. The tubing is just my way of insuring I don't short the guage circuit in the process of making the ajustment. What I recommend is boiling the sensor (while stirring) and measuring the resistance. This will be darn near exactly 212 degrees. The put the equivalent amount of resistance across the guage lead to ground and calibrate your guage for ~212*.

-dnult

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post #5 of 7 (permalink) Old Jan 14th, 03, 05:18 PM
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basiclly, what he was saying was to get a pot of boiling water and put a thermometer in there to get the real temp of the water. take the gauge out and hook it to the coolant temp sensor and place the sensor in water to compare readings of the two gauges.

the temp sensor is a thermoresistor. this means that it has little resistance when cool and the resistance gets greater as temp increases. if you have to tap the gauge, then the sensor is not bad, the problem is in the gauge or gauge circuit. check for a bad ground.
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post #6 of 7 (permalink) Old Jan 15th, 03, 02:53 PM
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<BLOCKQUOTE>quote:</font><HR>Originally posted by nate:


the temp sensor is a thermoresistor. this means that it has little resistance when cool and the resistance gets greater as temp increases.
<HR></BLOCKQUOTE>

Works the other way - as temp increases, sender resistance decreases; usually around 600-700 ohms at room temperature, about 100 ohms at 180 degrees, and the response curve isn't linear - it's a decaying exponential curve, which makes it very dangerous to "trick" the gauge with fixed resistors.



------------------
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'69 Z28 Fathom Green
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post #7 of 7 (permalink) Old Jan 18th, 03, 02:30 PM
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I turn my ignition to on before starting and get the pegged temp reading. Goes back down to actual temp after starting. Could really scare a guy stalling his car then checking readings before restarting. Freaked me a little. I knew it wasn't pegged before stalling the engine so I didn't freak too heavy.
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