The best way IMHO is to get your hands on an infra -red thermometer and once your engine is up to normal operating temperature (where your console gauge usually registers), take a reading from the thermostat housing and make a mental note of it. Minor differences in the sending unit and the gauge itself can/will cause each setup to read slightly different but using the above method will show you exactly what temperature your engine is running at for the console gauge indication at the time you take the reading.
I can't say if you could extrapolate it or not, I'm not sure the technology of the sending units and gauges back in the late 60's from the General were that hi-tech. They were better than the standard "idiot light" which by the time it lit was after the problem occurred. You could take multiple readings below normal to give an idea what the needle movement from the far left indicates but I wouldn't necessarily expect that to be a linear movement as far as temp steps go. Once you know what the usual operating temperature gauge position actually is by taking an IR reading, you know anything much beyond is going where you and your engine don't want to be and anything less indicates things aren't up to normal operating temp. Perhaps someone else will have additional information on this for you?
From a purely theoretical standpoint...YES, if you understand Ohm's Law and the theory behind the operation of a Wheatstone bridge using the balanced/unbalanced bridge concept, plus the operation of the D'Arsonval gauge movement technology of the "old" days. The design of the original factory circuit matched the gauge readout with the change in resistance of the water temp sensor mounted in the engine. Using the factory resistance specifications of each component, the theoretical location of the gauge needle at the exact center of the movement would indicate 260 Deg F. Why...there are 2 inductive coils in the temp gauge which control the location of the needle by magnetism. The coil in the "cold" circuit = 67 ohms. The coil in the "hot" circuit = 53 ohms. There is a series shunt resistor to ground added to the "cold" circuit providing a total of 150 ohms. The "hot" circuit utilizes the temp sensor in the engine to provide the variable resistance needed to position the needle. There is a total of 288 ohms across the operational range of the sensor (thermistor) which acts as a variable resistor as the water temperature changes. Therefore, when 12 volts is applied to the center tap of the inductive coils, each side must be equal in value to locate the needle on center. So, to equalize the EMF between coils, approximately 100 ohms added to the "hot" circuit coil value of 53 is needed to position the needle in the center. If you look at the specifications of the factory temp sensor, 102 ohms is equal to 260 Deg F. 53 + 102 = 155 which will, in essence, balance the bridge or equal the EMF between coils (150 in the cold circuit) and allow the needle to point to the center of the movement. The opposite effect is true with a completely cold engine. With 390 ohms @ 100 Deg F added to the 53 ohm "hot" circuit, the current limiting factor switches in favor of the cold side, thus, allowing the needle to be pulled to the maximum cold position.
Yes, what he said LOL. Good job Karl :beers:
But I'm remembering why they didn't let me take physics in high school.
Brian...if you wish to offset the scale by 25% or 1/4 of the gage movement, one would have to add resistance in series with the "cold" circuit. This would be a total of 67 (cold gage coil), 83 series factory shunt resistor already in place and an additional 144 ohms. Why...the total operational range of the engine sensor is ~288 ohms from 100 to 260 Deg F. To reach mid range at 260 Deg F with 50 percent scale movement of the needle, the entire current limiting resistance has been removed from the "hot" circuit, thus allowing the needle to deflect to it's maximum to the right which is "half way". Since a 288 change is required for 50% scale of the pointer to be achieved, it would seem reasonable that 1/2 of that amount (144) would be needed to affect a 25% or 1/4 change in scale. So, if you place 144 ohms in series with the "cold" circuit to ground, EMF is shifted to the "hot" circuit allowing the needle to move to the right by another 1/4 scale, placing it in the middle of the scale for normal operating temperature. Does that make sense?
Al...I tried to keep it simple so that I wouldn't have to splane the cosine of the angle to the dangle or was it the square root of the heat to the meat!
Not really any math, I've known this for years and actually tested it on my own factory temp gauge.
Holy Hanna Tim...slide rule, I haven't seen that since '69. I believe it's in the basement with rest of my textbooks. I had enough trouble with everyone's law like Ohm & Kirchkoff. Heck, I was a shy country boy my first semester in college and barely knew Thomas Edison...are you serious, I need more laws. Electrical Eng has more laws than the state of PA...what did I get myself into? :clonk:Oh well, it helped finance my fetish for classic cars and allowed me to put away foolish things like slide rules.l
