Need help solving C2 temp gauge problem
Burning Brakes
Joined: Oct 1999
Posts: 816
Likes: 37
From: Wilmington NC
Re: Need help solving C2 temp gauge problem (62fuelie)
I agree with 62fuelie.
I plotted the resistance vs temperature numbers that were in the "Troubleshooting Corvette gauges." document. The resistance in the sender unit is non-linear. At the higher temperatures it appears to be almost exponential. The problem you will have with the additional 'fixed' 25 ohms is that at higher temperatures (above 180) you could be off by as much as 30%. For example, a reading at the gauge of 200 degrees (83.5 ohms) it could actually be 240 degrees! If you are going to install a resistor then it could be more accurate if you use a NTC thermistor (negative temperature coefficient - higher temperature/lower resistance.). Also, you would want to install it as close to the engine as possible. It will not be as good as the sender unit w.r.t changes in temperature but, it would be better than a fixed resistor under the dash.
Also, for whatever resistance you add to the circuit, make certain that it can handle the wattage (@ 12 volts and 80 ohms == 1.8 watts.). You should double the worst case scenario.
jm2c
Gary
I plotted the resistance vs temperature numbers that were in the "Troubleshooting Corvette gauges." document. The resistance in the sender unit is non-linear. At the higher temperatures it appears to be almost exponential. The problem you will have with the additional 'fixed' 25 ohms is that at higher temperatures (above 180) you could be off by as much as 30%. For example, a reading at the gauge of 200 degrees (83.5 ohms) it could actually be 240 degrees! If you are going to install a resistor then it could be more accurate if you use a NTC thermistor (negative temperature coefficient - higher temperature/lower resistance.). Also, you would want to install it as close to the engine as possible. It will not be as good as the sender unit w.r.t changes in temperature but, it would be better than a fixed resistor under the dash.
Also, for whatever resistance you add to the circuit, make certain that it can handle the wattage (@ 12 volts and 80 ohms == 1.8 watts.). You should double the worst case scenario.
jm2c
Gary
Thread Starter
Melting Slicks
Joined: May 2001
Posts: 2,749
Likes: 776
From: Dadeville AL
Re: Need help solving C2 temp gauge problem (GaryC)
First, let me say thanks to all of you for taking the time to offer your ideas and concerns. I also recognized that the temperature/sending unit resistance relationship was not linear. It appeared more exponential to me too, so I ran a numerical analysis and projected the readings for temperatures above 211 F (where the helpful chart ended). That analysis predicts that the resistance will continue to drop at an average of about 10 ohms for every 10 degrees F rise in temperature. So, with the 22 ohm resistor I have added, the most I expect in the way of an error is about 20 degrees at very high temperatures. As I don't plan to run the engine above an indicated temperature of 220 F, I think it will be ok.
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