1966 L79 Temp Gauge
There was an article within the last two years about inaccurate temp senders and a theory about why and a fairly complicated fix of sorts. I communicated with the author off line but decided not to try to build the device he had designed.
I have had my car for 23 years and done a lot of “correcting” over the years to make it run in acceptable temp range, however the gauge on the dash reads higher than the temp I read on my IR gun and the higher it reads, the higher it reads. So I was tending to believe the theory in the article that says there is a point at which the thermistor becomes a heat source then
the resistance goes lower and the current goes up and so does the heat and on and on.
So I started with measuring the resistance of the sender at room temp (75) today. It reads 663 ohms which according to an old email with a graph and a table of Ohms vs Temp I got years ago from Jack Humphrey, is in the accepted range.
I started the car and observed the voltage at the lead which was on the temp sender. It was 14.6 volts. I’ll use this later
With the ohm meter connected to the sender I observed the resistance decline as the car warmed up. It finally settled at 140 ohms, plus or minus a couple as long as I wanted to let the car idle. I let it run long enough for the fan clutch to activate.
140 ohms corresponds to about 165 degrees according to the chart I have. I believe the car has a 160 degrees thermostat (which I think is wrong) which was installed by the previous owner thinking it would make the car run cooler!
Then I reconnected the wire to the gauge and the gauge read 160 but immediately started to climb to 200. The IR thermometer showed no increase in temp.
So, at 140 ohms and 14.6 volts the current is a bit over 100 milliamps. That translates to 1.4 watts the thermistor needs to get rid of and as the resistance drops the current goes up so at 83 ohms which is as low as my chart goes, the temp should be 210 degrees but the thermistor now needs to dissipate 2.6 watts and on and on.
The theory in the article from a few years ago was the newer senders are defective in that there is not good thermal connection between the thermistor and case of the sender. This means the case should be acting as a heat sink for the resistor and it isn’t. I think the author cut open a few to confirm that some has an air gap where there should not be one.
So the bottom line is I’m sure my car is running in the proper temp range but I wish the gauge would read correctly at the higher end. By the way, I have confirmed the gauge reads correctly based on the chart by using a resistor substitution box in place of the sender.
Any recommendations on a sender that might work better.
Rich
I have had my car for 23 years and done a lot of “correcting” over the years to make it run in acceptable temp range, however the gauge on the dash reads higher than the temp I read on my IR gun and the higher it reads, the higher it reads. So I was tending to believe the theory in the article that says there is a point at which the thermistor becomes a heat source then
the resistance goes lower and the current goes up and so does the heat and on and on.
So I started with measuring the resistance of the sender at room temp (75) today. It reads 663 ohms which according to an old email with a graph and a table of Ohms vs Temp I got years ago from Jack Humphrey, is in the accepted range.
I started the car and observed the voltage at the lead which was on the temp sender. It was 14.6 volts. I’ll use this later
With the ohm meter connected to the sender I observed the resistance decline as the car warmed up. It finally settled at 140 ohms, plus or minus a couple as long as I wanted to let the car idle. I let it run long enough for the fan clutch to activate.
140 ohms corresponds to about 165 degrees according to the chart I have. I believe the car has a 160 degrees thermostat (which I think is wrong) which was installed by the previous owner thinking it would make the car run cooler!
Then I reconnected the wire to the gauge and the gauge read 160 but immediately started to climb to 200. The IR thermometer showed no increase in temp.
So, at 140 ohms and 14.6 volts the current is a bit over 100 milliamps. That translates to 1.4 watts the thermistor needs to get rid of and as the resistance drops the current goes up so at 83 ohms which is as low as my chart goes, the temp should be 210 degrees but the thermistor now needs to dissipate 2.6 watts and on and on.
The theory in the article from a few years ago was the newer senders are defective in that there is not good thermal connection between the thermistor and case of the sender. This means the case should be acting as a heat sink for the resistor and it isn’t. I think the author cut open a few to confirm that some has an air gap where there should not be one.
So the bottom line is I’m sure my car is running in the proper temp range but I wish the gauge would read correctly at the higher end. By the way, I have confirmed the gauge reads correctly based on the chart by using a resistor substitution box in place of the sender.
Any recommendations on a sender that might work better.
Rich
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