Have 1 temp sensor send temperature readings to 2 seperate temp gauges

Question

My old truck (fuel injected '92 Ford Ranger) has 2 temp sensors, located where the hot coolant leaves the engine.

• One sensor goes to the ECU, which alters the amount of fuel which is delivered to the engine depending on the engine temperature.
• The other sensor goes to the analog temp dial on the dashboard. The temperature dial has arbitrary low/med/high levels, so it is not very helpful if I want to know exactly how hot the coolant is when it leaves the engine.

I installed a digital temperature gauge in the cabin. I cut the wire leading to the dial gauge on the dashboard and connected it to the new digital gauge. The digital gauge now displays the temperature of the hot coolant.

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This is where the question shifts over in the electrical realm.

I want to have both the old dial gauge AND the digital gauge running at the same time. But when I spliced the wire coming from the temp sensor and connected one wire to the dial gauge, and the other wire to the digital gauge, neither gauge displayed the correct temp.

It looks as if both the dial and digital gauge are displaying half the correct temperature. Eg: if the engine was running hot at 300 degrees, the dial would display medium, and the digital would display 150. They should be displaying hot and 300, respectively.

If I understand how the temperature sensor works, the resistance changes depending on how hot the sensor gets. Correct? In my case, this current is getting split to 2 separate gauges instead of one gauge.

So if I want to direct the current to 2 separate temperature gauges, I would need to double the volts/amps coming from the sensor, correct? Is there an easy way to do this? Is there a way to "do" the opposite of resisting a current?

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Parts

• Temperature sensor

• New digital temp gauge

Answer 1

You are correct about how the temperature sensor works. As the temperature rises, the electrical resistance changes. This usually happens in a reverse relationship. So as temperature rises the electrical resistance falls. And there is a known relationship between the resistance on the circuit, and the temperature the probe is experiencing. That's how they know what numbers to paint on the gauge face.

But the part you also need to consider is: How is the gauge reading that resistance? Most ohmmeters I am familiar with (and I believe automotive temp gauges, fuel gauges, etc) work according to Ohm's law I = V / R. So applying a known and constant current (I) to the circuit, you can passively measure the resulting voltage (V), and with simple arithmetic you can determine the resistance (R). In reality, a mechanical gauge isn't actually doing any calculations, it just relies on calibrated internal components/magnets/windings/etc so that the needle sweeps across the face according to the measured temperature. Sounds good so far, right?

OK, so lets say your OEM dash gauge works by applying 0.01 amp current to the sensor circuit, and it measures the resulting voltage. And if the gauge reads 1.0 volts, then (R = V/I) Resistance is 1.0 / 0.01 = 100 ohms. That level is equal to some temperature, lets say 200 deg F. So the gauge sweeps to read 200 def F. Everything is good so far.

Now, take that same circuit, same engine temp, and add another gauge to it. Lets assume the aftermarket gauge works exactly the same way as OEM gauge. It also applies 0.01 amps to the circuit, and then reads the voltage. So what happens now? Well, you've got an OEM gauge applying a 0.01 amp current, and an aftermarket gauge also applying a 0.01 amp current. So now you have a total of 0.02 amps. Put yourself in the shoes of (either) gauge. You are applying your 0.01 amp current, and you read the voltage. You don't "know" another device is also applying current. Your only sense available is to read the voltage. So what does the voltage read? Well, we already know that the temp is 200 deg F, so the resistance is known to be 100 ohms. V = I x R, V = 0.02 * 100 = 2.0 volts. So what number shows up on the gauge? We know for sure it won't be the correct reading, because we know that when the voltage reads 1.0 volts, it shows 200 deg F. So if the gauge reads 2.0 volts, we can also calculate what the gauge "thinks" the temp (resistance) is. R = V/I, R = 2.0 / 0.01, R = 200 ohm. And based on what we know about sensor readings (higher resistance = lower temp) we know that the gauge will read an incorrectly low temperature. Which is exactly what you're seeing.

It is much more complicated to figure out exactly what these numbers are in real life, because each gauge might use different amperage or voltage specs to do its job. And each sensor/application has different resistance/temp curve profiles. Its not a proportional relationship where any two gauges = temp reading cut in half. So the fact that you are seeing that is more of a coincidence than anything else.

But what we can say with certainty is that if you attach two gauges to the same sensor, both of them are going to read erroneously low by some amount.