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Automive Coolant Temp Warning Circuit/Device

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sdb

New Member
First post. Did a several hour search first with no complete answers to my quest. I want to build or purchase a warning device that will close a relay contact when a temp sensor sees 225-230 deg F. It needs to have a remote probe that could be fed through the firewall of the car. All I am trying to do is turn on a blinking LED and a Pizio buzzer to attract my attention to the coolant temp gage. It does not need to have a display and I would like to be able to adjust the set point. Thanks in advance.
Steve
 

MikeMl

Well-Known Member
Most Helpful Member
Find a suitable sensor first, and then post it's specs here. It needs to be sealed for use in a hot liquid, sort of like the OEM engine block temp sensor. Most of those are NTC thermistors.
 

sdb

New Member
MikeMI, thanks for the quick response. I was looking at grabbing the Engine Coolant Temp sensor information being fed back to the ECU. The car I am working with is a 97 Miata. I have found a system that someone developed for a Nissan product since I posted my original question up, but it doesn't have an audio output. If I posted up that schem would it help?
 

MikeMl

Well-Known Member
Most Helpful Member
You must determine what the existing sensor is outputting as the temperature changes before you worry about the back end circuitry. Can you measure the voltage across the sensor as a function of temperature?
 

sdb

New Member
OK, MikeMi and mohd.S.H, thanks for your responses. The more I learn about this, the more I need to learn, so I am going to stop this thread until I know enough to ask a good question. There is information that is not readily available for the Miata, that I think does exist, I just have to find it. So thanks for your help, I'll be back when I know a little more.
 

MikeMl

Well-Known Member
Most Helpful Member
OK, MikeMi and mohd.S.H, thanks for your responses. The more I learn about this, the more I need to learn, so I am going to stop this thread until I know enough to ask a good question. There is information that is not readily available for the Miata, that I think does exist, I just have to find it. So thanks for your help, I'll be back when I know a little more.

You may have to measure it as the car warms up. This way, the signal across the sender will be exactly as you will eventually use it. You can use an opamp to amplify the signal, and then use a comparator to switch your alarm.

To simulate the high temperature condition which is supposed to trip your alarm, you could warm up the car, and then starting at the normal operating temperature, clip a pot across the installed sender. Watch the voltage across the sender, and watch the dash-mounted temp gauge. Turn the pot until you fool the temp gauge into thinking that the car is overheated. Record the voltage across the sender/pot combination when you get to the desired reading. That voltage is what you want your alarm circuit to trip at...
 
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