Coolant Temperature Sender

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Bob Weir
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Re: Coolant Temperature Sender

Post by Bob Weir » Mon Sep 09, 2019 8:49 am

Have you used an infrared thermometer to read the temps at various locations?

Reference https://blog.thermoworks.com/tips/infrared-thermometry/

brokencase
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Re: Coolant Temperature Sender

Post by brokencase » Tue Sep 10, 2019 11:42 pm

When you say the reading is on the "high side of normal" what exactly do you mean?

john keefe
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Re: Coolant Temperature Sender

Post by john keefe » Wed Sep 11, 2019 1:46 am

Bob, I'd love to have one, but on a limited budget.

Brokencase, perhaps I should have written, "... on the high side, above NORMAL." The gauge had always registered between R & M in the letters "NORMAL" before the sensor disintegrated. Now its above the N, just touching or under the lower of the two hash marks indicating very hot (overheating).

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Re: Coolant Temperature Sender

Post by brokencase » Fri Sep 13, 2019 9:01 pm

John - You need to confirm coolant temp by another means. You could be running hot.

john keefe
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Re: Coolant Temperature Sender

Post by john keefe » Sat Sep 14, 2019 12:22 pm

Ha! I thought those lazer thermometers were all in the $250+ Snap-On tool range. I know an HVAC contractor who spent more than that on his to evaluate windows, doors, etc.. Didn't even think about the "home use/cooking" $20 models, so I'll pick one up later today. Thanks for the advice.

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Re: Coolant Temperature Sender

Post by john keefe » Sun Sep 15, 2019 9:35 pm

OK, thermometer reading is at 178deg., with coolant gauge needle at the edge of the "normal" box, just before it crosses over into the red. Since this is a dedicated circuit from the sender, and the gauge was working fine before, I conclude the Euro sensor is not calibrated to gauge.

So... again, would a resistor in-line before the gauge work to step down the signal voltage? Assuming its a 5V signal, what do you think the resistor should be to drop it 30% or so?

brokencase
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Re: Coolant Temperature Sender

Post by brokencase » Sun Sep 22, 2019 12:22 am

It is not 5v to the temp gauge. It is 12 volts.

Basically you have 12 volts going into the gauge through the sensor to ground. As the temperature goes up the resistance in the sensor drops and the more current runs through the gauge and the needle deflects more.

It is more current than you would suspect. The manual states that the sensor should read 19 ohms hot and 250 cold.
I=V/R says that is about 0.65 amps in the hot state.

To do what you want your going to need a high power resistor.

But I'm not so sure your sensor is incorrectly calibrated. Defective maybe, or perhaps there is something wrong with the gauge.
I find it hard to believe that they would purposely make a sensor that would go less than 19 ohms.

I say this because "19 ohms hot and 250 cold" = GE55 thermistor material.
It's not like they had a lot of options to choose from, especially at this low resistance level.

john keefe
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Re: Coolant Temperature Sender

Post by john keefe » Sun Sep 22, 2019 12:57 pm

Yeah, I'd not expect Ford would spec a different sensor when the (then) system worked well. Pennies per car adds up to a lot of $$$ cost in a production run.

My second suspect is the gauge, but it was reading OK prior to the sensor stalk cracking off. Would be a heckuva coincidence if both had gone out simultaneously. Unless it was something related to a voltage spike which damaged the gauge when the sensor broke?

IIRC, I'll need two resistors in the circuit. The 12V lead would Tee between the two resistors, one resistor going to ground, and the other going to the gauge.

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