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coolant sensor tricking computer?

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Hi, I'm new here and not a Cadillac owner but the mechanic of the family. My dad has a 1997 DeVille with the Northstar 4.6l. Last week he was driving and got the dash warning regarding overheating and to allow engine to idle followed by shut down engine. He did this until the temp cooled down enough to get him home at which time he called me regarding the problem. When I got to his house I topped off the coolant resivoir (1/2 gallon), by the time I had got to my place (about 6.5 miles away) It went into overheat mode again :angry: . I let it cool and decided to replace the thermostat . I noticed there was no steam, crackling noises , nor was it extremely hot to the touch of hand that you would think it would be if overheated 259*+. Well, the thermostat didn't do the trick. No further than 1 mile and it went into overheat mode again. Now it's parked. I did locate my pyrometer (digital thermometer) and at the time it was in overheat mode the upper radiator hose was 174 degrees, the valve cover at each of the forward spark plus was 128-130 and the lower hose was 210. sounds norm to me?? My questions are :

Has anyone else had this issue and what was the out come?

Could the coolant sensor be lying to the computer?

Is there a way to test the coolant sensor?

Is there a seperate waterpump belt (v belt)?

TIA for any responses,


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....My questions are :


Could the coolant sensor be lying to the computer?


Is there a way to test the coolant sensor?

Yes, but it requires the temperature vs. resistance chart and I don't have that.

This might be one of those cases where throwing a new OEM part at it is the 'easiest' diagnosis.



Drive your car.

Use your cell phone.


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Unplug the coolant temp. sensor and check the resistance of the sensor. At 40 degrees F, the resistance should be around 7280 ohms. The table below shows the approximate resistance of the sensor at a given coolant temperature.

41F = 7280 ohms

50F = 5670 ohms

59F = 4450 ohms

68F = 3520 ohms

77F = 2796 ohms

86F = 2238 ohms

95F = 1802 ohms

104F = 1459 ohms

113F = 1188 ohms

122F = 973 ohms

140F = 667 ohms

158F = 467 ohms

176F = 332 ohms

194F = 241 ohms

212F = 177 ohms

If the car sits overnight, the coolant temperature will be the ambient air temperature. When the car is up to operating temperature, the resistance will be approaching the 177 ohm value. Basically, if you monitor the resistance of the sensor, as the engine heats up, the resistance will go down.

'93 Fleetwood Brougham
'05 Deville
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