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Old 01-27-2022, 12:09 PM   #2
142 guy
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Join Date: May 2014
Location: Saskatchewan, Canada
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If the igntion is off then the ECT / MS reads the correct setting but as soon as the car starts coolant temps jump from 190ish to 250-290ish.
I am curious as to how you are reading a temperature via MS with the ignition off. Do you mean ignition on, engine running versus not running? If that is the case, then depending on how you have set up the ground connections I would first be looking for something in the fuel pump circuit or the ignition circuit creating a ground offset. If nothing pans out there I would then check your alternator / charging ground connections.

On my B20E, the D jet coolant sensor has a resistance of around 100 ohms at 100 C. The programmed in R vs T curve is approaching asymptotic at that point which means that small changes in sensor resistance or the voltage measured across the sensor cause big changes in the temperature that MS calculates. A small offset in the sensor ground reference can have a similar big change in the calculated temperature. Ground offsets might always be present; but, because of the highly non linear sensor curve shape, the offset error may be insignificant when the engine is cold, only becoming apparent when the sensor becomes hot.

Are you running an electrically isolated 2 wire temperature sensor? In order to affect the temperature sensor, ground offsets have to appear in the ground path for the temperature sensor. If the sensor has an isolated ground back to the MS terminal connector, ground offsets that raise the complete MS ground plane will not change the temperature reading. With a 2 wire sensor, in order to skew the temperature reading with a ground offset you would have to have an insulation failure on the wiring to the sensor to create that offset. If you are running a one wire sensor with ground return through the engine anything is possible. In addition to checking the sensor wiring, check the terminal plug.

If you have a pre 1988 Volvo and are using the original engine harness wiring for the temperature sensor I would examine the wiring carefully because of the known failure problem with pre 1988 wiring harnesses.

Last edited by 142 guy; 01-27-2022 at 12:19 PM..
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