![]() ![]() See Diagrams 1 and 2.Īt the extremes, an open-circuit ECT should indicate a scan tool data of approximately -40° F coolant temperature, since the PCM is receiving a zero return voltage. Most modern automotive ECT and intake air temperature (IAT) sensors are generally two-wire, negative temperature coefficient (NTC) thermistors in which the electrical resistance of the ECT and IAT sensors decreases as temperatures increase. Diagram 1: The coolant temperature should rise steadily as the engine warms up. Remember that the ECT input is part of the freeze-frame data that accompanies most diagnostic trouble codes. So, in some applications, an out-of-range sensor could affect the operation of many OBDII test monitors and the operation of many vehicle components without setting a trouble code. In many cases, early ECMs didn’t have enough capacity to rationalize the performance of the ECT sensor with other data inputs. Most pre-1996 OBDI and many early post-OBDII engine control modules (ECMs) and PCMs had only enough computing capacity to detect hard or intermittent circuit voltage faults. Much of an on-board diagnostic strategy depends upon the computing capacity of the PCM. The engine coolant temperature (ECT) sensor provides a good illustration of how many on-board diagnostic strategies have changed.Īn out-of-range ECT sensor can affect the PCM’s fuel and spark mapping, variable camshaft timing, transmission, radiator cooling fan and evaporative emissions functions, so it’s important to develop an awareness of how the PCM self-diagnoses the ECT circuit and how the ECT data is integrated into a vehicle’s operating strategy. Considering that roughly 1,500 or more different vehicle models are introduced into our domestic market each year, it’s becoming more difficult to predict how a powertrain control module (PCM) will utilize data from a particular sensor or detect an out-of-range sensor in any single vehicle platform. ![]()
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