Diagnosis of Engine Coolant Temperature Sensor

Diagnosis Of Engine Coolant Temperature Sensor (911 Carrera, Boxster, Boxster S):

General Description
Range checks for minimum and maximum temperatures are continuously performed.

A block-heater recognition function is included to retrigger the model calculations, as explained below.

A plausibility check is continuously performed by a comparison of the engine temperature with a reference temperature. A plausible temperature range for the coolant temperature is formed by adding and subtracting a calibrated safety margin around this reference temperature. The result is compared with the temperature range determined. If the measured coolant temperature does not increase by the amount expected (e.g. parallel circuit or short circuit to a plausible voltage level) and therefore leaves the calculated range, the error flag is set. During the retrigger phase, induced by blockheating, the plausibility check is disabled.

Monitoring conditions for plausibility check:
- block-heater is not detectable
- engine start has been finished for a period of time
- coolant bypass stop valve closed (time delay after valve switching)
- engine speed below threshold (time delay after exceedance of threshold).

It is also checked, whether the engine temperature exceeds the switch-on threshold for closed loop control. The engine temperature must exceed this threshold after a given period of time; dependent on the engine start temperature. The time delay depends on air mass flow. Countdown of the time delay is stopped during fuel cut-off mode. If the temperature threshold to enable lambda control is not reached within a specified time, an error is set. The error path is suppressed after a start with a preheated engine block (blockheater) as long as the condition that the measured engine temperature is below the engine start temperature (maximum time 2 minutes).

Model Temperatures
A temperature model supplies a calculated reference temperature for diagnosis and a calculated substitute temperature used with a temperature sensor failure. The characteristics of these calculated temperature values are independent of one another.

Reference temperature is calculated as a function of:
- engine coolant start temperature,
- rising or falling gradient, map dependent on air mass flow,
- delay time after start or condition `blockheater`,

the calculation of the substitute temperature model works similar to the reference temperature model, but the start temperature is taken from engine oil temperature or intake air temperature or constant substitute value. The map dependent gradients are different for both models.

Diagnosis of Engine Coolant Temperature Sensor