Final answer:
A poor ground causes a lower than normal voltage in an engine coolant temperature sensor, due to increased resistance reducing the sensor's output voltage.
Step-by-step explanation:
The effect of a poor ground on the signal voltage of an engine coolant temperature sensor would be c. lower than normal voltage. A poor ground can introduce resistance in the sensor's circuit, which would reduce the voltage that is seen at the sensor's output. This occurs because the sensor's reference voltage would drop across the added resistance of the poor ground, resulting in a lower voltage signal being returned to the engine control unit (ECU).
To illustrate this point, if we consider Ohm's Law (V=IR), where V is voltage, I is current, and R is resistance, an increase in resistance with a constant current will result in a higher voltage drop across that resistance, thus leaving less voltage for the remaining parts of the circuit, such as the signal voltage.