Usage:
All Machines With Sensors And Switch Inputs On Electronic Control Modules
Reference: Service Manual For System Under Test
The following is an explanation of "pull up voltage" found on switch and sensor inputs of electronic controls.
What is "Pull Up Voltage" ?
"Pull up voltage" is a voltage supplied by the control module through a resistor which pulls up the "signal" lead of the device. See the Illustration. The signal lead is high except during the time the sensor takes the signal to a low level.
Typical Pull Up Circuit
Where Are Pull Up Circuits Used?
Pull up circuits are typically used on most sensor and switch inputs of electronic controls. Frequency sensor inputs to the control module do not typically have a "pull up voltage".
Under What Conditions Will I See "Pull Up Voltage" ?
When the sensor is disconnected at the harness connector, the signal lead should be pulled high by the "pull up circuit". When a voltmeter is placed between the signal lead and signal ground on the harness connector, (control side), the voltmeter should read the pull up voltage. A voltmeter reading of zero volts would indicate a broken wire or short to ground between the control and the sensor harness connector, or the control is faulty.
How Much Voltage Will Be Seen From Pull Up Circuits?
Whatever the control uses on inputs. The exact voltage can be found in the reference material.
Explain What Is Meant By Analog And Digital Sensors
Analog Sensors are sensors which provide a signal which can vary smoothly over a range such as fuel level senders. Digital sensors have only two output levels either high or low.