sensors diodes transistors...
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Common SensorsUnderstand the following sensors:
Pull Up sensorPull Down sensor
PotentiometerThermistor
Pull Up Switch (sensor)VERY low current
12 volt
Pull Up Switch (sensor)VERY low current
12 volt
12 volt
Pull Down Switch (sensor)VERY low current
Pull Down Switch (sensor)VERY low current
Test the Switch - Sensor
Check Scan tool PID data for sensor
Should show ON/OFF or Open/Closedas you operate the switchIF No change, check voltage at switchLook for open /shorted wires (use voltmeter and logic to see if wire is open or shorted)
Potentiometers
A potentiometer is a Position sensor
4 volt
PotentiometerVery low current
2 volt
PotentiometerUsed as a sensor
2.5 volt(should be 2 volt)
Potentiometerpoor ground
5 volt(should be 2 volt)
Potentiometeropen ground
Potentiometers sense motion
check for: reference voltagegood groundun-interrupted signal
0.52 V MIL lamp lit. TPS code
What is wrong?
0.0 V MIL lamp lit. TPS code
What is wrong?
Thermistors
Thermistors change resistance with temperature
Negative Temperature Coefficient thermistors decrease in resistance when they heat
Commonly used to sense changing temperatures
High Resistance when Cold!
Low Resistance when Hot!
4.52 VHot or Cold?COLD!
1.53 VHot or Cold?HOT!
1.45 V1.15 V
Poor gas mileage, runs rich, may set oxygen sensor code
Poor gas mileage, runs rich, may set oxygen sensor code
0.49 V
Poor gas mileage, runs rich, may set oxygen sensor code
0.02 V
Thermistors
Thermistors are checked with an Ohmmeter or a Voltmeter
A temperature probe or thermometer should be used when checking
Compare temperature at measures at sensor temperature displayed on Scan Tool
Semiconductors
Elements with four valence electrons are not good as insulators or conductors
Silicon, when is a crystal form, shares valence electrons to make a good insulator
Silicon can be “doped” to add electrons to the valence ring or remove electrons from the valence ring
Semiconductors
Silicon doped with phosphorous (which has five electrons in the valence ring) will create an “N” type semiconductor
Silicon doped with boron (which has three electrons in the valence ring) will create a “P” type semiconductor
Semiconductors
All Semiconductors will be destroyed by:
Too much Current
Too much Voltage
Diodes
Placing an “N” type semiconductor with a “P” type will create a diode.
In a forward biased diode current will flow with a small amount of resistance
Too much current will overheat and destroy the diode
Diodes
Reverse biasing a diode will create very high resistance at the center of the diode and current will not flow
Too much voltage will overcome the internal resistance and the diode will short out
12.6 V
Arrow shows conventional theory (hole flow)
Diodes wired like this will block current
0.0 V
Arrow shows conventional theory (hole flow)
Diodes wired like this will block current
Arrow shows conventional theory (hole flow) Diodes wired like this will allow current to
flow with a slight voltage drop
12.6 V
Arrow shows conventional theory (hole flow) Diodes wired like this will allow current to
flow with a slight voltage drop
12.0 V
Diodes use (Drop) about .5 volts
Arrow shows conventional theory (hole flow) Diodes wired like this will allow current to
flow with a slight voltage drop
0.1 V
Testing Diodes
Diodes require .2 - .6 volts to forward bias
Digital ohmmeters may not supply this voltage with ohm check
use diode check feature when using DVOM
Testing Diodes
Shorted diodes on alternators will allow A/C voltage that may confuse a control module
Routine checks with oscilloscope are easy
Anti- Spike Diodes
Clamping or Anti-Spike diodes protect sensitive control modules from …..
….Too much VOLTAGE
Semiconductors
All Semiconductors will be destroyed by:
Too much Current
Too much Voltage
Failure in Anti- Spike Diodes
An open, or shorted diode will damage computer control modules
Shorted relay coil windings will cause too much current to flow and burn out switching transistors in computers
Shorted Diodes allow too much current
Check resistance with an OHM meter
Failure in Anti- Spike Diodes
An open, or shorted diode will damage computer control modules
Open diodes allow spike voltages to harm electronic components (too much VOLTAGE)
Oscilloscope will show an OPEN diode for any coil
Types of Diodes
Positive diodeNegative diode
Zener Diode
Light Emitting Diode (LED)
Transistor acts like a very fast On/Off pull down switch
Transistors
Transistors can act as a relaythey use a small signal current to control a larger working current
Transistors can act as a signal amplifier
PNP transistor
Points In Permanently
Will conduct current when the base sees a Negative polarity
NPN transistor
Never Points In
Base must be Positive to allow transistor to conduct current
Transistors can act as a Signal Amplifier
Small changes in current flow through the Base will result in large current flow changes through the Emitter and Collector
TransistorsTransistors are damaged by too much current
check resistance of the LOAD (The load is what is being turned ON or OFF)
Transistors are damaged by high voltage
Coils turning OFF create high voltageCharging systems that allow Overcharging can damage control modules and computers
Diagnose Control ModulesElectronic control modules can not be directly tested
Before replacing, verify proper operation of PowerGroundSensors (inputs)Actuators (outputs)
Remember… Electronics are damaged byhigh voltagehigh current
Diagnose Control Modules