sensors diodes transistors...

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