MAP
Make - Toyota
Model - 4AFE
Signal name - MAP
Volt/division/range - 1v
Time/division/range 1s
Explain in detail an electrical fault that would make this unit operate incorrectly.
A bad earth due some grease or dirt, gives a resistance in the earth wire. This would give a lower out put signal voltage this would effect the air/fuel mix, spark advance and injector pulse with.
Explain why this condition would cause this unit to malfunction/ Use mathematical equations to back up your explanations.
Because there is an extra resistance in the earth wire more voltage will be use up in the MAP sensor circuit. This will cause a lower out put signal voltage. This is a simplified example of a MAP wiring diagram with a resistance in the earth wire.
Vd = Vin(R1/R1+R2)
5(10k/10k+28k+10k) = 10000/48000 = 0.2
5x0.2 = 1.04
5-1.04 = 3.9
V1 = 3.9
Vd = 5(10k/10k+4k+10k) = 10000/24000
=0.416
5x0.416 = 2.08
5-2.08 = 2.92
V2 = 2.92
Vout = (V1-V2) Rf/Rin
= (3.9 - 2.92) 1000/5000
0.98x2=1.96
Vout = 1.96
So the difference between good earth and bad earth is 2.5v
With the bad earth you can see point A voltage is lower than the first graph at point B.
Explain how this condition would affect the operation of the vehicle?
Because the MAP sensor measures the air intake volume it has a major part in determining how much fuel to inject and also the ignition timing so a low voltage signal would make the engine idle rough or make the engine to stall. Or hesitate when accelerating.
MAF
Signal Name - Mass Air Flow
Volt/division/range - 1v
Time/division/range - 500ms
Explain an electrical fault that would make this unit operate incorrectly.
The PTC resistor or platinum hot wire has been contaminated with grease and crud. This has increased the the resistance of the PTC and would effect voltage output and effect engine performance.
Explain why this condition would cause this unit to malfunction/ Use mathematical equations to back up your calculations.
If the PTC had a higher resistance more voltage would be needed to push across it. This would lessen the voltage to the non-inverting side of the differential op-amp decreasing the output voltage to the base of the power transistor. So this would turn the transistor less on supplying less current to the PTC which would not heat back up as much and send a lower signal to the ECU effecting engine performance.
These are my calculation for a MAF circuit with a high resistance in the PTC with the transistor in the active region.
25000/25000+4000 = 25000/29000 =
0.86
Vce = 2.5 so 2.5 x 0.86 = 2.15
2.5 - 2.15 = 0.35v
Vout = 0.35v
These are calculations are with normal resistance at R3 = 10k
10000/10000+4000 = 10000/14000 =
0.714
Vce = 2.5 x 0.714 = 1.785
2.5 - 1.785 = 0.72
Vout = 0.72v
So difference in Vout voltages is 0.37v you can see an example of what this might look like on an oscilloscope below. Point A is the lower signal voltage when opening the throttle caused by the higher resistance in the PTC.
Explain how this condition could effect the operation of the vehicle.
Because the MAFs output is a variable voltage that changes in direct relation to the mass of air flowing to the engine. A low output signal from the MAF would effect the fuel injection time and upset the air/fuel mixture. This can cause a lack of power when accelerating or engine stutter.
Injector
Make - Toyota
Model - 4AFE
Signal Name - Injector
Volt/division/range - 10v
Time/division/range - 2ms
Point A shows the supply voltage to the injector in this case 14.4v. Point B is when the injector is switched on, because the injectors are earth triggered the voltage drops to 0v for approx 2ms this is wile idling. Point C is the voltage spike induced when the earth is switched off approx 60v.
Explain in detail an electrical fault that would make this unit operate incorrectly.
The injector plug has a corroded and dirty electrical connection.
Explain why this condition would cause this unit to malfunction.
A corroded and dirty connector plug could cause the injector to miss fire because the supply voltage or earth switching wire signal are intermittently broken.
This is what a fuel injector miss firing signal might look like.
Explain how this condition would effect the operation of the vehicle.
This condition would cause poor engine performance, lack of power and rough idling also it could effect engines fuel economy.
(TPS) Throttle Position Sensor.
Signal Name - TPS
Volt/division/range - 2v
Time/division/range - 1s
Point A shows 0.5v this is when throttle is closed as I open the throttle you can see the voltage goes up to just over 4v at point B. Then I closed the throttle about the same speed that I opened it point C is half open throttle approx 2v.
Explain in detail an electrical fault that would make this unit operate incorrectly.
The signal voltage drops to zero because the wiper arm on the signal wire fails to make proper contact with the carbon resistor this is called "drop out".
Explain why this condition would cause this unit to malfunction.
Because the ECU uses the variable signal voltage to determine the throttle position 0.5v being closed and 4.5v being wide open throttle. If the signal voltage goes to zero volts because the wiper loses contact with the carbon strip the ECU will think the throttle is closed.
