Multimeters are very handy tools and are used for a vast number of jobs. They can also be used to check inputs and outputs on cars. Cars today have sensors on them that communicate with the ECU and eventually it determines on how a vehicle operates. We were given the task to check these inputs and outputs using a multimeter. Some of the sensors on cars are MAP (Manifold Absolute Pressure sensor), TPS (Throttle Position Sensor), Crank Sensor, CAM Sensor, O2 Sensor, Air Temperature Sensor, Coolant Temperature Sensor. You can also check the Fuel Injector duty Cycle %.
All of these sensors are important and should be working effectively for the car to perform well. These sensors communicate with the ECU in different ways. Some communicate by sending a voltage (usually between 0-5V) to the ECU. Some communicate using AC voltage or HZ.
Coolant Temperature Sensor
The coolant temperature sensor gives out a voltage reading to the ECU, which indicates how cold or warm the engine coolant is. (usually on a scale from 0.5 to 3 DC Volts). The ECU needs to know this information so then it knows how much extra fuel the engine would need. As we know fuel condenses when the engine is cold and we need fuel to be in gas form for it to ignite. That's why when the engine is cold the ECU richens up the air/fuel mixture (so that not all the fuel condenses and we still have some in gas form). A normal reading when the engine is cold is usually between 2.5 to 3.0 DC volts.
To do this test we switched the multimeter to DC volts, put the black lead to earth and put the red lead to the coolant temperature sensor signal. (engine off, ignition on). The reading we got was 2.523V. Then when the engine was warm we did the same test again to see the voltage reading. The reading was 0.53V. These were good readings as it indicated that the sensor is working. The higher voltage reading meant that the engine was cold and that more fuel will be required. The lower reading meant that the engine was now warm and did not need the extra fuel.
Now if the coolant temperature sensor was giving a false reading such as 2.00V when the engine is warm, the engine will be running rich and wasting the extra fuel as it is not needed. This will mean that the car will now be running less fuel efficient. This is because the false reading indicates to the ECU that the engine is cold when in fact it is not. The ECU will use this false reading and richen up the air/fuel mixture.
TPS (Throttle Position Sensor)
TPS is used by the ECU to tell how ope or closed the throttle is. The sensor works on a scale from 0.5 - 4 DC volts. The higher the reading the more the throttle is open. The ECU needs to know this information because it needs to know how much air is coming into the engine, and therefore knows how much more fuel it needs to add. The wider the throttle being open the more air that comes inside the engine, therefore more fuel is required.
This test was also done with the meter set on DC volts. The black lead was grounded and the red lead was on the TPS signal. This test was done with the ignition on and the engine off. The reading we got with the closed throttle was 0.502V and as we opened the throttle the voltage reading rose till 3.87V. These were good readings. It indicates the throttle position accurately to the ECU.
Now if the TPS was faulty and was giving a reading of 2.5V when the throttle is wide open, the car will experience a lack of power. This is because the low voltage reading is indicating to the ECU that the throttle is not widely open and that not a lot of air is coming into the engine. Therefore is will not be adding the proper amount of fuel needed and the car will end up running lean. The car will then experience lack of power.
Crank or RPM Sensor
The crank sensor is used by the ECU to tell the engine speed and position. The crank sensor uses AC voltage or a HZ reading. It scales from .05 to 5 AC volts. Its very simple the higher the voltage (AC) or the HZ reading, the faster the engine is spinning. The ECU needs to know this when to fire the spark from the spark plug to ignite the air/fuel mixture. It also uses this information to know which cylinder to fire the spark in.
For this test we set the multimeter on both AC volts and HZ, and got a reading with both. The black lead was grounded and the read lead was to the Crank sensor signal. This test is done with the engine turned on. At idle rpm the reading we got was 0.8 AC volts and 37HZ. The reading we got at 2500rpm was 1.58 AC volts and 82.8HZ. These are good reading as the voltage reading and the HZ increases as the engine speed increases. Thus indicating to the ECU that the engine is now spinning faster and that it needs to fire the sparks more rapidly as well.
Now if the crank sensor wasn't working or was giving false reading like 1.0 AC volts when the engine is at 3000rpm. The engine would have difficulty starting up (most of the time it won't), and if it does start up it will run rough and wont run for too long and the ECU doesn't know the correct engine speed or engine position. It wont spark at the right time.
MAP sensor (Manifold Absolute Pressure sensor)
The MAP sensor tells the ECU how much vacuum is there in the intake manifold. The MAP sensor works on a scale of 0.5 - 4 DC volts. The higher the vacuum the lower the voltage reading. The voltage reading goes up as the air pressure inside the manifold increases. The ECU needs to know this information because it needs to know how much more or less fuel is required. The more the air pressure inside the manifold the more fuel is required by the engine.
To do this test we set the multimeter on DC volts. WE grounded the black lead and the red lead was put onto the MAP sensor signal. First we recorded the result for when the engine was on and at idle (vacuum being created), the reading was 1.58V. We then turned of the engine (ignition on) and then recorded a result of 3.61V. These are good readings. When the engine is on idle vacuum is created inside the manifold as the cylinders keep drawing in air. Since at idle the throttle is only open a little bit and therefore there is not a lot of air pressure inside the manifold. Thus the vacuum, which was shown by our reading. When the engine was turned off (ignition still on) the air pressure inside the manifold equalized to the atmospheric air pressure. Thus the reading went up as now there is no vacuum created inside the manifold.
Now if the MAP sensor was faulty and was giving a reading of 1.5V under acceleration, the car would experience lack of power. This is because under acceleration the air pressure inside the manifold increases, reducing the vacuum and therefore more fuel is required. But since the MAP sensor is now telling the ECU that there is higher vacuum and lower air pressure, the car would run lean as the ECU wont supply the correct amount of fuel needed. Hence the car experiences lack of power.
Air Temperature sensor
The air temperature sensor senses the temperature of the air that is coming into the engine. This is important for the ECU to know because colder air is more dense and requires more fuel to burn efficiently compared to warmer air which is less dense and doesn't require as much fuel to burn. The air temperature is also checked using DC volts. The higher the voltage reading the colder the air and therefore more fuel is required.
To do this test the multimeter was set on DC volts. The black lead was grounded and the red lead was onto the air temperature signal. The reading we got when the engine was cold was 2.474V. When the engine was warmed up the reading was 2.293V. The reading didn't change much because the engine was in a room and the air temperature in the room was the same. What made a difference was the heat from the engine that warmed up the air a bit. But overall these readings were good as it indicated that out air temperature sensor is working.
If the air temperature sensor was faulty and was giving us a low voltage reading on a cold night, the car would be experiencing lack of power. As we know colder air is more dense and therefore it requires more fuel to burn. The low voltage reading on the air temperature sensor will tell the ECU that the air coming inside the engine is warmer than it really is. The ECU will then use this false reading and will supply less fuel. This will mean that the car will run lean and will experience a lack of power.
Cam sensor
Cam sensor does the same thing as the crank sensor, and uses the same units to AC volts and HZ. It tells the ECU the engine speed and position. It lets the ECU know the position of the valves and so the ECU uses this information on when to spark and which cylinder to spark.
The test was done exactly the way it was done for the crank sensor. The reading at idle rpm was 0.25 AC volts and 20HZ. The reading at 2500rpm was 0.5 AC volts and 22HZ. Thus showing that the sensor is working correctly.
Fuel Injector Duty Cycle %
The fuel injector duty cycle % shows us the on time for the fuel injectors. It shows us how much percentage of the time the fuel injector is open to inject the fuel. The longer a fuel injector is open the more fuel its lets into the combustion chamber. Thus resulting in a higher duty cycle.
For this test the multimeter was set on duty cycle mode. The black lead was grounded and the red lead was onto the fuel injector signal. (only one fuel injector was used for this test). At idle rpm the duty cycle % was 5%. Under acceleration the duty cycle % rose to 13%. These are good results as it shows us that at idle not a lot of fuel is needed and that under acceleration more fuel is injected (shown by the rising duty cycle %).
If we had a problem with the fuel injector system and the fuel injector duty cycle was higher than normal. E.G 10% at idle. The car would be running very rich and will be less fuel efficient.
