ME 117/220 Lab #4 Automotive Sensors Lab

Sherman Lo Richard Stephenson Alan Taber May 26, 1998

Lab Set up: Vehicle and Reference

Vehicle: 1989 Acura Integra LS 2 door coupe; 5 speed manual
Manual: Chilton's Acura Integra/Legend/Vigor 1986-1993 Repair Manual part no. 8426

1. Electronic Engine Control Module

The Electronic engine control module for emissions control is located underneath the front passenger seat (right seat). It is encased in a hard plastic case and the wiring is protected from below by the vehicle floor board. The box is approximated 10 inches by 7.5 inches and fits comfortably underneath the seat. It is not accessible unless the entire passenger seat assembly is removed. This is probably to prevent mishandling and accidental tampering. Chilton's warns: "The electronic engine control system is extremely complex, diagnosis and repair of this system requires that care be taken and all precautions be followed. Failure to follow all directions carefully, will result in expensive computer damage ....". Removing the seat to gain access to the ECU (electronic control unit) is reasonably easy and requires the removal of 8 bolts. Removing another 4 screws allows us to take the plastic cover off of the ECU. Hence the ECU is placed in a place where it is protected from accidental tampering but can be easily accessed when necessary.

Figure 1. Engine Control Module
There are many reasons why the designer would chose to place the ECU underneath the front passenger seat. Accessibility is one consideration. Placing it in the front seat makes the unit easy to access when necessary while making it sufficiently inconvenient to accidentally damage. Furthermore, since the Integra is a small car, the engine compartment is tightly packed and there would be little room for such a unit in the engine compartment. The passenger compartment is shielded from the temperature variations, EM fields, vibrations, and other disturbances generally found in the engine compartment. For example, the high temperatures would adversely affect the computer's operation. Temperature variations may affect instrument calibrations. Vibrations and EM fields would have bad effects on electronics. Hence placing the ECU in the passenger compartment makes sense. The temperature generally only ranges from 10-35 C or 50 - 95 F (throughout a year) when the car is in operation and it doesn't vary much during a normal trip (a few degrees).

We did not endeavor to take apart the car to access the ECU. However, I did probe around and after extensive neck craning, I managed to move into a position where I could locate the LED display used for diagnostics. The LED blinks based on the problem detected. If there is a problem, generally the check engine light would go on (though not all faults trigger this). However if the problem goes away, the check engine light would not light up but the ECU would still retain the fault code in memory. I cycled the engine and gave it a try. I didn't see any blinks (other than the initial blink when I turned on the engine) but that could be due to not triggering the self diagnostic. If I did the self diagnostic correctly, this would indicate that there is nothing wrong with the systems my ECU is monitoring (yeah!!!). Later models have a jumper wire which can trigger the self diagnostic however Chilton's make no mention of this for my model. See figure 2

2 . Emissions Control Sensors

.Figure 3. Locations of various sensors
Manifold Absolute Pressure (MAP) sensor. The MAP sensor is located at the top center of my engine. It is housed in a small box and it outputs an electronic signal to the ECU. There is one visible cable that enters into the MAP sensor, though there may well be two wires inside the cable, one for voltage transmission and the other to ground.. Since the sensor needs to sense absolute pressure in the air manifold, it probably uses a silicon diaphragm calibrated to read 0V at 0 psi as a pressure sensor. The noticeable absence of a lot of wires also makes us think that it relies on a voltage output, and the size of the sensor (the whole box is about 2 inches by 2 inches by 1 inch) seems to indicate that the sensor itself is fairly small. A silicon diaphragm set up as a capacitor could be put on one side so as to not interfere with the airflow through the manifold and still react to the pressure in the manifold.

Figure 4. Manifold Absolute Pressure (MAP) Sensor

Throttle Position (angle) sensor: The sensor is located at the top center of my engine directly behind the throttle. By stepping on the throttle, we rotated a cylindrical actuator which is (mechanically) linked to the throttle position sensor. Although we don't know for sure how it works, we believe that since the actuator is cylindrical with the round end pointed towards the sensor, it would be possible to attach a small piece of ferro-magnetic material to one point on that circle and have a magnetic sensor detect its position. This would then transmit a voltage back to the ECU. The ECU would then act on that knowledge along with the current oxygen sensor reading and other readings such as the distributor rotation sensor to increase or decrease fuel injection as needed. Another method is to have a sensor that is a gear motor or an encoder.

Figure 5. Throttle Angle Sensor

Oxygen sensor: The O₂sensor is located in the exhaust manifold right next to the radiator. It is inaccessible and thus we could not get anymore information about it other than what is available in the Chilton's manual. Chilton's states that the sensor is a hollow shaft of zirconia with a closed end. It forms a platinum electrode since the inner and outer surfaces are coated with platinum. The inner surface is exposed to the atmosphere while the outer one is exposed to the exhaust gas. Voltage is induced when ever there is a concentration difference and the ECU monitors the changes in induced voltage. The sensor is temperature sensitive (see Nernst Equation) and apparently my Integra is one model that does not heat the oxygen sensor.

3 .Other Sensors

Coolant thermostat: The coolant thermal sensor is located inside the engine block on the right side. Since I can't see it, I can't speculate on its operations. However, Chilton's mentions that it is a temperature dependent diode (thermistor) that measures the differences in t he coolant temperature. Its resistance decreases with increasing temperature implying that the thermistor is an NTC thermistor. The thermostat on a car should ideally be able to survive temperatures of up to 300 F and down to -50F without needing to be replaced. This is the range of temperatures coolant will see in the course of a year of normal operations, and so this is minimal. The sensor should be able to have a threshold set above which a warning light will go off on the dashboard to warn of high engine temperature. This could be done by constructing a bridge circuit with the thermistor in one corner and the threshold voltage as the input voltage. Once the thermistor's resistance drops below a certain level, the output voltage would be high enough to pass a logic gate and trigger the light.

Distributor rotation sensor: This is probably the CRANK/TDC (top dead center) sensor and the CYL sensor. The CYL sensor is used to detect the position of the no. 1 cylinder whose signal is used to trigger the sequential fuel injection.. The sensor is located on the right side of the engine and is difficult to see. However it looks like a big motor leading one to think that it is functions like an encoder or geared motor. Chilton's confirms that the signals are generated by the rotation of toothed wheels passing through pick up coils.

Figure 6. Distributor Sensors

Air bag Deployment sensor: N/A I don't have an airbag

4. Other Systems

Anti-lock Braking System: N/A. I have disc brakes but no active braking system (other than the driver).
An anti-lock braking system would need to sense slipping. That would require a measurement of velocity and wheel rotation rate. If there is no slipping,

v = 2(pi)(r)(w) where v = velocity, r = wheel radius, w = angular/rotation rate of the wheel.

If v - 2(pi)(r)(w) > a > 0, then we would need to have the anti lock braking system release the brake. "a" can be a set threshold. This could be set independently for each wheel and the braking pressure could then be regulated separately for each wheel. An accelerometer and four (one per wheel) wheel rotation sensor (and clock) or a wheel rotation rate sensor are necessary for such a system.

A second way of constructing the anti-lock braking system is to simply compare the acceleration of all
four wheels and see if one is decelerating faster than the rest of them. If that is the case, then the
hydraulic pressure on the locking brake could be pumped to keep it from locking up. This would be
cheaper, but it has the dangerous flaw that if all four brakes lock at once, the system will think that they
are not locked and this would be a system failure.

Active Suspension: N/A. They would be nice. For active suspension, one could incorporate more damping in the form of piezoelectric dampers which dampens more when there is roughness in the ride.

Cockpit Temperature Regulation: N/A. I wish I had this. Cockpit temperature sensor could probably work like a household thermostat. One would ideally locate this perhaps in the ceiling above the driver's head and perhaps another sensor by the driver's leg. Simple pyroelectric sensors would be enough to get the job
done. Thermistors or even bimetallic strips may also be adequate.