90-92 Speed Density TPI Systems
Below is a list of all the needed sensors to install a speed density TPI setup, and each of their functions.
Manifold Absolute Pressure (MAP) Sensor: This sensor is responsible for measuring manifold air pressure. In simple terms, it measures engine vacuum inside the intake manifold. The lower the vacuum reading, the greater the load on the engine (i.e. heavy acceleration). The higher the vacuum reading, the lower the load on the engine (i.e. cruising or light throttle). The ecm needs this information to calculate how much fuel the engine needs. An engine under heavy load will require more fuel than an engine under light load. The MAP sensor is usually located on a bracket on the passenger side of the plenum. It may however be mounted on the firewall, or somewhere close by.
The ecm will see a signal from the MAP sensor which will vary from 0.0 to 5.0 volts. Under full throttle (0" of vacuum), it should read close to 5.0 volts. The voltage should drop depending on engine vacuum as described by the chart below:
Oxygen Sensor: Responsible for determining the amount of oxygen in the exhaust manifold. Depending on how much oxygen is in the exhaust, the ecm can determine whether or not the air/fuel mixture is rich or lean. The signal sent to the ecm by the ecm varies between 0.0 and 1.0 volts. An ideal mixture (also known as a Stoichiometric mixture) of 14.7:1 is represented by .450 volts. If the oxygen sensor voltage is below .450, then the air/fuel mixture is lean. Anything over .450 means the mixture is rich. Since the sensor is essentially just a switching device, it will be fluctuating alot between lean and rich. This is normal, and an indication that the sensor is in working properly.
Keep in mind however that oxygen sensors (except wideband oxygen sensors), are not very accurate below or above .450 volts. Exhaust gas temperature will affect the oxygen sensor reading as well. The sensor will not read properly until exhaust gas temperature reaches approximately 600 degrees Farenheit. If you have headers installed, it is a good idea to use a heated oxygen sensor (3 wire) instead of the usual single wire sensor. Headers usually place the oxygen sensor further down the exhaust stream, where exhaust temperatures are cooler. A heated oxygen sensor will heat itself, allowing a more reliable sensor reading than a single wire sensor. If you have factory exhaust manifolds, then the single wire sensor is adequate.
Oxygen sensors are a regular maintenance item, and should be replaced every 30,000 miles. When an oxygen sensor goes bad, it tends to read lean, and will not fluctuate very much. The ecm will attempt to correct this false lean condition by richening the mixture. This will cause poor driveability, and high gas consumption.
Knock Sensor: Also known as detonation sensor, it is responsible for sensing spark knock. Basically, thats when the fuel mixture ignites before the spark plug fires. The piston is moving upwards as this premature combustion takes place. Since fuel is used to cool down the combustion chamber, a lean condition causes the temperature to rise, and ignites the fuel mixture prematurely. This is very abusive on the engine internals, and reduces the life of any engine. The more powerful the engine, the greater the potential for damage. Detonation can be cause by a variety of things. One of the more common causes on TPI retrofits where prom changes have been made to the fuel or spark tables is a lean condition. It isn't always loud enough to be heard, so just because you don't hear any pinging, doesn't mean its not happening.
Detonation will cause a vibration to travel through the engine block. The sensor listens for this vibration at a certain frequency, and sends a signal to the ecm when the frequency is heard. This frequency is different depending on engine size. To prevent possible engine damage, the spark timing needs to be retarded when detonation is present. The sensor itself does not pull the timing back however. The ecm is in charge of retarding the timing, and will do so according to a series of settings inside the prom. The knock sensor is located on the passenger side of the engine block on factory applications. It can however be relocated to the driver side of the block if needed (header clearance for example). They are different depending not only on the size of the engine, but also the ecm being used. It is important that the correct sensor is used to avoid problems. Although it is possible to run the car without one, I strongly suggest against this. I have had customers come to me looking for a $45 knock sensor after spending several hundred dollars and an extra month of work rebuilding a blown engine due to detonation.
Throttle Position Sensor (TPS): Responsible for reporting to the ecm the position of the throttle blades. The ecm will receive a signal which can vary from 0.0 to 5.0 volts. At idle, the TPS should be read .54 volts (factory specification) unless it has been set to a different value inside the prom. If it does not read .54 volts and the idle TPS voltage setting has not been modified in the prom, then it should be adjusted. Under full throttle, it should output close to 5.0 volts. Throughout its range of motion, the voltage should climb steadily, without any jumps or falls. If it is not steady or has some fluctuations as it is moved through its range of motion, it should be replaced. This sensor is located on the passenger side of the throttle body.
Coolant Temperature Sensor (CTS): This is basically a thermistor (means that it changes resistance with temperature) that supplies the ecm with the temperature of the engine coolant. This temperature reading is used for several important functions. The most notable is that the ecm adds extra fuel to an engine when its cold, and as the engine warms up, the extra fuel is reduced. This sensor mounts at the front of the intake manifold. The chart below shows the approximate resistance for this sensor in relation to temperature.
Intake Air Temperature Sensor (IAT): This sensor is also a thermistor (means that it changes resistance with temperature) that supplies the ecm with a temperature reading of the air being drawn into the engine. It is the same as the coolant temperature sensor on 86-92 models. The 1985 intake air temperature sensor used a different connector and cannot be used as a coolant temperature sensor because it had an exposed bulb. This sensor mounts underneath the plenum. The chart above shows the approximate resistance for this sensor in relation to temperature.
Idle Air Control (IAC) Valve: Although this is technically not a sensor at all, people often treat it as one. It is responsible for regulating the amount of airflow being admitted into the engine to adjust engine speed, particularly at idle and deceleration. The ecm controls the IAC at its discretion. The IAC works by moving a cone shaped pintle, which can extend and retract as needed to admit or block off incoming air. The valve moves the pintle in "steps". These steps are numbered and range from 0 to 160. A properly adjust throttle body should be idling when warm between 15-25 steps.
The IAC is used under a variety of conditions, not only at idle speed. The valve mounts on the bottom coolant plate of the throttle body.
Vehicle Speed Sensor (VSS): This is a magnetic speed sensor mounted on the transmission that sends a 4k ppm (pulse per mile) sine wave signal to the ecm. It is responsible for suppling the vehicle speed to the ecm. This is needed for a variety of functions. It is absolutely critical for the ecm's learn mode, timing retard, emissions, torque converter lockup (automatic lockup transmissions only), idle speed control, and to avoid stalling on deceleration. It is possible to run without one. However, your car will NOT be street legal if you are required to retain emissions equipment, the ecm will not control the torque converter lockup, the ecm will not retard timing if you run into detonation, and it is possible to run into stalling /idle speed issues. In addition, the ecm will not adjust the fuel table properly as you drive (known as its "learning ability"). If the ecm does not know the vehicle speed it is assumed to be 0 mph.
If you still insist on not running a vss, I very highly suggest that the minimum vehicle speed for timing retard be brought down to 0 mph in the prom. The factory setting is 2 or 3 mph. If you don't bring this value down, and you do not run a vss, the ecm will NOT retard your timing under detonation.