Sensors, their Functions, and Interactions
MAP (Manifold Air Pressure) Sensor
The MAP sensor is a pressure sensor chip which puts out a voltage proportional to engine vacuum, from zero to 5 volts. The computer uses the voltage to decide if the throttle is open and how far, so it can adjust the fuel mixture. The less vacuum, the higher the voltage, as shown below:
By applying vacuum from a hand pump, and voltage either from the car’s harness or a separate 5V supply, you can measure these points to see if they are correct and linear. The sensor can fail by fracture or electrical damage (no voltage output), or it can drift, which shifts the voltage output. Usually when it drifts, the voltage is higher, the computer thinks the engine needs more fuel for accelerating, and sets the mixture richer. This causes poor gas mileage (1/3 to ½ drop), rough running or idle, stalling and stumbling, with black smoke.
It can also cause the oxygen sensor to see too much raw fuel and it can then cause a fault code for the oxygen sensor, when the MAP sensor is really the cause.
Fault codes are 13 (loss of vacuum) or 14 (beyond min or max voltage).
Replacement of the MAP sensor is the fix. It costs about $30 to $40 and is a 5-10 minute job for the amateur.
Throttle Position Sensor (TPS)
The Throttle Position Sensor is a potentiometer (variable resistor) which attaches to the throttle shaft and sends a voltage signal to the computer, telling it how far open the throttle is. The resistance across it is about 5,000 ohms. At idle the voltage is about 0.7V and wide open throttle is about 4.0V. You can measure the voltage with ignition on and engine off, using a voltmeter. You can also unplug it with ignition off and use an ohmmeter to measure the resistance change as you open the throttle. It should sweep cleanly from 0 to 5,000 ohms between the center and one of the outer pins, and from 5,000 to 0 ohms from center to the other outer pin. Usually these fail by having a “dead spot” where the contact breaks down inside, and it can cause stumbling or missing on acceleration, or at a certain throttle opening.
Fault code of 24 is only set if the min or max voltage is exceeded, so often there is no fault code set.
Replacement of the sensor is the fix. It costs about $20 to $30 and is a 5-10 minute job for the amateur.
Coolant Temperature Sensor (CTS)
The Coolant Temperature Sensor is a thermistor (temperature-dependent variable resistor) which screws into the thermostat housing and sends a voltage signal to the computer, telling it how far open the throttle is. The resistance across it is about 13,000 ohms with engine off at room temperature. With engine fully warm, it is 600 to 1,000 ohms. The resistance drops rapidly as the engine warms on a steep curve. You can unplug it with ignition off and use an ohmmeter to measure the resistance change at these temperatures. It should sweep cleanly and rapidly from about 13K ohms to 1,000 ohms or less as the engine warms. Usually these fail by not responding to increasing temperature, so that it fools the computer into thinking the engine is still cold. The result is good cold starting, but poor warm/hot starts, or failure to start when warm, and rich, rough running when warm.
Fault code of 22 is only set if the min or max voltage is exceeded, so sometimes there is no fault code set.
Replacement of the sensor is the fix. It costs about $20 to $30 and is a 5-10 minute job for the amateur. It involves unplugging it and unscrewing it from the thermostat housing. Teflon tape is recommended for sealing the threads.
The Oxygen Sensor is a feedback device that senses how much oxygen is in the exhaust. The zirconium coating in the sensor produces a small voltage when exposed to depleted levels of oxygen, so a lean exhaust which has excess oxygen yields a signal of about 0.2V, whereas a rich mixture (all oxygen consumed) produces over 0.7V. The sensor works best above 600F, so modern cars have a heater element to bring it into operation faster, to reduce emissions and improve gas mileage. Since its output is ignored for about the first two minutes of cold operation, a failed oxygen sensor cannot cause a car not to start when the engine is cold.
Failure can be a failed heater element, or being damaged and running too rich or too lean, which will give a code 51 or 52. A head gasket failure and exposure to steaming coolant can kill a sensor, so it should always be changed after a head gasket failure. A failed sensor can cause poor gas mileage (1/3 to ½ drop), rough running or idle, stalling and stumbling, with black smoke.
Replacement of the sensor is the fix. It costs about $40 to $90 and is an easy job for the amateur *if it unscrews easily*. It involves unplugging it and unscrewing it from the exhaust manifold. Teflon tape is NOT recommended for sealing the threads. A little anti-seize compound works well, and is often supplied with it, but DO NOT get any on the sensor tip, or it will fail prematurely due to the sulfur in the past attacking it.