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COOLANT SENSOR. Usually located on the cylinder head or intake manifold, this sensor is used to monitor the temperature of the engineís coolant. Its resistance changes in proportion to coolant temperature. Input from the coolant sensor tells the computer when the engine is warm so the PCM can go into closed loop feedback fuel control and handle other emission functions (EGR, canister purge, etc.) that may be temperature dependent.
Coolant Sensor Strategies: The coolant sensor is a pretty reliable sensor, but if it fails it can prevent the engine control system from going into closed loop. This will result in a rich fuel mixture, excessive fuel consumption and elevated carbon monoxide (CO) emissions - which may cause the vehicle to fail an emissions test.
A bad sensor can be diagnosed by measuring its resistance and watching for a change as the engine warms up. No change, or an open or closed reading would indicate a bad sensor.
OXYGEN (O2) SENSOR. Used on both carbureted and fuel injected engines since 1981, the oxygen (O2) sensor is the key sensor in the fuel mixture feedback control loop.
Mounted in the exhaust manifold, the O2 sensor monitors the amount of unburned oxygen in the exhaust. On many V6 and V8 engines, there are two such sensors (one for each bank of cylinders).
The O2 sensor generates a voltage signal that is proportional to the amount of unburned oxygen in the exhaust. When the fuel mixture is rich, most of the oxygen is consumed during combustion so there is little unburned oxygen in the exhaust. The difference in oxygen levels between the exhaust inside the manifold and the air outside creates an electrical potential across the sensorís platinum and zirconium tip. This causes the sensor to generate a voltage signal. The sensorís output is high (up to 0.9v) when the fuel mixture is rich (low oxygen), and low (down to 0.1v) when the mixture is lean (high oxygen).
The sensorís output is monitored by the computer and is used to rebalance the fuel mixture for lowest emissions. When the sensor reads "lean" the PCM increases the on-time of the injectors to make the fuel mixture go rich. Conversely, when the sensor reads "rich" the PCM shortens the on-time of the injectors to make the fuel mixture go lean. This causes a rapid back-and-forth switching from rich to lean and back again as the engine is running. These even waves result in an "average" mixture that is almost perfectly balanced for clean combustion. The switching rate is slowest in older feedback carburetors, faster is throttle body injection systems and fastest in multiport sequential fuel injection.
If the O2 sensorís output is monitored on an oscilloscope, it will produce a zigzagging line that dances back and forth from rich to lean. Take a look at the waveform on the opposite page - thatís what a technician wants to see when he checks the O2 - think of it as a kind of heart monitor for the engineís air/fuel mixture.
O2 Sensor Strategies: Unheated one- or two-wire O2 sensors on 1976 through early 1990s applications should be replaced every 30,000 to 50,000 miles to assure reliable performance. Heated 3 and 4-wire O2 sensors on mid-1980s through mid-1990s applications should be changed every 60,000 miles. On OBD II equipped vehicles, the recommended replacement interval is 100,000 miles. The O2 sensorís responsiveness and voltage output can diminish with age and exposure to certain contaminants in the exhaust such as lead, sulfur, silicone (coolant leaks) and phosphorus (oil burning). If the sensor becomes contaminated, it may not respond very quickly to changes in the air/fuel mixture causing a lag in the PCMís ability to control the air/fuel mixture.
The sensorís voltage output may decline giving a lower than normal reading. This may cause the PCM to react as if the fuel mixture were leaner than it really is resulting in an overly rich fuel mixture.
How common is this problem? One EPA study found that 70 percent of the vehicles that failed an I/M 240 emissions test needed a new O2 sensor.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR. This sensor is mounted on or connected to the intake manifold to monitor intake vacuum. It changes voltage or frequency as manifold pressure changes. The computer uses this information to measure engine load so ignition timing can be advanced and retarded as needed. It performs essentially the same job as the vacuum advance diaphragm on an old fashioned mechanical distributor.
On engines with a "speed density" type of fuel injection, the MAP sensor also helps the PCM estimate airflow. Problems here may cause an intermittent check engine light (light comes on when accelerating or when the engine is under load), hesitation when accelerating, elevated emissions and poor engine performance. The engine will run with a bad MAP sensor, but it will run poorly. Some PCMs can substitute "estimated data" for a missing or out of range MAP signal, but engine performance will be drastically reduced.
MAP Sensor Strategies: Some MAP sensor problems are not the fault of the sensor itself. If the vacuum hose that connects the MAP sensor to the intake manifold is loose, leaking or plugged, the sensor canít produce an accurate signal. Also, if there is a problem within the engine itself that causes intake vacuum to be lower than normal (such as a vacuum leak, EGR valve thatís stuck open or leaky PCV hose), the MAP sensorís readings may be lower than normal.
THROTTLE POSITION SENSOR. Mounted on the throttle shaft of the carburetor or throttle body, the throttle position sensor (TPS) changes resistance as the throttle opens and closes. The computer uses this information to monitor engine load, acceleration, deceleration and when the engine is at idle or wide open throttle. The sensorís signal is used by the PCM to enrich the fuel mixture during acceleration, and to retard and advance ignition timing.
Throttle Position Sensor Strategies: Many TPS sensors require an initial voltage adjustment when installed. This adjustment is critical for accurate operation. On some engines, a separate idle switch and/or wide open throttle (WOT) switch may also be used. Driveability symptoms due to a bad TPS can be similar to those caused by a bad MAP sensor: The engine will run without this input, but it will run poorly.
MASS AIRFLOW SENSOR (MAF). Mounted ahead of the throttle body on multiport fuel injected engines, this sensor monitors the volume of air entering the engine. The sensor uses either a hot wire or heated filament to measure both airflow and air density.
MAF Sensor Strategies: The sensing element in MAF sensors can be easily contaminated causing hard starting, rough idle, hesitation and stalling problems.
VANE AIRFLOW SENSOR (VAF). The VAF has a mechanical flap-style sensor that is used on Bosch and other import multiport fuel injected engines. The function is the same as a mass airflow sensor, but air pushing against a spring-loaded flap moves a rheostat to generate an electronic signal.
VAF Sensor Strategies: The drivability symptoms for the VAF are the same as those of a mass airflow sensor if the sensor fails.
MANIFOLD AIR TEMPERATURE (MAT) SENSOR. Mounted on the intake manifold, this sensor changes resistance to monitor incoming air temperature. The sensorís input is used to adjust the fuel mixture for changes in air density.
MAT Sensor Strategies: Problems with the manifold air temp sensor can affect the air/fuel mixture, causing the engine to run rich or lean.
CRANKSHAFT POSITION SENSOR. Used on engines with distributorless ignition systems, the crankshaft position sensor serves essentially the same purpose as the ignition pickup and trigger wheel in an electronic distributor. It generates a signal that the PCM needs to determine the position of the crankshaft and the number-one cylinder. This information is necessary to control ignition timing and the operation of the fuel injectors. The signal from the crank sensor also tells the PCM how fast the engine is running (engine rpm) so ignition timing can be advanced or retarded as needed. On some engines, a separate camshaft position sensor is also used to help the PCM determine the correct firing order. The engine will not run without this sensorís input.
There are two basic types of crankshaft position sensors: magnetic and Hall effect. The magnetic type uses a magnet to sense notches in the crankshaft or harmonic balancer. As the notch passes underneath, it causes a change in the magnetic field that produces an alternating current signal.
The frequency of the signal gives the PCM the information it needs to control timing. The Hall effect type of crank sensor uses notches or shutter blades on the crank, cam gear or balancer to disrupt a magnetic field in the Hall effect sensor window. This causes the sensor to switch on and off, producing a digital signal that the PCM reads to determine crank position and speed.
Crank Position Sensor Strategies: If a crank position sensor fails, the engine will die. The engine may, however, still crank but it wonít start. Most problems can be traced to faults in the sensorís wiring harness. A disruption of the sensor supply voltage (Hall effect types), ground or return circuits can cause a loss of the all-important timing signal.
KNOCK SENSOR. The knock sensor detects engine vibrations that indicate detonation is occurring so the computer can momentarily retard timing. Some engines have two knock sensors.
Knock Sensor Strategies: A failure with the knock sensor can cause spark knock and engine damaging detonation because the PCM wonít know to retard ignition timing if knock is occurring.
BAROMETRIC PRESSURE (BARO) SENSOR. The baro sensor measures barometric pressure so the computer can compensate for changes in altitude and/or barometric pressure that would affect the fuel mixture or timing. Some MAP sensors also perform this function.
VEHICLE SPEED SENSOR (VSS). The vehicle speed sensor, or VSS, monitors vehicle speed so the computer can regulate torque converter clutch lockup, shifting, etc. The sensor may be located on the transmission, differential, transaxle or speedometer head.
Vehicle Speed Sensor Strategies: A problem with the vehicle speed sensor can disable the cruise-control system as well as affect transmission shifting and converter engagement.
Replacing a sensor wonít solve a drivability or emissions problem if the problem isnít the sensor. Common conditions such as fouled spark plugs, bad plug wires, a weak ignition coil, a leaky EGR valve, vacuum leaks, low compression, dirty injectors, low fuel pressure or even low charging voltage can all cause driveability symptoms that may be blamed on a bad sensor. If thereís are no sensor-specific fault codes, these kinds of possibilities should be ruled out before much time is spent on electronic diagnosis.
Hello everyone. I own a 2000 Volvo S40 with around 150,000 miles that has been having some trouble recently. Based on what I've read it sounds like it could be the Fuel Pump Relay and am looking for confirmation before switching it or possibly some other ideas for what is wrong. Below are its symptoms:
Disclaimer - it does have a constant check engine light on related to the transmission so some of these symptoms might be related to that. Also, I thought it was battery related at first so I had that checked and it came back as "bad battery" and I installed a brand new one this week. The problems have continued but it does start "better" than before
1. Car has trouble starting but usually will if I hold the ignition key for a few seconds
2. Car occasionally dies while driving when foot is not on the gas peddle 2a. I notice it die mostly within the first 1/2 - 1 mile of driving and after that it tends to drive better
3. If the car feels like its going to die and I use the gas, it often will continue to drive
4. If I'm coasting (on highway simply for example but it happens at different speeds) and the RPM should be idling around 3RPM, it will suddenly drop down between 0-1RPM. Again, if I use the gas it will often continue to drive
5. The car seems to "drive itself" meaning without using the gas it will sometimes engage and drive along as if I had the gas peddle down. The brakes always work to stop the car despite the engine still trying to run so it is not dangerous to drive.
6. When the car does die, I lose automatic steering or whatever it is called. I can still steer the car it is just much harder to turn.
7. The car tends to start up almost right away after it dies. I do not need to wait for it to cool down or anything.
Like I said, from some of what I've read it sounds like maybe the Fuel Pump Relay? Which would be awesome because thats an easy and inexpensive fix. I am trying to avoid putting a few hundred dollars into this car due to the transmission problem. I'm hoping to just fix this problem and drive it out until it dies completely.
Any and all advice or thoughts is SO appreciated! Thank you!
I'm looking to buy my first car, and I've heard that Volvos were the most reliable and safest. I'm 22, female and don't know much about cars, so I'm a bit nervous about this whole car buying thing. The volvo I'm looking at is a 1991, four door, white, black leather seats, about 175,000kms. I believe it's a front wheel drive. I will forsure be bringing a male friend with me to the dealership, but he's already told me that he doesn't know much about those cars. Any information you could give me about them would be VERY much appreciated.
Can anyone advise if I had a flat trye on the FRONT ( 205/55/R16 ) & I replaced it with my spare ( which is 205/50/R16 )....being 5 ml smaller in Profile, whether this will damage TRANSMISSION or other... Just thought different size tryes driving car on either side may cause some damage..???
Im taking a look at a 2002 Volvo s40 for my first car. It has 98k miles. What determines the reliability of a car? The amount of miles or the year ATEQ VT55 OBDII TPMS diagnostic tool? assuming it was regularly maintained. Please help. Do you think it could do a 6 hour road trip drive?
Please I need help on how to resolved this problem, my car volvo v40 2002 less than 100km has a pulled back air pressure when i accelerate and sometimes in response jumped forward I have been experiencing this in the last 2 weeks. I took the car for scanning and I have this code P0420 Catalytic converter below threshold bank 1. I am completely novice on how to solve this problem. PLEASE HELP.
Hi, I have a Volvo s40, 2003. I have been having problems with the acceleration especially going up hill (sometimes i can only reach 30mph) or over taking cars. There just doesnt seem to be any power there. I think it started when I went really low in gas, although this maybe a co-incidence. It does drive and can reach proper speeds but its like the power has dropped, it is also hard to start in the mornings - not sure if this is the battery or the starter. I have had it plugged in and Camshaft sensor came up and also speed sensor...? I bought a second hand camshaft sensor but that didnt make any difference. Anyone any ideas. I really know nothing about cars so any advice would be very helpful.. Im not sure if its worth spending too much more money on her..especially if it is a bigger issue. Thanks!