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Here’s a quick and dirty rundown of various engine sensors. I’ll try and explain what they do, and why they are on the engine to begin with, but as far as checking to see if they’re bad, there are so many different ways to do so that I’m not going to get into it right now.
Way back in the early days of motoring, I mean back when the horseless carriage was a new-fangled thing, there were no sensors at all on any automobile. They were unnecessary. People were expected to do things like adjust their own fuel trims and ignition timing as they drove along. They were expected to check their engine oil every time they got gas. They were expected to keep half an eye on the thermometer that was built into the exposed radiator cap to make sure that their engine wasn’t overheating. People accepted this as the cost of not having to ride a horse all over the place. Over time, people essentially got lazy and didn’t want to do all that stuff as they drove. They just wanted to get in and drive their cars. Enter the very early, largely mechanical “sensors”. Things like the vacuum advance distributor, which would adjust ignition timing for you so that you didn’t have to. Time moved on, people got lazier until they didn’t want to do anything to their cars other than drive them. That’s why we have all the sensors now, which automatically keep an eye on everything from incoming air flow and temperature to the amount of unburned fuel in the exhaust. The sensors are also there for emmisions reasons, and for ease of starting in the cold. That’s the Why part of my explanation.
I’ve got to leave for work now, but when I get home in about 13 hours or so I’ll continue on with a rundown of the various sensors you’ll find under the hood of a modern car and what those sensors do.
Ok. It’s a little later than I wanted it to be before I got back here, but oh well. Now, onto my quick and dirty list of the various sensors and what they do. Not every car will have all of these, this is just a list of all the sensors that I can think of. It’s by no means a complete comprehensive list. Also, presume that all of the names have the word sensor after them, just so I don’t have to type it over and over again. I apologize in advance if I use language that’s overly technical, I’ll try my best to keep this in layman’s terms, but I can’t promise anything.
Mass Air Flow (MAF)
This is usually located on the air filter housing. It measures the amount of incoming air which will let the computer know how much fuel to spray at any given time. These will get dirty over time and not work as reliably, in which case they will need to be either cleaned or replaced. Cleaning them requires a dedicated MAF cleaner, which is available at most auto-parts stores. Whatever you do, make sure that you do NOT use carburetor cleaner on one of these. carb cleaner is way to aggressive and will destroy the sensor.Manifold Absolute Pressure (MAP)
This is another sensor that the computer will use to determine how much air the engine is taking in, so that the computer can determine the appropriate amount of fuel to spray at any given second. When they go bad, the car will most likely not run, or it may run real bad.Knock
Used to detect engine knock, or pinging, caused by incorrect ignition timing or poor fuel. When this sensor detects knock, it will tell teh computer to retard the timing, meaning that the spark plugs will fire later during the piston’s travel inside the cylinder. This is done to make sure that your engine’s internals stay inside the engine. Knock, or ping– either term is correct– is the sound that your engine makes right before it does something very expensive. It sounds like a hammer striking an anvil, hence the term ping.Crankshaft Position (CKP)
Used to determine the position of the crankshaft during it’s 360 degree spin. The computer needs to know where the crankshaft is so that it knows where the pistons are, which is required knowledge in determining when to fire the spark plugs. Commonly used in conjunction with the CMPCamshaft Position Sensor (CMP)
Like a crank sensor, only for a camshaft. It’s what tells the computer when to fire the fuel injectors.Air/Fuel Ratio (AFR)
Used to measure the amount of fuel left in the exhaust stream to help the computer keep the engine at a stoichiometric air fuel ratio. That ratio is 14.7:1 at sea level on unleaded gasoline. Meaning, 14.7 parts air to every 1 part fuel.Oxygen (O2)
Used in a nearly identical fashion as the AFR, for very nearly the same reason. I believe that it’s possible to tell the two apart by looking at them, but they both look the same to me. One of them cycles between lean and rich faster than the other, but I forget which is which.Steering Angle (SAS)
Used to tell the adaptive suspension/stability control/moving headlights how far the steering wheel is turned in either direction. Commonly only seen on high-end luxury cars like BMW and Mercedes-BenzVehicle Speed (VSS)
Also known as the ABS (Anti-lock Brake System) sensor, it measures the speed of each wheel using a toothed wheel and a magnetic pick-up mounted just inboard of the hubs. You may also find a VSS in the rear differential, with the toothed wheel mounted on the ring gear.Airbag
Tells the computer that you’ve just crashed and that it should fire the airbag out of the steering wheel/dash board so that you don’t die.Coolant Temperature (ECT)
Tells the computer how hot the coolant, and by extension the engine itself, is running. The heat difference is used to determine how much fuel is needed. Engines at operating temperature require less fuel to keep running than engines that have been cold-soaking over night.Oil Pressure
Used to tell the computer when to illuminate the oil pressure light on the dash, that little red light that looks like a watering can. The one that comes on usually right before something very expensive happens to the engine.Oil Temperature
Like the ECT, only for oil.Fuel Level
Hopefully self-explanatory, found in the gas tank.Ambient Light (ALS?)
Used on cars that have automatic headlights to know when to turn the headlights on.There are other sensors, but I can’t think of them at the moment. Like I said, this isn’t meant to be a comprehensive list. It should be enough to get you started though.
Quoted From Trcustoms719:
This is one of my top favorite threads so far!
Keep ’em coming.I agree, this needs to be made a sticky….
Here is my small contribution seeing as 3SheetsDiesel covered the majority-
Intake Air Temperature Sensor (IAT)
Has a thermistor sensing element that is exposed to the intake air and influences air/fuel ratio’s, spark timing and idle speed control on engines that use the IAT.
Thick Film Ignition Module (TFI)
Used on Fords and is used to turn on and off the primary ignition coil circuit as well as controlling primary circuit dwell time.
Exhaust Gas Recirculation Valve (EGR)
The EGR valve is used to allow a measurable amount of Inert gas into the combustion chamber to help reduce emissions. This Inert gas is also cooler than cylinder temperature. They also help to reduce NOx (oxides of nitrogen)
I agree that the amount of sensors and crap on a car is crazy but, us techs are going to have to adapt and live with it.
My own personal vehicles are a 87 mazda B-2000 truck with a carb, no A/C and no power steering, and
a 88 mazda mx-6 GT turbo that’s just for a fun project. And before that I had my 70 Vw bug.
I like old cars for my personal cars.Why all of the sensors? So the engineers can talk to each other and say: “Dude, wait til the techs get ahold of THIS one!” Freakin B@$t@rd$. No car in the history of Earth actually NEEDS any of this electronics garbage. Give me a Sloburetor and a Distributor, I’ll be just fine. You can tune the whole engine with a piece of pocket change in a pinch.
That’s why i ask about the sencers what they do. I work on cars last 15 years. Older ones. I put my baby (92 fulsize Blazer in the lake in fefruary. Dads idea go ice fishing. That was bad day. Took the two weeks to get it out and 30k later. Morow of that story is DO NOT GO ON ICE.
Anyway got a 98 grand prix with 3100 changing head gaskets and intake gaskets well redoing the top of the engine. All the sencers i had to unhook. Made me think why and how? Before is a gessing game what sencers and throw bunch of money at it. I took ericthecarguys advice from his videos and youtoob and learn how to chec stuff. with the ohm meter found out that my aic is no good.
I love older cars they are simple. The newer one just more and more electronic and one sencer could make car run like crap. Then stressing if car will pass or fail emission test. It’s nice to know now how to check and fix that. So i’m pressy happy.
I do agree with you that simplicity is nice but I also remember the cars of the early 80’s when I started driving in Maine that were a pain to start in the winter. It seemed that every car had a special technique that only the owner knew that would get a cold car started. But now it seems that these new cars with all these sensors start reliably every time regardless of how cold it is.
Also I remember going in to Providence, RI as a kid in the 70’s with my dad and wondering why it was so dark over the city when it was sunny elsewhere. There was that much smog. I’m sure other large cities were the same back then. The sensors have help solve that too.
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