"Mercedes 'A' Class"
Components Parts

Sensors Locations

Vehicle, engine runs rough and back-fires Please see the content of Question 211 for further information.

We are told that an important part of sensors systems in today's vehicles is to determine positions. The wide range of mechatronic systems used in modern vehicles requires position sensors both for reporting the drivers' intentions (measurement of relevant values )and for determining the state of the engine or vehicle (Measurement of actual values oil water brake fluid etc )
Gone are the days when you removed a plug and brought a particular piston to top dead centre or TDC to confirm the engine crankshaft position. In this day & age drivers are even encouraged by the provision of sensors to sit in their car and check their oil!

As well as check the outside temperature, almost implying that if it is too cold outside you won't want to leave your car?
No in this instance it is a useful asset indicating to the driver when the outside temperature drops below freezing and hopefully warning him /her that road surfaces may be slippery/icy, this sensor is situated along side the front towing eye. A

However a number of the sensors are not easily accessible but are situated in such a way that if/when they themselves go defective, they are difficult and expensive to replace.

The electronic oil level sensor is a fine example, it costs nothing to pull on the dip stick once a week and check the oil level and providing the car is on level standing, you cannot be wrong. And yet when the electronic sensor goes defective, Oh they do! And require the engine to be at the very least dropped just to replace the defective sensor it does not make either mechanical or financial sense. Also if the defect on the sensor is not suspected or detected and the only way this can be done is by a manual oil level check, excess oil will put into the engine as demanded by the sensor, can put the engine at risk causing untold pounds worth of damage, as indeed can too little oil. in both cases damage can be caused costing hundreds if not thousands of worth of damage, for instance the catalytic converter is at high risk if the engine is overfilled with oil, adding hundreds of pounds to the overall costs.

Be Damned to sensors, lets get back to hands on mechanics.

The Crankshaft Position Sensor
The crankshaft Position Sensor is located at the front face of the engine as shown,(where the engine meets the gear box) Mercedes-Benz service information indicates that the connection should be removed when inspecting the poly -V-belt, i.e. turning the engine to inspect the belt throughout its length, for wear and damage, it being re-connected on completion .
It was also interesting to note that one owners car keep stopping approx every 15-20 minutes and wouldn't re-start for a further 20 minutes or it appears until the engine had cooled.
This problem was diagnosed as a faulty crankshaft position sensor, relatively easy to replace once diagnosed & located star diagnostics although it appears there were no warning light indicating that a problem existed.

Crankshaft position sensor. A

Crankshaft position sensor A

Crankshaft position sensor A To locate this unit look below and to the left of the gear linkage, quite difficult to locate and I suspect even more difficult to work on albeit is only secured by one small bolt. the cable connector is the same as is is used on connection such as the brake cylinder level indicator, and reversing light switch. quite difficult to locate and I suspect even more difficult to work on albeit is only secured by one small bolt. the cable connector is the same as is is used on connection such as the brake cylinder level indicator, and reversing light switch.

Should it become necessary to remove this component then a small 'TORX' (star drive) C socket is required, it is possible to get away with standard hexagonal socket on bolts of this size but take care not to damage it trying to remove or difficulty will be experienced even with the right tools.
This sensor can be reached from the front of the engine bay

Where the crankshaft sensor is suspect, the rev counter, on the Instrument cluster display may not register the revolutions of the engine as it normally does.

The Camshaft Sensor

The Camshaft Sensor is located on the O/S end of the engine and as it name applies indicates to the ECU the status of the camshaft at any given time, I cannot claim to have much knowledge of such sophisticated electronics.
However if either is defective or the connector has not been reattached after repairs the engine may not start or run erratically, this will be down the the ecu not getting a signal from the components as to the position of the crank or cam shaft, information which is vital if the spark is not produced or fuel induction is not carried out at the correct time. This sensor is reachable and located on the o/s end of the engine

The sensor B connection is of the same type as above and the securing bolt is a star C above drive.

Coolant Temperature Sensor

This sensor is impossible to photograph due to its location I suspect also that it would be impossible to get at it to replace without again lowering the engine, albeit not by much. To locate it look for the thermostat housing shown in the photograph and then look to the left and back the arrow is pointing in the general direction, you will see there a small corrugated tube carrying the electrical supply to the sensor, it is possible to just see and feel the sensor.
Consideration to replacement of parts such as this on this car seems to have been completely over looked in the design & development stages with the result that the engine now needs to be mounted on elastic for ease of removal.
This sensor in my opinion cannot be reached without the engine be lowered to some degree

Oil Level Sensor

Its difficult to see the relationship between the diagram below and the photograph of the oil sensor but be assured that's what it is! It is not removable/replaceable without first lowering the engine

If your car is causing you problems by constantly telling you to check your oil level then check the connection to this sensor, if it is defective consider replacing it on the next oil chance as the oil needs to be drained to do so. that is always assuming that it can be removed without having to drop the engine?

If you can see the bottom of the dip stick tube then this unit E is located very near its base.

Knock Sensor

Located very near to the bottom of the dip stick tube, I really am playing the guessing game with these sensors , But I suspect that if you had the slightest problem with the crankshaft bearing Big end or small this sensor would give you warning!

My having made this comment prompted Matt to offer some technical information as to what this sensor is actually for and does. Its pleasing to me that owners are reading the content of the site, and I'm always pleased to receive information that will enhance the content of the site. Thank you Matt. Quote
Just to clarify, and so that you need guess no longer, a knock sensor on an 'intelligent' EFI system is there to detect correct ignition combustion. In the case of low octane/quality fuel, the air/fuel mixture may not ignite 'cleanly' causing knock, (sometimes referred to as detonation, ping, pinking or pre-ignition). When the knock sensor detects this condition, it will retard the ignition timing to counteract this. Detonation can, and will, eat away at the piston crowns, valve seats and spark plugs. If left unattended, it can cause catastrophic mechanical failure, resulting in a full engine rebuild or replacement. Most noted on turbo charged engines where it can easily destroy a piston in a very short space of time. Knock mostly occurs when the engine is under load. i.e. full throttle or uphill, of if the trying to accelerate too hard in too lower gear. In extreme cases, or on older cars, it can be audible and sounds like something small rattling in the exhaust.
Your analogy with the distributor is exactly correct, except these days, we do not have them. Well, not on many cars anyway. As an aside, had you ever wondered how Shell and BP make their claims about more power, responsiveness and economy from using their 'Optima' or 'Ultimate fuels'? - it is all down to intelligent fuel injection and knock sensors. The ECU will always advance the ignition timing as far as possible until the knock sensor detects detonation, the ECU will then retard the ignition to a safer amount of advance. The more ignition advance an engine can take, (within reason), the more efficient it can run. As 'Optimax' has a higher RON, it is less susceptible to detonation, thus gives more power/economy. Kind regards
Matt. Unquote.
So my assumption was wrong in this instance, thanks Matt for putting us all in the picture,
If instead of calling it a 'knock sensor' they had called it an 'Electronic Advance and Retard Sensor', 'EARS' ,for short, I suspect a good many of us would have understood its purpose more easily. This sensor cannot be reached without firstly lowering the engine

Air Intake Temperature Sensor

Located on the front of the engine left hand side and level with the bottom of the MAFS cover. it obviously has something to do with the air flow temperature going to the engine but do not know in what way. We do know of course that warm air flowing into an engine helps it run more economically and therefore the engine runs smoother but as with so much of the technology on this car one has almost to guess as MB give nothing away!
For more information see question 211
This sensor can be reached from the front of the engine bay

Front Axle Speed Sensors

Components of the ABS/ESP , care should be taken not to damage these components when working on brakes hubs, brake discs etc the transmitter surrounding the drive shaft neck marked F may need to be cleaned from time to time , I use an old tooth brush.
All components and sensor can be reached , wheels need to be removed for ease of access.

Rear Wheels Speed Sensor.

Rear wheel speed sensors, part of the ABS/ESP system (Active Braking System/ Electronic Stability Program
Sensors can be reached from under the rear of the car.

Fuel Sensor

See page 12 for more information on the Fuel tank sender unit.

Accelerator Control Sensor

I received this mail from an owner who has had an intermittent problem with the accelerator on his car :-

Owner :- Last week my A-Class suddenly developed a fault i.e. power from the throttle is reduced and the accelerator does not respond properly to the pedal. I got it checked and found out that the pedal position sensor is faulty. The cost for this sensor is around 110 + taxes.

Although I have added this information to the page I do feel you would be wise to get a diagnostic test carried out before spending that sort of money on a spare part.

Where it becomes necessary to remove the connection press down on tab A to release the connector.

In a similar vein I have attached information from another owner whose car kept stalling, the 'star' diagnostics pointing to the crank angle sensor which has now been replaced, it is doubtful if these defects can/could be picked up without the use of such diagnostic tools. It is worth remembering the AA (Automobile Association) in the UK do carry 'Star' diagnostics.

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