Conference terri::cars_uk

    Could be a coil or hall effect electronic switch. Electronic monitoring
    equipement could use it by measuring the rotational speed (time
    interval between rotations or ignition pulses) and the time lapse
    between the ignition pulse and TDC from the sensor.
    
    Not so easy for you to use for DIY purposes.
    
    Richard

when replacing the timing belt or chain it's useful to line things up using
tdc if any of the pulleys have moved.

Dave

Modern electronic ignition systems use a TDC sensor, plus temp sensors etc, to
get the ignition timing right, as there is no conventional distributor.
You will find the distributor
is just a rotor arm on the end of the camshaft for distributing HT.  All the
LT stuff is done electronically, using aforementioned sensors.  The leccy ign
unit works out �BTDC itself, based on engine speed and conditions.  My unit
even has a vacuum transducer (or whatever high-tech name they've given it) to
calculate the vacuum advance electronically too.

Scott

The TDC sensor, as previously described, is a part of the electronic engine 
control system found on many late-model fuel injected cars.  Here are some of
the other components of an electronic engine control system.

o  ECU (Electronic Control Unit) - This is the central processor of data taken
   in from the various sensors. The ECU will gather data as you drive and adjust
   the engine constantly.  It is like having a good mechanic under your hood.
   The ECU will store data from the last 20-30 starts, plus a certain number 
   of hours of running time.  If the weather changes abruptly and your car 
   does not run well anymore, you may need to reset the ECU by disconnecting
   the battery for 10 minutes.  The ECU brings the engine back to a factory set
   condition after the ECU is reset.

o  Oxygen Sensor - This device fits into the exhaust manifold and determines
   whether the engine is running lean or rich (ie: which side of the 14:1 air:
   fuel ratio the engine is on).  It operates as an on/off switch depending
   on which side of the 14:1 you are on.  Cars with oxygen sensors must run
   unleaded petrol to prevent fouling the sensor.  When an oxygen sensor fails,
   the car runs rough, gets poor gas mileage, and puts out black smoke.  Oxygen
   sensors are one of the most common sensors to fail.

o  TDC (Top Dead Center) Sensor - This sensor tells the ECU when the #1 piston
   is at TDC.  Some engines use a CAS (Crank Angle Sensor) instead.

o  TPS (Throttle position sensor) - This tells the ECU how far open the 
   throttle plate is.

o  MAP (Manifold Absolute Pressure) Sensor) - This senses the amount of vacuum
   in the intake manifold.  When this one goes bad, the car will shake and put
   out black smoke when it idles.  The MAP sensor is one of the most common
   sensors to fail.

o  CTS (Coolant Teperature Sensor) - This one tells the ECU whether the car is
   warmed up or not.

o  Knock Sensor - This one sits on the cylinder head and listens for knocking.
   It is usually found on turbocharged cars.

o  WOT (Wide Open Throttle) Sensor - This one alerts the computer when you put
   your foot to the floor.  The computer must enrichen the mixture to prevent
   the engine from running lean and knocking.

o  MAT (Manifold Absolute Temperature) Sensor - This senses the temperature of
   of the air and fuel going into the engine.

o  Airflow Sensor - This one is just behind the throttle plate and determines
   how much air is flowing into the engine.  Most of them are heated wires that
   are cooled by the flow of air.  The more flow, the cooler it gets.  As the
   wire cools, its resistance goes down.  Multiport fuel injected cars have 
   one of these, whereas the throttle body F.I. cars do not.

Did I miss any?  All cars sold in America will have all of these with the
exception of the knock sensor.  If you drive a car in the UK and it must have
unleaded petrol, you probably have all these with the exception of the knock
sensor.  Cars that burn leaded petrol will probably have only some of these.

Jim

    re .4, three points:
    
    Ford calls ECUs ESCs (Electronic Spark Control)
    You say they're on FI cars; they're also on carb cars
    The air temp sensor may be in the air cleaner rather than the inlet
    manifold.
    
    I didn't realise they "remembered" the last few hours running time; a
    great leap forward for artificial intelligence!  Do they use on-the-fly
    calculation to set the ignition advance, or have they got loads of
    look-up tables inside?
    
    Scott

    Whilst .4 is very informative....
    
    The actual car referred to does not have fuel injection, and the TDC
    sensor conects only to a diagnostic plug for external sensing. (I know,
    I have the same engine) Therefore my comments about being used to
    diagnose still apply in the case of this particular car.
    
    Richard