Engine Speed Sensor Questions

[betsy325e]

Dear thirtiers,

From what I understand, the engine speed sensor is "counting" or "sensing" the flywheel teeth as they pass by the sensor to tell the tach, ecu, and (in my case, idle control module) how many RPMs the engine is currently running.
Diagrams show yellow wire, black wire, and a "shield" wire.

(I've done the bentley resistance tests on the sensor: all seems good)

Question #1: What's the yellow wire (power? ground?)?
Question #2: What's the black wire (power? ground?)?
Question #3: I understand the "shield" insulates the signal to the ECU from all the spinning thingamabobs sending out noise in the engine...is this "shield" grounded...how exactly does it work...or is the shielding not connected to anything? The diagrams confuse me.

Question #4: How exactly is this thing "counting" or "sensing" these flywheel teeth?

Notice: I'm not particularly bright, you know, so...like, don't be using them big words.

Thanks,

J

[TobyB]

If you have an ICM, do you have an M10 318?

On the M20 325i engine, the sensor generates voltage pulses of varying
timing AND amplitude, so the ECU has to 'condition' them and look for the missing one.
And then generate rev and crank position info from them.
Yellow and black are 2 sides of a 'black box' circuit that do the signal conditioning-
I wouldn't be surprised if they are isolated from power AND ground.

On the M10, it's a 2 pulse per rev circuit, and it's only telling the car when to fire
and how fast the motor's spinning.

On the Eta, it's got 2- they work differently than either of the other systems,
one for position and one for speed...

The big word? Reluctive, as in the spinning bit generates a voltage pulse in the sensor,
which is the odd bit about the whole thing. Eventually, everyone goes to simple magnetic switches,
but in the '80's, those weren't deemed durable enough yet.


hth

t

[betsy325e]

Thanks!