Inductive proximity sensors

[khwilson]

I recently had the 911 quit on me again and found it was the reference sensor thanks to my service manual and browsing this board (thanks to all that helped even though you didn't realize it).

I purchased two sensor units from our host, replaced both the reference and speed sensors, the car runs now, and now that all is well I got to thinking - since the ECM only uses the reference sensor for initial start-up, why can't it be "fooled" into thinking all is OK during an emergency situation?

From the web, the following is an operational description of an inductive proximity sensor:

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"Inductive proximity sensors operate under the electrical principle of inductance. Inductance is the phenomenon where a fluctuating current, which by definition has a magnetic component, induces an electromotive force (emf) in a target object. To amplify a device’s inductance effect, a sensor manufacturer twists wire into a tight coil and runs a current through it.
An inductive proximity sensor has four components; The coil, oscillator, detection circuit and output circuit. The oscillator generates a fluctuating magnetic field the shape of a doughnut around the winding of the coil that locates in the device’s sensing face.
When a metal object moves into the inductive proximity sensor’s field of detection, Eddy circuits build up in the metallic object, magnetically push back, and finally reduce the Inductive sensor’s own oscillation field. The sensor’s detection circuit monitors the oscillator’s strength and triggers an output from the output circuitry when the oscillator becomes reduced to a sufficient level."

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Since the ECM is looking for a reduced voltage from the sensor, my question is can a simple unit consisting of a 9V battery, a resistor, and the connector from my bad sensor be used to fool the ECM and allow the car to start? Something like this would cost about $15 as opposed to carrying around a $150 cable that takes significant effort to install even in the best of conditions let alone in a parking lot somewhere.

[dfink]

I will take a stab.
First they rarely fail so I don't think you need to carry a spare. Get a spare DME relay instead.
Second I believe the reference sensor tells the system where to start counting the teeth on the ring. If that sensor failed the system wouldn't know where it was in the rotation cycle. Some systems use a missing tooth to perform the same function but for whatever reason porsche uses the dual sensor.
If I am wrong on this someone will correct me promptly.

[Porsche_monkey]

You could easily fool the ECM but I think the point made above is correct. So you have no way of fooling it in a manner that will leave you with a driveable vehicle.

[khwilson]

Oh, I've got it all... the spare DME relay, the spare fuel pump relay, and now a spare reference sensor - all in a nifty little box labeled "Emergency Porsche Parts" tucked snuggly away in the trunk. Along with all the tools I need to install said parts.

It makes sense that the fooling gadget wouldn't work - I forgot that the reference sensor "sees" where TDC is as it is cranking and once it knows it allows the car to run.

[Porsche_monkey]

Originally posted by khwilson
Oh, I've got it all... the spare DME relay, the spare fuel pump relay, and now a spare reference sensor - all in a nifty little box labeled "Emergency Porsche Parts" tucked snuggly away in the trunk. Along with all the tools I need to install said parts.

Seem to me like that would taker a lot of the fun out of a long drive...

[khwilson]

You ain't kidding... I have been lucky so far in that both times the car took a dump on me it was in the garage and since I have other transportation I wasn't left stranded.

I have been working on cars and bikes a long time and one thing that I have found is that they all have human attributes. Some are humble and well-mannered and really don't give a lot of grief other than the occasional maintenance item. Some are arrogant prima donnas that will do everything they can to test your patience. I believe the latter is the one I have right now. She's only been around me for 6 months and already has shown her a*@ twice. Luckily, I am older now and have developed a healthy stockpile of patience. Hopefully, I will relent. I cannot attest to my continued good nature if she ever leaves me stranded somewhere remote.

Thank goodness for USAA Roadside Assistance.

Thank goodness for this board, too.

[ChrisBennet]

Good luck changing and speed or reference sensor on the side of the road or even the paddock. I carry a few resistors to fool a head temp sensor in an emergency, jumper wire and a spare DME relay.
-Chris