Proximity switch/sensor testing?

New to the group, so apologies if this question has been asked before.
My car is fitted with a sensor that tells the ECU that manages the auto
gearbox what RPM's the engine is running at. The error code on the car points to the sensor not sending the RPM's to the ECU Before I fork out £60 for another sensor is there a way of using digital multimeter to test it? The unit is a plastic moulding with two wires, and has quite a strong magnetic field.
Anyone know of a layman's guide to testing gadgets and gizmos found on modern cars on the web??
My level of expertise?....Novice!
All help appreciated.......ttfn........Alistair
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Alistair Ross wrote:

Firstly, it may not be the sensor but the electronics pack to which it is connected. However:
The simplest of these just has a magnet with a coil of wire wrapped around it and works by producing a small electrical output as the teeth of a gear wheel rotate into and out for the magnetic field produced by the magnet. As this couldn't be a lot simpler, they seldom go wrong - but their position relative to the gear wheel is very critical. It may be that it doesn't require replacement but simply needs to be set correctly in relation to the gearwheel teeth. It may have vibrated out of adjustment. The vehicle maintenance manual will have the setting up instructions.
One of these sensors can be tested for basic functionality by simply by putting it across the terminals of an analogue microammeter and moving the sensor towards and away from a steel object - the needle of the meter should swing as you do so. It is less easy to do with a digital meter but you may be able to see that the meter is reading /something/ as you move the sensor.
You may have a different type of sensor though - some use the two wires to power an active electronic, self-compensating, sensor from a constant current supply. The sensor gives an output by varying the impedance of the device as seen by the power supply and hence changes the supply output voltage. These units can only be tested whilst connected to a suitable supply. I can't think of any harm you can do to one by simply connecting it across a microammeter, though - to see what it does. These more complicated sensors are used because their exact positioning is not so critical and their output is amplified and low impedance and thus far less effected by ambient electrical noise - of which a lot exists in an engine compartment. Because they produce a much larger output, the electronics pack to which they connect can be made correspondingly simpler and less likely to go wrong.
Testing these needs special equipment - although a 'scope will do the job nicely.
In my experience, these seldom go wrong but much more frequently are knocked or vibrate out of position.
--
Sue
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Sue };-) I have a whole new respect for you .
From: snipped-for-privacy@hotmail.com.invalid (Palindr☻me) Alistair Ross wrote: New to the group, so apologies if this question has been asked before. My car is fitted with a sensor that tells the ECU that manages the auto gearbox what RPM's the engine is running at. The error code on the car points to the sensor not sending the RPM's to the ECU Before I fork out £60 for another sensor is there a way of using digital multimeter to test it? The unit is a plastic moulding with two wires, and has quite a strong magnetic field. Anyone know of a layman's guide to testing gadgets and gizmos found on modern cars on the web?? My level of expertise?....Novice! All help appreciated.......ttfn........Alistair Firstly, it may not be the sensor but the electronics pack to which it is connected. However: The simplest of these just has a magnet with a coil of wire wrapped around it and works by producing a small electrical output as the teeth of a gear wheel rotate into and out for the magnetic field produced by the magnet. As this couldn't be a lot simpler, they seldom go wrong - but their position relative to the gear wheel is very critical. It may be that it doesn't require replacement but simply needs to be set correctly in relation to the gearwheel teeth. It may have vibrated out of adjustment. The vehicle maintenance manual will have the setting up instructions. One of these sensors can be tested for basic functionality by simply by putting it across the terminals of an analogue microammeter and moving the sensor towards and away from a steel object - the needle of the meter should swing as you do so. It is less easy to do with a digital meter but you may be able to see that the meter is reading /something/ as you move the sensor. You may have a different type of sensor though - some use the two wires to power an active electronic, self-compensating, sensor from a constant current supply. The sensor gives an output by varying the impedance of the device as seen by the power supply and hence changes the supply output voltage. These units can only be tested whilst connected to a suitable supply. I can't think of any harm you can do to one by simply connecting it across a microammeter, though - to see what it does. These more complicated sensors are used because their exact positioning is not so critical and their output is amplified and low impedance and thus far less effected by ambient electrical noise - of which a lot exists in an engine compartment. Because they produce a much larger output, the electronics pack to which they connect can be made correspondingly simpler and less likely to go wrong.     Testing these needs special equipment - although a 'scope will do the job nicely. In my experience, these seldom go wrong but much more frequently are knocked or vibrate out of position.
--
Sue
--
outstanding !
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Roy Q.T. wrote:

LOL. Thanks. Eau de gasoline doesn't do much for my love life though..Neither does being better at mending a range than actually /using/ it.
--
Sue


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Yes.....Thanks Sue best answer to a question I've ever had. Also thanks to other's who replied direct.
ttfn.....Alistair

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