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Discussion Starter #1
For my swap I'm using a 91 non ic 6bt backed by a 92-93 a518. I got isspro gauges and need to know how to link the speedo up. The speed sensor on the tranny is elec and has a 3 wire connection. What are these 3 wires? And no Im not using the factory harness? The speedo is electric btw.
 

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Made some calls....nobody has any idea(including isspro), I have a 2 wire speed sensor from an older 727 tranny that I think will fit in the 518, that's my plan for now. Any input on a 2 wire?....I'm guessing 1 as a sender, the other to ground?
 

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I am not familiar with your particular sensor or application but a typical 3 wire sensor has a power input, signal out of it, and ground.

As for a two wire the circuit would go as follows. Power to to the dash guage, across the guage and down to the sensor, across the sensor and to ground. The two wire is most always used full voltage where a three wire will frequently be in a cut voltage cicuit like 5 volts or something dependeing on the application.
 

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Thats pretty much what i have found out so far. because i dont know which wire does what on the 3 wire or what voltage is runs at...heard 5-8 volts (but no one seems to know) im gonna try to 2 wire one. ill update when i try it...not gonna have any time this week
 

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I used the old 2 wire one I had from the 727, bolted right up, 1 wire from the larger terminal to signal on speedo, ground the other. Works perfect. Hope this helps someone down the road, still couldn't find any info on a 3 wire set up.
 

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2 wire sensors are variable reluctance devices. They are essentially just a coil of wire and a magnet. The tone wheel decreases the reluctance of the magnetic circuit when a tooth approaches the sensor and increases it when the sensor is between teeth. The result is an approximately sinusodal output of varying frequency and magntude, depending on the speed of the tone wheel. They have the advantage of not needing power, and are generally rugged and reliable. Their disadvantage is low output voltage at low speeds, and for this reason the wires from the sensor to the connected device (PCM, TCM, speedo, etc.) are configured as a twisted pair to give better immunity to external noise pickup.

3 wire sensors are Hall effect devices. I won't get into the physics of how the Hall effect works, but the nature of it requires dedicated, sensitive electronics to detect the voltage changes produced, so they are typically manufactured as an integrated sensor with the necessary electronics built in. As mentioned above, the 3 wires are power, ground, and output. The output is normally a non-zero crossing square wave of 5 or 12 volts. Assuming you know which wires are power and signal, such a sensor is easy to interface to most aftermarket speedos designed to be driven off of a speed output from a PCM/ECM. Hall effect sensors can function down to zero RPM, so they are commonly used for things like crank and cam position sensors, where low speeds during cranking may not generate enough signal from a variable reluctance sensor. In some cases, like the OP's, they are also used for speed sensors.
 

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Discussion Starter #7
Ya I would have liked to use the 3 wire....but it seems no one in the world knows which wire is what coming out of the sender. It turned out to be accurate so I'm happy.
 
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