Trying to locate the tachometer transducer, "sending unit", on my 5.9 Cummins, Allison AT542, 4 speed tranny. Might the indicated, mounted in the forward/input end of the housing, be the piece. Thanks.

That is where mine is located (looks the same also) but I have a MD3060 6 speed trans.

Don't think that make a difference

Same here. It's on the engine not the transmission.

Thank you gentlemen. And I imagine it functions by sending an electrical pulse from a magnetic origin, similar to the old style cruise control sending unit.

Most of the older style sensors consist of an inductive coil which indeed generates a pulse of current when a magnet passes by. These are usually simple to troubleshoot, as the sensor is a sealed coil with two leads. The coil resistance can be measured with an ohmmeter and will have a specific resistance. Open or shorted coils will show higher or lower resistance respectively.

Newer designs are often based on Hall effect sensors. Hall effect sensors are semiconductors of the FET (field effect transistor) type. They usually have three leads, or two leads plus ground. The sensors respond to a magnetic field by changing their conductivity. An applied voltage allows current to flow through the sensor when in the presence of a magnetic field. They usually have a power lead, ground lead and a sense output lead. These are more difficult to diagnose since they consist of semiconductor diodes, not easy to check with standard meters. Common applications include crankshaft sensors and keyboard sensors.

I am fairly certain the Allison transmission sensors are of the older inductive coil type.

I think it is the inductive coil type with the two leads. I understand fully open reads 1, and shorted very close to 0 resistance. Would it be unusual to be partially open or partially shorted? Meaning if not 1 or near 0 then probably a good sensor. I have no idea what the proper resistance is for a good sensor.

Correct. Prob a couple hundred ohms normally. Sometimes they are intermittent when they heat up....

what we are talking about here is a "hall effect sensor" You can google it or go here---https://www.electronics-tutorials.ws/electromagnetism/hall-effect.html They are quite commonly used to sense proximity of ferromagnetic objects. and have been around for years (nearly back to the vacuum tube days. By nature they are extremely reliable, the unreliable part comes in when the cheapest offshore source is used to reduce cost and to enhance the dealer repair income. Because of this they are quite overpriced to the consumer. john

Why don't we have a saluting emoji? My point made... and with no drift intended.

Like looking for something in the fridge. My wife's selfi -