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• The tach sensor for the 6.9/7.3L IDI is located towards the front of the engine on the injection pump gear housing (red arrow in image).
• If you'd prefer to test the tach sensor before replacing, check resistance across the two sensor wires with a multimeter. If the resistance measures between 2,000 and 3,000 Ohms, the sensor should be good.
• This is the tach sensor, Ford part number E5TZ-17B384-A. It's a very primitive, simple sensor that reads a magnet on the injection pump gear as it rotates. Every time the magnet passes this sensor, it triggers it and this pulse is what drives the tachometer.
• To remove the sensor, first disconnect the connector (red arrow, hidden behind the heater hose in the image). There are two clips that secure the connector; a small flathead screwdriver can be used to release the two safety clips.
• At just under $75 for a new sensor, it is not a cheap item. As you can see, this sensor has seen better days, and the previous owner(s) have performed some fancy tape work in an attempt to keep the original one working. Once the sensor has been disconnected, use a 1 inch wrench to remove it.
• Apply a small amount of fresh motor oil to the o-ring (red arrow in image) before installing the new sensor; this ensures a positive seal.
• Installation of the new sensor is reverse. Thread it in by hand to ensure it does not cross thread, then snug it up with your wrench. Finally, plug the connector into the harness. You may also want to wrap the sensor's wires with electrical tape to protect it from chafing in the future.