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My tach will not register when at idle. If I accelerate, it will register but when I go back to idle, nothing?
 

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Had same thing and turned out to be the Tach sensor. The 2 wires had started to come loose and after replacing it the tach shows proper RPM. From what I have read replacing the Tach sensor can also fix some hard shifting issues because it shifts based on RPM. Have a project 1990 7.3 IDI I am working on and I havn't gotten to the point I can test that on the road but I had both hard shifting and the tach didn't show proper RPM. It would show zero at idle and would climb to around 1.8k with a light foot on the pedal lol I don't remember the RPM when I floored it.

Followed this: 6.9L & 7.3L IDI Tach Sensor Replacement
 

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Thank you rosston, I am aware of those wires and will check them out. They are connected to the wire harness with a slip on connector so it shouldn't be difficult. I always wonder whats under that big nut that the go to. Magnetic?
 

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Yeah the tach sensor is a quick replacement. I am still learning the IDI but my understanding is your timing is under it for the fuel injector pump? and the sensor is magnetic to sense the RPM.
 
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