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The infamous tachometer


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This is (almost) the last thing not working on my truck at the moment.


The 2.2k resistor checks out. Ive pulled the instrument cluster to examine. The yellow wire is getting voltage. But that must not be all it needs?


any thoughts?


after pulling the tach from the cluster, it appears to be in such excellent shape and I find it difficult to scrap such a fine piece of engineering and manufacturing.  

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I have resurrected over a dozen 720's due to the quasi ECU or "brain" failing. I have not gotten into the details of what the brain actually is but from loose information it as a shitty relay system, I basically bypass the brain and directly wire the coils/Distributor. One of the casualties of a broken brain is the tachometer,  just like the coils/distro, I just directly wire the tachometer up which is very easy as it has own plug in the back of the gauge cluster. Just like Mike said, black to ground, yellow-red to a acc power source and blue-black to the negative side of either coil. If you have not bypassed your brain yet, I strongly recommend doing so as it is just a matter of time before it goes bad

Edited by Logical1
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What 'brain'?


The California emission '85s  had a ECC carburetor controlled by an ECU under the driver's seat. This is a carburetor with an adjustable primary jet/solenoid several sensors and O2 feedback to keep air/fuel near optimum. The tach signal didn't go through it although it was connected to the coil negative in order to know the engine RPM. 

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  • 7 months later...

an interesting update; I was having issues with brake lights.  I was tracing the connections and trouble shooting that system, and may have jostled something just right.  The tachometer has been resurrected.

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