jamesh Posted June 28, 2020 Report Share Posted June 28, 2020 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. 1 Quote Link to comment
datzenmike Posted June 28, 2020 Report Share Posted June 28, 2020 Should be a black ground wire, a Black/Blue from the resister and a Yellow/Red power from the 4th fuse from the left on the fuse box 1 Quote Link to comment
jamesh Posted June 28, 2020 Author Report Share Posted June 28, 2020 I see all of these going to the wire harness. 1 Quote Link to comment
Logical1 Posted July 1, 2020 Report Share Posted July 1, 2020 (edited) 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 July 1, 2020 by Logical1 1 Quote Link to comment
datzenmike Posted July 1, 2020 Report Share Posted July 1, 2020 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. 1 Quote Link to comment
jamesh Posted February 11, 2021 Author Report Share Posted February 11, 2021 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. 2 Quote Link to comment
datzenmike Posted February 11, 2021 Report Share Posted February 11, 2021 I'd rather be lucky than good. 1 Quote Link to comment
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