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Tachometer Reading Too High-123 Ignition


sgaffman
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Hi, I've got a small issue I'm hoping to get some direction with.  I just upgraded the ignition system on my 72tii to a 123 distributor and a Bosch red coil and couldn't be happier, my car runs fantastic with this setup!  Lately I've been daily driving it and I've noticed the tachometer begins to drift from the RPMs displayed in the 123 app.  At idle, the tach in the car is maybe 100 RPMs more than the 123 app, but the difference between the two grows as RPMs increase.  At 60mph, the 123 app displays 3k RPMs which I'm inclined to believe, but my tach displays 4k RPMs.  If I'm in neutral and rev it to what I think 2500 RPMs is on the tach, I'll pull out the 123 app and see I'm really at about 2k RPMs.  In a nutshell, my in-car tach is as accurate as possible at idle speed, but becomes progressively more inaccurate as RPMs increase.

 

I found this article while searching for a tach calibration procedure...is this what I need to do now? 

This weekend I will have some time to work in the garage so I'd like to have some things to check (will start at the connections/grounds).  I've also found articles talking about building the Y-grounding harness which I'm planning to do as well.  Thanks in advance!

 

 

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I've found that 2002 tachometers are usually off by 80-100 rpm at idle, when compared to a digital, inductive timing light. I suggest you check, using a timing light with rpm function....

 

And since we're here: I *HIGHLY* recommend you double check everything with an adjustable timing light, to verify that the timing shown by your 1-2-3 is actual, and not off by a few degrees, across the board. If you're not 100% spot on, when setting it up, all your computerized readings will be off - and useless. 

 

 

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The factory tachometer will be full of 40 odd year old analog electronics that pulse an output to the meter coil to drive the needle whenever it detects the 'high voltage ' of the back emf caused by the coil field collapsing. The 123 is counting the actual revolutions of the distributor so is bound to be more accurate. 

 

An instrument specialist can probably recalibrate for you but you can certainly check for good connections throughout. 

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