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I just rolled the dice and ordered a factory tach(oer) to install on my 69. I am going to switch to console gauges. I think it looks alot cleaner. My dist. # 8361, will (crossing fingers) the tach work, do I need to install a tach adapt? Hey, with the sale classic industries is running rioght now, the tach was only like $177, i figure I can get about 75 outta my 5" autometer with shift light, so, Price was not too bad. I just have to think of a clever place to install a shift light!
 

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My theory is that the tach signal gets overloaded by the flyback voltage from the coil. The higher the energy the higher the flyback. I believe a diode breaks down in the input stage of the tach and instead of only stripping the positive spikes, the negative spikes also get through and foul the reading.

It's not good for diodes to be broken down in reverse bias unless they are zener diodes. I personally would make a filter. The stickies at the top of the Electrical forum has a description of a home-brew. It's adjustable and it will work. Think of it as making it possible to turn the volume up before you blow your speakers 'cept your trying not to blow your input diode.
:beers:
 

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I ran the MSD box and coil with my GM HEI, but it had the MSD module in it, with an OER factory tach and had no issues with it at all. I'm not sure if the MSD dist would make a difference. I used the tach output at the MSD box. I know the original factory tach had issues and would require a tach filter but the OER don't seem to need them.
 
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