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I have just purchased a 68 XR7 390. It has an MSD ignition and the factory tach will spin all the way up to about 450 rpms! I thought I saw someone post a message about some sort of ballast with an aftermarket coil.

Can anyone explain it for me? I'd love to get the tach working...
 

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which box

Which msd box do you have? If its the 6A series I can help the msd boxes have a special tach ouput do to the multiple sparks it releases up to 3500 RPMs. It is a pushon connector (like a speaker connector) mounted flush on the box.
 

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Aliright here is what i know 1.have to figure out what type of trigger your tach uses 2. after that you may need the msd adapter p/ns are 8910&8920 and hopefully you get lucky because the tach in my 73XR7 wouldn't work no matter what I did,also if you think maybe your msd tach output is bad use a voltmeter and read it,it should be between 1-5 volts or so at least thats for a MSD6A.
 

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I have an MSD tach adapter p.n. 8920 if your looking for one. New, never mounted, with instructions. I could send the instructions so you could see the install, or go to msd.com and download them.

Rick
 

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Soon after I replaced my Stock coil with a Jacobs coil my tach. quit working. It just hovers around 650 and moves a little, but nothing close to the actual RPM's. Any thoughts? or ideas?.
 

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1KAZYKAT.. Did you also change your points out ?
 

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Well ok you have the MSD 6A UNIT and it has its own tach output on it ,you need to check and see if your tach connection goes to that .If it does then check and see if tach output is good with a voltmeter read the tach output. Now with an MSD THE TACH CANNOT hook up to the coil.If you need the installation instructions for the MSD visit their website. Also it is always a good idea to have the wiring diagrams for your car they making tracing any wires alittle easier.Hope some of this helps you out
 
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