You need a filter. Go to the MSD website, or do a search on this forum.

 

Actually, it's a 'Tach Adapter',
And it's MSD p/n 8920

http://store.summitracing.com/partdetail.asp?autofilter=1&part=MSD-8920&N=700+115&autoview=sku

 

will the jeep still run without this tach box or do i need it because the tach is in series?

 

I guess my question is, do I absolutely need the tach box to run the MSD 6A in my jeep?

 

No, you only need the tach adaptor if your tach does not work when connected to the tach output terminal on the side of the MSD-6A module. Your ignition will run fine without the tach adaptor. There are two tach adaptors used with AMC/Jeep, the 8910 when using the white wire trigger mode, and the 8920 when using the magnetic pickup mode.

read page seven on the instructions for the MSD 6A below:

http://www.msdignition.com/uploadedFiles/MSDIgnitioncom/Products/Ignitions/6420_instructions.pdf

Jim

 

Your jeep will still run, the tach just won't work properly.

There is NO WAY I know of to hook up the factory Jeep tach to an MSD (or any other CDI module) and have the tach work without using a Tach Adapter.

See,
Your jeep tach is a SERIES type tach,
Meaning your key switch runs the ignition power through the tach before it goes to the ignition coil/module.

When the coil/module fire the ignition, the tach 'Counts' how many times the engine is firing and gives you a reading on the tach.

When you install an MSD (or other CDI module) the tach isn't seeing any 'Pulses' in the electrical current,
It's just supplying enough STEADY (not pulsing) current to keep the module switched 'ON'...

No pulses, no reading.
By hooking up a tach adapter, you can add 'Pluses' to that signal for the tach, without screwing up the CDI module's power supply... And your factory tach works.
--------------------

With an aftermarket type tach (Parallel type) the tach simply 'Counts' how many times the coil is grounded, and that produces a signal.
Since that's what most factory and aftermarket tachs do to get a reading, that's what MSD originally put on their units, and that is what all the 'Clones' use also...

The Parallel or 'Grounding' circuit is the easiest to reproduce for the CDI module,
And it's the most widely used, so that's what MSD and other CDI makers use.

 

Ok, so do I just run a wire from the series wire on the back of the tach to the msd tach box?

 

Think I'm just gonna get a aftermarket tach, it will be cheaper.