I installed an MSD 6AL ignition control box and connected it to my MSD HEI Blaster coil with MSD's wiring harness. Now when my factory tach hits 4000rpm, the needle drops to 1000rpm and bounces around like crazy. If I am cruising around at a low RPM, sometimes the tach will drop to zero (engine still running fine of course, exhibiting none of the behavior indicated by the tach). I have a feeling that MSD's multiple spark technology may be to blame. If so, is there a way I can keep my factory tach by connecting it to my MSD box directly, or will I have to live with an aftermarket tach.Any help is appreciated.

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1990 Firebird Formula 350 speed density TPI; K&N, air foil, throttlebody coolant bypass, underdrive pullies, MSD 6AL, MSD HEI coil, 8.8mm wires, adjustable fuel pressure regulator, Hypertech Thermomaster chip, 160* thermostat, fan switch, B&M stage 2 shift kit, Edelbrock 1&5/8 headers, gutted dual cats, Flowmaster 80 series muffler, 3.73 posi rear end

 

I have heard there is a filter you have to buy, and it's put in between the box and the tach. The MSD boxes have been known to screw with tach readings..

 

The part number from Summit is MSD-8910 (it's called a "tach adapter").

 

Before purchasing the filter, try connecting the tach to the tach output on the MSD6AL. The tach output is a male spade terminal on the side of the 6AL. The white wire that goes from the harness to the MSD6AL (it used to go to the minus side of the coil) is the tach wire - connect it to the tach output of the MSD6AL. Leave the white wire in place that goes from the distributor to the MSD6AL trigger input (it used to go from the distributor to the minus side of the coil). I have had a 6AL hooked up, as indicated above, in my '92, and in my '84, & have not had any problems with the tach.

Tim

 

The factory tach should work fine when using an MSD. It says so in the directions that come with the MSD. Do hook the tach up to the box, the istructions say to never hook anything to the negative terminal on the coil when you have the MSD hooked up.

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'82 Firebird, dead stock, 9 bolt disc rear, over 200,000 miles and still going strong, more to come...