Tachometer bouncing (MSD ignition wiring question)
So I am trying to run down an issue with my '71 Duster 340 4-speed. The problem is that the aftermarket original-looking rallye tach jumps quite a bit at high RPMs. It seems accurate at idle and low speeds, but whenever I get on it, it's all over the place. It will usually settle down at the higher RPMs when I am decelerating. I am going through the wiring trying to determine if the tach is bad and needs replacing, or if perhaps there is a simpler fix.
The ignition system on the car was inherited from a previous owner. It has an MSD 6AL box and a Blaster 2 coil. The distributor is vacuum advance with a magnetic pickup. The tach is fed from a dedicated tach output from the MSD box. I'm not at all familiar with the MSD system, but I downloaded a manual and it talks about some tachs needing an adapter. The first thing I wanted to try was moving the tach feed from the box to the (-) coil terminal to see if that improved anything.
BUT, the first thing i found when running down the existing wiring is that I am 99% sure the coil is connected backwards. There are two wires running from the MSD box to the coil. The black wire is connected to the (+) coil and the orange to the (-). Logic and the manual say this is backward, but the car seems to run fine. A little Googling on the subject yielded results all over the map. Some sites say it is the end of the world, others say it's no big deal. I tend to believe the second thing, but I also don't want to fix something that doesn't seem to be broken.
So this seemed like the perfect question for the brain trust here on FABO. I want to just reverse the wires and see what happens. Does my tach bounce go away?
Or does the car immediately burst into flames? :lol:
Any thoughts?
Thanks!
So I am trying to run down an issue with my '71 Duster 340 4-speed. The problem is that the aftermarket original-looking rallye tach jumps quite a bit at high RPMs. It seems accurate at idle and low speeds, but whenever I get on it, it's all over the place. It will usually settle down at the higher RPMs when I am decelerating. I am going through the wiring trying to determine if the tach is bad and needs replacing, or if perhaps there is a simpler fix.
The ignition system on the car was inherited from a previous owner. It has an MSD 6AL box and a Blaster 2 coil. The distributor is vacuum advance with a magnetic pickup. The tach is fed from a dedicated tach output from the MSD box. I'm not at all familiar with the MSD system, but I downloaded a manual and it talks about some tachs needing an adapter. The first thing I wanted to try was moving the tach feed from the box to the (-) coil terminal to see if that improved anything.
BUT, the first thing i found when running down the existing wiring is that I am 99% sure the coil is connected backwards. There are two wires running from the MSD box to the coil. The black wire is connected to the (+) coil and the orange to the (-). Logic and the manual say this is backward, but the car seems to run fine. A little Googling on the subject yielded results all over the map. Some sites say it is the end of the world, others say it's no big deal. I tend to believe the second thing, but I also don't want to fix something that doesn't seem to be broken.
So this seemed like the perfect question for the brain trust here on FABO. I want to just reverse the wires and see what happens. Does my tach bounce go away?
Or does the car immediately burst into flames? :lol:
Any thoughts?
Thanks!
