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Old 09-09-2003, 06:16 PM
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Help MSD /Tach Question

I understand that in some applications; MSD ignitions use a tach adapter to get the signal to the tach correct.
Is this because the signal that comes out the tach port of the MSD 6AL is a square wave. I just don't understand.
Would some one please explain. What symptoms does the tach read ? too high,too low, bouncing , or just all over the place.
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Old 09-09-2003, 06:33 PM
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A conventional tach mesures the break in the negative side of the coil. A voltage on and off.

msd switches more often than once per fire, so it won't work there.

I think the reason why a regular tach won't work on the tach signal port, is that the signal is a very low voltage signal(like the one from the distributor pickup) and the conventional tach simply isn't sensitive enough.

Don't be fooled into thinking you need to spend money on an autometer though. A less expensive tach, like the equus pro series, will work on the tach signal port. Less than half the price of an autometer.
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Old 09-09-2003, 06:48 PM
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Thanks Dave ; The reason I asked is the dwell/ tachometer (Snap-On), I was using to set the idle on my new 340 was a little crazy.
I borrowed a buddy's 5" tach, he said it probably didn't need this tach adapter thing.His engine was a GM if it makes a difference.
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Old 09-09-2003, 11:17 PM
rguggisb rguggisb is offline
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Hey JimmII-

Just in case you have a stock tach in the car, these can be modified to work with MSD (6al at least). If you pull off the back, the first component is a resistor, just bypass this resistor. As Dave571 said earlier, MSD fires more often and louder. With the resistor in place, the tach has a hard time seeing the "on" and "off". When the resistor is jumped, the signal isn't crystal clear, but the tach sees the difference.

I have a 69 Runner with factory in dash tach and MSD. Had the tach rebuilt by reputable speedo shop. They would be the source of this little "trick"

Good Luck!
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Old 09-09-2003, 11:23 PM
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I think the adapter amplifies the signal from the tach output port, so a standard tach will read it.

Don't know for sure, just thought I'd throw that in.

Remove the resistor eh? Sounds like a good idea (If it works of course)

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Old 09-10-2003, 05:10 AM
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Thanks for the replies; I think I understand about the weak voltage that was spoke of earlier. These tachs worked well on the cars. The 340 on the engine start stand was equipped with the MSD. The Snap-On tach was my dads hate to think I screwed it up after all these years.
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