Join Date: Mar 2002
Location: Austin, TX, USA
Wow...burned the foil? Is this the flexible mylar PC board? The tach should not have allowed enough current into it to burn the foil. I have a 69 tach I'm working on now. It has a large enough resistor in series with the tach line that it could not possibly draw enough current to burn the trace even if it was connected to 12 volts directly. I can't explain it.
I have learned a few things about 1st gen tachometer technology recently though. They depend on a flyback voltage from the coil to make the meter read. In other words, they must see more than 12 volts on the tach lead for a short period of time to get a reading. What I suspect happens with HEI and MSD ignitions is that the flyback voltage is large enough that when it goes negative it causes and input diode to break down and makes the tach read irratically.
The filter does a couple of things. It suppresses high frequency noise from the coil and reduces the amplitude of the signal to prevent the diode from breaking down. The optimum filtering would probably be just above the point where the tach stops reading. A potentiometer would be nice since you could adjust it for the point of no reading, back up a bit and then build a filter using the same value.
I appologize if this isn't clear. What I'm trying to say is that the GM engineers probably did a cut and try job with the tach. They kept tweaking component values and when it worked they shipped it. Aftermarket ignitions change the formula making it necessary to add filters. Problem is, that each type of ignition could benefit from a different filter. So I think you will need a filter, but it is possible that your filter is working a bit too good.
Try this. Put a variable resistor in paralell with your filter (say a 20K ohm pot). Adjust the filter until the tach reads right and then replace the pot with a fixed value resistor. The extra resistor will reduce the filters resistance and allow a bit more signal through.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI