HEI Filter Assistance for Firebird Tachometer (Page 1/1) rbell2915 FEB 08, 09:40 PM Mickey_Moose posted a write up in this thread saying this... quote Ok, this may apply to other dash swaps out there with people running an engine swap using a HEI ignition system (ie: 4.9, carb SBC, etc). Sympton: tach bounces, or work fine till the higher rpms. Why: the modern tachs are driven off either the ECM or a DIS system and supplies the tach with a nice clean signal - the older systems such as the HEI have quite a bit of noise on the signal and the 'modern' tach can not deal with this noise. This circuit should eliminate any noise and send a cleaner square wave to you tach input. Edit: fixed picture I kind of took the lazy way out by hacking up an old Fiero tach to get the filtering circuit out of it. I also have no ideal if the component values are the same as the above diagram (never cheacked). Sorry, I didn't have a working digital camera to take pictures as I cut up the board, but you should be able to get an ideal of where I cut it from the pictures. Front view, the large round hole in the lower right is the input to the tach, the resistor pack (white chip) is part of the one that get removed when doing the V8 tach mod. Rear view, shows a better ideal of the cut in relation to the traces. The brown wire is ground and the upper yellow wire will feed your tach. The lower yellow one is the input from the distributor. Anyways, there you have it. Tim But the photos are not loading. Does anyone have any guidance on what exactly I am supposed to do? My tach is not reading correctly at RPMs higher than 3k and I am quite interested in getting this fixed.