Originally Posted by OTT

I've search so don't give me ****.

The car is a 93 with jacob ignition, if it matters.
My tach will "dance" between 2.5 and 3k except when very cold (like 40 or below). I replaced both capcitors and resoldered all joints but still doesn't work right, any ideas.

Steve

Originally Posted by bumpstart

bouncing tacho?



something you may find handy

- its variations of the "HEI corvette tach filter"

they are simple , low pass filters, and nothing complex
( stereo and electronics people will recognise the circuit and its brutal simplicity )
there are more complex types of filter designs
but this one GM found suitable for quelling the bell-ring and RFI that HEI coils would feed to the old points spec tacho

with the bosch HEI DLIDFIS and solid or spiral HT wires, the stock tacho can pick up the occasional noise, and flicker

the monster tach in particular seems even more vulnerable
( with so many variations over the years, its a little wrong to lump them all as one )
- actually a sign the tacho actually has fast response sensitivity
( but unfortunately more susceptible to bell ring and RFI false pulse )

only a few cents worth of components, repro tach filter is $25 US and $15 postage !!!!
( the original discontinued and in huge demand in US and dismantlers know it )

if using polarized one way type caps get them around right !!
( i use unpolarised polyester film type )


ima using similar to this ( 3.3 + 10 + .047mf )
( that's going to filter out signals beyond what a V8 single coil needs cut off , V8 single coils work hard )
[ highest frequency cut, IE softest filter ]

but also i have some fall backs if that not good enough
same at .082mf ( a much lower cut off )
[ lowest frequency cut, IE hardest filter ]

and another at 0.047 and combined 20K ohm
( ranked low, medium and high cuts )
two of them if installed reversed, will slightly alter the cut

- all cuts beyond frequency a rotary leading coil would fire at 10000 rpm with divider factor 2
( wastespark )
( actually up around 22000 don't ask for math, its crude and modified around the practical values of components avail at DSE )

knocking out this obviously bell ring or RFI signals will limit the bounce
( the tacho displays an average voltage, and responds to the high frequency flickers )

going TOO nuts with the resisters, may slow down response
( they are required to stop some surges and things on shut down also )
[ but also have other effect in the calculation, but be fee to alter within reason to trim your filter ]

going less than .047 mf is going to raise the cut off and allow lots of noise through
( factory actually uses similar values to quell alt and old dizzy noises )

some recommend cut offs using .1mf for 4 cyl applications
i would consider them as hard a filter as you would like to try

have fun !