decieded to install an alternator on the race car..used a constant voltage regulator and a factory alt. after installed, charged fine BUT, the tach went crazy!! it now idles from 4000 to 4200 rpm and when you raise the idle it goes outa sight!! someone said to use a "noise filter??" on power line..would that be on the tach line or the power line to the msd??..have seen both described..help!!

A "noise filter" is just a big capacitor.

A cap basically is a filter that a) blocks DC (like from your car's battery) from passing through, b) allows AC (like the electrical noise generated by your alternator) to pass through, and c) will store an amount of DC charge (this is why you don't poke around inside the back of TV sets or tube amplifiers unless you know what you're doing!).

The negative (-) side of the cap connects to ground.

Wire the + terminal of the cap to the output of the alternator, or the BATT terminal of your ignition switch. I don't recommend putting it right on the input of something like the tach or ignition system, because it stores a charge, which can take a couple seconds to bleed down.

I like to see filters "upstream" or "before" the ignition switch, so that if you switch the ignition off, that pesky charge doesn't keep the igntion hot for that extra second or two. (Of course, there may be enough load so that it drains the cap in a fraction of a second, depending on your car's wiring... but I wouldn't want to promise you wouldn't have that momentary 'run on'.)

This way, everything downstream of that filter cap has (most of) the noise bled off the 12V power coming to it.

Crane sells a 16V 38000 uF cap; available online for $35. Mouser sells that same value cap for $19.45 here.

I'd like something with 20+ volts worth of insulation myself; 16V is cutting it kinda' close when a strong alternator output is crowding 15V. I'd expect any cap 22000 uF or more to be sufficient; more doesn't hurt, but costs another buck or two. Here's another.

Hope this helps. If not, PM or email; I'll try to help y'all out.

-Bill

Bigger is not always better for filtering hash. Caps can exhibit what is known as "series inductance" and if the bothersome hash is high enough frequency, what might be more effective is LOWER value disc ceramic caps, with short leads.

This noise can be difficult if it's superinposed on a high-current conductor--like the main output lead. In that case one "help" is to hold the alternator--to battery lead fairly short and use the battery itself as a filter. Something amiss in the alternator can also cause excessive noise/ hash/ ripple---like open or shorted diodes, or open or shorted--or partly shorted--windings

Sometimes a "split ferrite" around the power lead to the tach can help

Welcome to the modern world of electronics--and a high rf noise floor.

had an alt on the car but wouldn't work..got a rebuilt under warrenty and installed..found regulator wasn't working either..replaced with m/p constant voltage reg. and now charges like a bear..but as soon as we installed the new alt,which was tested befi=ore we left the store, it went nuts,,msd said to install a noise filter on the powqer lead for the tach, but someone else said no..thats why i was up in air..lost a meet 1st place because i didn't have time to charge the battery between rounds..hense the alt!!

When I put an alt. in my drag car (1wire) the
shift light would blink on and off and tach read
high at idle like yours.
Moved the alt output to drivers side of engine
away from dist wires and it worked fine.
Good Luck Steve