Tach Filter

[js292]

Anyone have a good source for a tach filter? Wilcox is the cheapest I found @ $37, still a little steep for what it is.

 

[mrvette]

5 bux worth of Rat shack parts, or I can send you them, as I have plenty....but have to look around....make y our own.....but it's not OEM, it's what's on my car for 15 years now...

:friends:

 

[DWncchs]

If your talking about the little silver thing the tach wire plugs into that bolts to the intake , I was told years ago to throw that piece of crap away and plug the tach wire direct into the distributor. The tach "board" in the dash has a filter built into it.

 

[js292]

Thanks guys, yeah I've also heard about just by-passing it.

 

[Artsvettes]

I have used these with good results

http://www.egauges.com/vdo_send.asp?Sender=Classic_Tach_Filter

 

[mrvette]

If your talking about the little silver thing the tach wire plugs into that bolts to the intake , I was told years ago to throw that piece of crap away and plug the tach wire direct into the distributor. The tach "board" in the dash has a filter built into it.

Hate to say, but a 'stock' GM tach of the shark era needs that filter, new stuff don't....

the old tachs will read funny and do it at various RPM's due to that filter is a pulse integrator and so takes the spikes off the tach output signal....it was done at the engine so AM radios didn't have such a HUGE buzz signal...same as the silly chrome on a breaker point ignition, shielding...not a issue with a steel car,....

the thing about AM stereo was to put the bass buzz in the left channel and the HI freq buzz in the right channel....

:amazed::hissyfit::thumbs:

 

[DWncchs]

I've never seen a tach read funny from bypassing what everybody calls the "tach filter". In this diagram its referred to as a radio noise capacitor and its on the battery wire not the tach wire. I will add that back in 79 or 80 I was told by the guy I bought my Chevy parts from to get rid of it or it might damage the tach board. I bypassed all of my customers cars and I have never replaced a tach board in a car that I bypassed. I've only replaced 2 bad tachs ever and both had the filter plugged into the tach wire. One other thing , John from Redline Gauge also says get rid of them.

 

[mrvette]

I've never seen a tach read funny from bypassing what everybody calls the "tach filter". In this diagram its referred to as a radio noise capacitor and its on the battery wire not the tach wire. I will add that back in 79 or 80 I was told by the guy I bought my Chevy parts from to get rid of it or it might damage the tach board. I bypassed all of my customers cars and I have never replaced a tach board in a car that I bypassed. I've only replaced 2 bad tachs ever and both had the filter plugged into the tach wire. One other thing , John from Redline Gauge also says get rid of them.

the middle of your diagram there is a radio cap to ground off the B+ to the coil....fine...

but then on the dizzy cap out to TACH....white wire is what lead to that filter I talking about...it's not a power thing, a SIGNAL from the 'points' side of the coil to the tach...the other end of the primary coil.....

and that used to plug into that manifold mounted custom made 'filter' or what I refer to as a pulse integrator....


:friends:

 

[big2bird]

I've never seen a tach read funny from bypassing what everybody calls the "tach filter". In this diagram its referred to as a radio noise capacitor and its on the battery wire not the tach wire. I will add that back in 79 or 80 I was told by the guy I bought my Chevy parts from to get rid of it or it might damage the tach board. I bypassed all of my customers cars and I have never replaced a tach board in a car that I bypassed. I've only replaced 2 bad tachs ever and both had the filter plugged into the tach wire. One other thing , John from Redline Gauge also says get rid of them.

the middle of your diagram there is a radio cap to ground off the B+ to the coil....fine...

but then on the dizzy cap out to TACH....white wire is what lead to that filter I talking about...it's not a power thing, a SIGNAL from the 'points' side of the coil to the tach...the other end of the primary coil.....

and that used to plug into that manifold mounted custom made 'filter' or what I refer to as a pulse integrator....


:friends:

Yep. Sorry Roger, the tach filter is NOT shown in your diagram. It's on the WHITE lead. BTW, the filter is an "RC" circuit.

 

[big2bird]

Anyone have a good source for a tach filter? Wilcox is the cheapest I found @ $37, still a little steep for what it is.

Ecklers?? has a cheaper non-stock filter. You CAN make your own, but I forget the values. It's a simple "RC" circuit.

 

[DWncchs]

I've never seen a tach read funny from bypassing what everybody calls the "tach filter". In this diagram its referred to as a radio noise capacitor and its on the battery wire not the tach wire. I will add that back in 79 or 80 I was told by the guy I bought my Chevy parts from to get rid of it or it might damage the tach board. I bypassed all of my customers cars and I have never replaced a tach board in a car that I bypassed. I've only replaced 2 bad tachs ever and both had the filter plugged into the tach wire. One other thing , John from Redline Gauge also says get rid of them.

the middle of your diagram there is a radio cap to ground off the B+ to the coil....fine...

but then on the dizzy cap out to TACH....white wire is what lead to that filter I talking about...it's not a power thing, a SIGNAL from the 'points' side of the coil to the tach...the other end of the primary coil.....

and that used to plug into that manifold mounted custom made 'filter' or what I refer to as a pulse integrator....


:friends:

Yep. Sorry Roger, the tach filter is NOT shown in your diagram. It's on the WHITE lead. BTW, the filter is an "RC" circuit.

Are you sure the radio noise capacitor shown on the Bat wire is not the one we are referring to as the tach filter ?

Think about it there is no cap on the bat wire in the actual car. Where is the cap shown on this diagram ?

Is it possible the "tach filter" (as we refer to it)was supposed to be installed on the bat wire and not the tach wire ?

 

[mrvette]

I've never seen a tach read funny from bypassing what everybody calls the "tach filter". In this diagram its referred to as a radio noise capacitor and its on the battery wire not the tach wire. I will add that back in 79 or 80 I was told by the guy I bought my Chevy parts from to get rid of it or it might damage the tach board. I bypassed all of my customers cars and I have never replaced a tach board in a car that I bypassed. I've only replaced 2 bad tachs ever and both had the filter plugged into the tach wire. One other thing , John from Redline Gauge also says get rid of them.

the middle of your diagram there is a radio cap to ground off the B+ to the coil....fine...

but then on the dizzy cap out to TACH....white wire is what lead to that filter I talking about...it's not a power thing, a SIGNAL from the 'points' side of the coil to the tach...the other end of the primary coil.....

and that used to plug into that manifold mounted custom made 'filter' or what I refer to as a pulse integrator....


:friends:

Yep. Sorry Roger, the tach filter is NOT shown in your diagram. It's on the WHITE lead. BTW, the filter is an "RC" circuit.

Are you sure the radio noise capacitor shown on the Bat wire is not the one we are referring to as the tach filter ?

Think about it there is no cap on the bat wire in the actual car. Where is the cap shown on this diagram ?

Is it possible the "tach filter" (as we refer to it)was supposed to be installed on the bat wire and not the tach wire ?

I am unsure about that cap shown above, I don't recall ever seeing one in a actual HEI install, OEM or aftermarket...just a heavy ~10 ga pink wire to the ignition ckt the TACH filter is a little black crapassitor looking thing about 5/8 diameter and 2" long with white leads coming out the ends that plug into the tach slot on the dizzy, and then the wiring harness, it's ground is done by the bolt to the intake manifold and clip on the cap.....

I have a published schematic from y2k yet, but wife is at work, so I will get it scanned for here later on....

the description is.....tach out from dizzy goes through two resistors to the harness....first one is 5.6k ohms resistance, then a .1 mfd capacitor to ground, from that point another resistor/mentioned above, is ten K ohms with 100 pf cap to ground, from that tie point it goes to the harness.....

in electronics, as mentioned above it's called a PIE network, or pulse integrator, what is does is take out the spikes and unnecessary signals from the spark coil, done that way because vettes are plastic, otherwise it's done at the tach itself....as explained above in another post...

I stay away from wife's computer, I"m still alive....:thumbs::rofl::hunter:

 

[Artsvettes]

That drawing is pretty generic it does not show the tach filter at all. And I have never seen a cap. in the B+ wire

 

[js292]

Good info guys, I had a generic understanding what it was for. It's nice to get this much detailed info.

 

[mrvette]

Finally dumped about a meg worth of stupid file size....here it is....

:shocking:

 

[big2bird]

Are you sure the radio noise capacitor shown on the Bat wire is not the one we are referring to as the tach filter ?Positive

Think about it there is no cap on the bat wire in the actual car. Where is the cap shown on this diagram ?Haven't a clue.

Is it possible the "tach filter" (as we refer to it)was supposed to be installed on the bat wire and not the tach wire ?

Not a chance

 

[big2bird]

Finally dumped about a meg worth of stupid file size....here it is....

:shocking:

There ya go. But that stuff at an electronics store for $3.00, solder it up, and stuff it in a can with epoxy, done.

 

[mrvette]

Finally dumped about a meg worth of stupid file size....here it is....

:shocking:

There ya go. But that stuff at an electronics store for $3.00, solder it up, and stuff it in a can with epoxy, done.

I used RTV....:shocking:

 

[DWncchs]

That drawing is pretty generic it does not show the tach filter at all. And I have never seen a cap. in the B+ wire

That drawing is from the 80 Corvette Electrical Troubleshooting Manual , Ignition.
Here is a link to a 77 Corvette manual. Same diagram. http://www.europeancorvette.com/photogallery/Electrical Troubleshooting Manual 77/Page009.jpg

 

[DWncchs]

Are you sure the radio noise capacitor shown on the Bat wire is not the one we are referring to as the tach filter ?Positive

Think about it there is no cap on the bat wire in the actual car. Where is the cap shown on this diagram ?Haven't a clue.

Is it possible the "tach filter" (as we refer to it)was supposed to be installed on the bat wire and not the tach wire ?

Not a chance

Why ? In the GM manual the picture of what WE are referring to as the "tach filter" is actual called the "radio noise capacitor" , same as what is in the drawing above.Here is the matching page for the diagram.