Tach circut 78'
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Le Mans Master
Joined: Mar 2001
Posts: 6,640
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From: Ann Arbor Michigan
Tach circut 78'
There is a capasitor looking thing in series with the wire that goes from the Tach terminal on the HEI to the tach. What's this for and do I need it? The wire is about ready to break off and I don't see the part in the catalogs.
Le Mans Master
Joined: Jan 2000
Posts: 8,918
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From: New Hill NC
Re: Tach circut 78' (Duke94)
It is the tach filter. Frankly I dont know what the real reason for this device is. I guess it just filters out noises on the signal line. Any way the tach will work without it.
Le Mans Master
Joined: Aug 2000
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From: Semper ubi, sub ubi
Cruise-In VI Veteran
Cruise-In VII Veteran
Re: Tach circut 78' (Duke94)
Do a search for "tachfilter" in the C-3 section and you'll find all the information you could want on them. Under most circumstances, they are necessary for your tach to work correctly. The filter is just a simple band pass type filter that uses capacitance and resistance to smooth the electronic signal. Without the filter, electronic noise will cause spikes that can cause your tach to jump around.
If your wire does break off, don't throw it away. Save the connectors so that if you have to make your own filter you can reuse the connectors/
Here is a schematic of the tachfilter circuitry, the parts can be found at most any electronics parts place, or on the internet. The values for the capacitors and resistors are not really all that important, any value that is close will work. So if you can't find a .10 mfd capacitor, .05 thru .20 will probably work just fine. Same thing for the resistors, if you can't find a 5.6k or 10K resistor, anything in the ballpark will work, just so there is a differential between the 2 resistors.
In fact the circuit itself is probably overdesigned and would probably do the job with only one resistor and one capacitor.
If your wire does break off, don't throw it away. Save the connectors so that if you have to make your own filter you can reuse the connectors/
Here is a schematic of the tachfilter circuitry, the parts can be found at most any electronics parts place, or on the internet. The values for the capacitors and resistors are not really all that important, any value that is close will work. So if you can't find a .10 mfd capacitor, .05 thru .20 will probably work just fine. Same thing for the resistors, if you can't find a 5.6k or 10K resistor, anything in the ballpark will work, just so there is a differential between the 2 resistors.
In fact the circuit itself is probably overdesigned and would probably do the job with only one resistor and one capacitor.
