[melsman]

I figured there were just not enough threads on here about tach filters...

Anyway, after doing numerous searches on using my new OER tic-toc-tach with my GM HEI ignition, I decided I should play it safe and build a filter to place in-line between the "tach" terminal on the HEI and the tach itself.

I read JimM's excellent work on the his tach filter and I also read about other folks simply placing a rectifier diode in-line; both with varyin degrees of success. After studing the design a little bit and exchanging a couple of messages with JimM, I decided to try and combine both concepts - mostly because both concepts do different things. And I'm very pleased with the results - accurate rpm readings and a dead-steady tach needle.

As many of you have correctly pointed out, the tach pulse from the HEI tach terminal is an A/C signal that is not very clean. It consists of a 12v A/C pulse, a lot of trash frequencies, and the occasional strong negative "fly-back" pulse. The resistor/capacitor portion of the filter eliminates the trash signals from the pulse and the diode clips the negative side of the pulse - including the large negative fly-back pulses - so that the tach sees only a clean positive pulse.

First off - let me take a moment to recognize the work JimM put in to figuring out the proper capacitance and resistance necessary to make the first stage of the filter work. The resistor is connected in front of the capacitor so that the trash signals decide to go through the capacitor to ground, rather than through the tach. Note also that the diode has the cathode band on the tach side.

Parts used (and Radio Shack number) -
- 1 uf electrolytic capacitor, non-polarized (272-996)
- 270 ohm resistor, 1/2 watt (271-1112)
- rectifier diode, 400v, 30A (IN4004)
- Male and female crimp connectors, two sets
- 6" 3/16" heat shrink, and 3" 3/8" heat shrink
- ground wire with terminal connector

So - here's how I built mine. (This would be a lot easier (and shorter) if I could post my pictures, but I've already used my measly 125kb of "attachment space" for my speedometer post.)

First, I twisted the leads together for the resistor and the capacitor. Then, I twisted together the other resistor lead and the non-banded lead of the diode. Then, I twisted the other lead of the capacitor to the bare end of the ground wire. each component in preparation for soldering. Be sure to add 3" of the 3/16 heat shrink to the ground wire before soldering to the capacitor, and double check that the banded end of the diode is NOT connected to the resistor.

Time to solder all of those twists together. Now, add the other piece of 3/16" heat shrink to the diode/resistor leg, and slide the heat shirk on the ground wire up to cover the solder joint between the ground wire and capacitor lead. Apply heat to shrink.

Now - use a meter to check that the components are connected well. Remember - the negative test lead goes to the banded end (tach side) of the filter.

Time to crimp on the connectors. Use the smaller female connector on the resistor capacitor end, and the larger male connector on the diode (tach side) end. This is the hardest step because crimping doesn't give a real good connection. If your filter doesn't work when everything is complete, it'll probably be because one of the crimps is bad. Be sure to test resistance through the filter once again with the multimeter when the ends are crimped on. (Actually, I recommend stripping the insulation from the connector crimp-area and solder the leads into the connectors)

Once everything checks out good with the meter, time to add the 3/8" shrink wrap to cover the filter from one connector to the other.

Install the filter by cutting the tach signal wire as close to the tach as you can get it (to ensure thaqt the filter removes any stray signals that might jump on board between the HEI and the tach). Crimp the opposite connectors to the tach wire and install the filter in-line. Now, connect the ground wire to a convenient screw or bolt.

Give'er a shot. It should work well for you.

Melsman

[melsman]

... added a picture that shows how the components are wired together before soldering.

[JimM]

I like it!

Gonna have to add that to mine!

[69Z28]

Hi. I was curious to know if this is the case for all the tach's including the reproductions, aftermarkets. When I first bought my car it had an old repro tach with a plastic back plate with what appeared to be vinyl printing and silver tape around the tach face. I had to use a filter on that with a small body HEI I had done years ago, but I since replaced it with a repro, I didn't need the filter and using the small body HEI it appears to work great with a nice steady needle. I thought I read somewhere that the repros will work both ways, points or HEI.

[dnult]

I'm not sure how the repros are circuited internally. I can tell you that the OEM design was more or less a tuned circuit that was specifically designed to work with the signal off a breaker point ignition. HEI presented a different signal that would upset the tuning an could damage the relatively low-tech internal components. If you read the stories of OEM tach users with HEI you'll find it's a mixed bag. Seems like 1/2 work fine and the other half don't

I would expect a modern replacement tach to have better signal conditioning than the OEM unit. Many modern tach circuits use an Integrated Circuit to do the frequency to voltage conversion. But beyond this basic hunch, I can't say for sure. Hopefully the repros didn't just clone the original circuit design.