Interesting side note on HEIs and tach filters - Team Camaro Tech
Electrical & Wiring Troubleshooting electrical.

 
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post #1 of 6 (permalink) Old Mar 8th, 04, 03:51 PM Thread Starter
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I know some of us have had issues with tachs not working correctly with HEIs and filters seem to solve the problem.

I had a problem with the HEI on my wife's 69 that caused my tach needle to bounce eractically until about 3000 then smooth as silk!

Well, I had the Accel Super Coil installed from the get-go and just recently added a new Accel module to go with. The tach now works correctly?

So, it is dependent on the module you have for the tach working correctly! I haven't dissected one of these, but sure would like a schematic of a plain jane rebuilt one vs the Accel one I have now.

By the way....the Accel Super coil and hipo module produces a smoother idle and I can even get a lower idle out of my XE274 cammed 10.5 CR, iffy Road Demon carbed 357 than I could before?

Steve "Jack'stands" Jack

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post #2 of 6 (permalink) Old Mar 9th, 04, 01:24 PM
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Here is an email I sent to a fellow Team Camaro member that might explain some of the mystery behind the tach filter issues.

Quote:
In general, here is an overview of what is going on. The hot side of the coil is always at 12volts for HEI or about 9 volts in the case of a breaker point ignition system with a ballast resistor or resistor wire.

The negative side of the coil is in common with the negative end of the high voltage circuit. The breaker points or ignition module connect the negative terminal of the coil to ground briefly to build up a magnetic field in the coil. The time the negative terminal is shorted to ground is known as the dwell which you may have heard about before. Then, at the moment the plug is supposed to fire, the points open or the ignition module disconnects the negative terminal which results in a high voltage spark. In addition to a spark on the high voltage lead, the voltage on the negative terminal will ring. In fact, when the points open, the voltage on the negative terminal will ring like a bell going as much as a few hundred volts positive and negative just like a spring vibrating and eventually coming to rest at 12 volts again. This is why you have to be careful with a DVM. Those voltage spikes can exceed the meter rating and blow the input circuit. They can also mess with your tach which brings me to the next topic.

An analog tach relies on this ringing voltage to trigger it's circuitry. But there are two problems that tach filters are meant to solve. First, if the ringing is too aggressive, it can cause incorrect readings. This is often the case with an HEI or MSD system which generates much higher voltages and much more ringing on the negative terminal. That is where the filter comes in. It reduces some of the ringing on the signal reaching the tach input lead.

The other problem with an analog tach is that the voltage spikes can be too high. If so, they can cause diodes in the tach to break down and let current flow in the opposite direction. Diodes are like electronic check valves that are supposed to only let current flow one way. But if you put too much voltage across them in the reverse direction, they'll give up and conduct when they're not supposed to. This maybe what is happening in your case. I think it is this high voltage breakdown problem that causes the jumpiness in the tach reading on HEI and MSD ignition systems. Well the tach filter helps with this problem also by attenuating or reducing the input signal to a safe level. So the tach filter does two things. It reduces some of the ringing and attenuates the signal to a safe level.

When I spoke about an adjustable filter, what the adjustment does is allow you to set the level where the tach works the best. Too much attenuation and the tach doesn't work at all. Too little and the high voltage spikes cause the diodes to break down and mess up the reading. Think of it as a squelch control on a CB radio (if you remember those). The best squelch setting eliminates the noise except when someone is talking.
Here is a design for a universal tach filter. Create a T out of a 3.3K ohm resistor, a 0.1uF capacitor and a 20K ohm potentiometer. The 3.3K ohm resistor must be 1/2 Watt. The coil should connect to the 3.3K ohm resistor. The other end of the capacitor should connect to ground. The wiper on the potentiometer should connect to the tach. The right hand terminal of the potentiometer should connected to the T.

Turn the potentiometer fully counter clockwise and start the motor. Then turn the pot clockwise with the motor running until the tach begins to function. Take it for a drive and tweak the potentiometer until it works reliably. If you find it is still irratic, add a 0.01uF capacitor between the tach line and ground. That should work for just about every application.

Dave
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68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #3 of 6 (permalink) Old Mar 9th, 04, 02:02 PM
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HI Dave,

could you draw a quick picture of how the componants go togeather for us imagination challanged people with tach problems please, If you wish you could hand draw one and fax or email it to me if this is easier.

thanks alot.

Dan

68 Camaro 383, afr, roller cc, 700-R4, 12 bolt, etc.
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post #4 of 6 (permalink) Old Mar 9th, 04, 02:57 PM Thread Starter
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DAve, I completely agree with your post and understand the ergonomics of the tach circuit and what the HEIs generate. Simply put, it's the noise level that is causing all the ruckus!

The profound curiosity here is with the hipo Accel module tho? I would think that a module that actually works better with the Accel super coil (my idle is better and high end seems a tad stronger...so I think it's producing a pretty fat spark) would generate like "ringing" in the circuit too. Perhaps the signaling to the coil is so different that the ringing is different. I should like to see it on a scope to compare to the cheap stock modules.

I would encourage anyone that has a tach issue with an HEI to try a different module first. That may fix it? Then again.....

Steve "Jack'stands" Jack

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post #5 of 6 (permalink) Old Mar 9th, 04, 03:08 PM
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I can imagine how the module affects the signal. That would be an interesting investigation - how are the modules different and which one is better.

But it comes down to this. The circuit in the tach isn't high tech at all. I'm sure the old-time GM engineers fiddled with part values until they got something to work. Change the coil, the ignition module, etc. and you've got to recalibrate the thing. So I'd say you got lucky in that the module / coil combination worked. But I'm sure others can take advantage of your experience in that you have found a combination that works.

I have drawn a simple bitmap schematic. Unfortunately I have no place to post it. Dan, you have mail.

Dave
========================
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #6 of 6 (permalink) Old Mar 10th, 04, 02:48 AM
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Angry

Thanks for the help Dave, perhaps this year I will have a working tach! Yahooo


d

68 Camaro 383, afr, roller cc, 700-R4, 12 bolt, etc.
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