jcb999

any suggestions? I got a solid state handheld dwell-tach hooked up to the same wire (brown tach feed) and it reads half.

http://www.holley.com/HiOctn/ProdLin...C/800-100.html

implies that it produces a 12volt signwave.

Rustydawg

I took a look at the Holley article and it said "Connects easily to any electric tachometer that uses a 12 volt square wave signal".

A factory HEI ignition produces a short pulse when the module fires the coil. The Holley box is outputting a square wave which (technically) has a duty cycle of 50%, the duty cycle of the signal from the HEI is much less than this.

I think the tach is sensitive to the duty cycle of the signal. Is it the factory gauge or an aftermarket?

Recently I was checking a bagful of tachometers ranging from new to ancient and some are horribly sensitive to this duty cycle, or dwell.

jcb999

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Rustydawg

Putting a capacitor in series with the Holley tach output and your factory tach might clear up the problem, say 0.05uF or 0.1uF @ 1000 Volts. Radio Shack or an electronics outlet should be able to cough something like this up. It will pass the 'kick' of the ignition firing, but not the entire length of the pulse.

It's cheap to try anyway. Maybe Holley has some answers.

jcb999

but i might try that

thanks

jcb999

and ofcourse.. holley never replied.. any other ideas?

Vader

Perhaps the line on their web page...

"Multiple Sparks - Each spark plug is automatically fired multiple times from idle to 3,000 RPM for 22½° of crankshaft rotation."

...has a little to do with the problem.

Holley? NOT answering a technical question? I'm friggin' SHOCKED! Whay do you think I hammer on them any chance I get, and stopped using their shìtty examples of fuel management systems in the early-mid '70s? I don't expect their other products to be a lot different, either. Too bad Comp Cams doesn't make the parts you need, or you would have been able to call them for free and talk to a real, knowledgable person.

Then again, most serious racers use Jacbos ignition boxes....

That's not a slam on your decision to get one - I'm sure you expected it to work without a hitch, as would I.

As for the immediate problem, you may need to breadboard a 555 IC and set a minimum threshold time with the capacitor selection. And since it is a CD box, you'll probably have to use a 741 IC with an inverted input (output of th e555) to correct the phasing of the signal to the tach. You can run the whole thing at 12V and tape or silicone-pot it into a small project box. The factory tach filter isn't going to help you much with the CD output, since it operates just exactly opposite of the theory of a standard Kettering ignition cycle.

jcb999

I got a response back from holley.. (they said that their mail server had crashed and lost my original note).
They say the condition is comming from Ignition RF. And.. I do have jacobs solid core wires..

Guess I need to find a suppressor for the tach.

Rustydawg

Yeah, I've had trouble with solid core wires too. I ran 'em on a four cylinder for a while and it set my radar detector off continuously. Guess they radiate like crazy, especially if your tach is picking up the additional sparks and counting 'em.

I don't know how hard it would be to replace the tach lead with a shielded piece of coaxial cable but that would screen the tach lead from noise coming from your ignition wires. Just ground one end of the shield, say at the Holley box end.

I think the hard part would be getting to the back of the tach where the brown lead goes right now, but shielded cable should be very effective.

jcb999

http://store.summitracing.com/partdetail.asp?part=9162

If you experience radio noise after installing your MSD unit then you may need to install this noise filter. With this filter in place the MSD unit will draw through the noise filter instead of from the battery allowing all 12 volt accessories to not be affected. The noise filter comes with a cover.

Sounds like it should go in the hot line for the ignition box.

Rustydawg

I took a look at that link, the filter is a large capacitor and goes in parallel (ground one side, battery on the other) with the power leads to the MSD or Holley ignition box. It is the same thing that guys with huge amplifiers use in parallel with the battery.

Installing the capacitor can't hurt but I don't see it making much difference - especially if your wiring is in good condition with clean, solid connections and heavy gauge wire.

Do you have a set of stocker-type or helical wires to try? If they fix or reduce the problem then it is radiated ignition noise.

Does the Holley box run a heavy RED lead direct to the battery and a heavy BLACK lead to ground like a MSD box, using a separate wire to turn the system on? If so, it is important to run them straight to the battery with good, solid connections.

jcb999

No,, not a good set (burned up a set of taylor 409s is all)

Quote:

Does the Holley box run a heavy RED lead direct to the battery and a heavy BLACK lead to ground like a MSD box, using a separate wire to turn the system on? If so, it is important to run them straight to the battery with good, solid connections.

yep two separate pairs of lead wires.

Holley replied that they thought that would fix it.. but at 30bucks,, really want to be pretty sure.

Rustydawg

30 bucks sounds like a lot for a capacitor, unless there is something else that I didn't catch.

You could try phoning around or dropping in to some electronics places and ask for a 200 000 microfarad capacitor good for 15 volts. If they have something larger than 200 000, go for it (if you have space for it), but don't go much smaller. The voltage rating isn't important as long as it is 15 or better. It should have the heavy-duty screw mount terminals just like the one in the pic.

Sometimes they call them 'computer capacitors' too. If you know of an electronics surplus place you could likely find one for about 5 bucks.

Rustydawg

jcb999

I'm curious, have you had any luck fixing this problem yet or is the car parked for winter?