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Old 05-27-2006, 06:51 PM
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Ahhhhhhhh!

Guys, I have a 69 Camaro with a 350. Has the factory gage package and a repro Dash Tach. I am running a HEI ignition with a MSD module and coil. Problem is the tach pegs out as soon as I turn the ignition on. MSD had sent me a "filter" that didn't work. They sent me a larger "filter" that did work. Tried to fire up the car today and the tach pegged again! This filter goes in the wiring harness between the tach and the tach terminal on the HEI. Has anybody else encounted this problem and if so what was the final solution? Thanks.

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Old 05-27-2006, 07:52 PM
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Doc here,

How is the tach wired?

And are you using a filter/Buffer AND Tach adapter?

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Old 05-28-2006, 10:51 PM
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hey Doc,

I was hoping you would reply. You seem to know all that is electrical. I called Rick's Camaros and they told me I needed a "tach adapter" and Jegs and Summit had them. I called both of those companies and nobody there could tell me exactly what I needed. MSD sent me a "filter" which plugged into the wiring harness between repro factory tach and the tach terminal on HEI. That did not work. MSD then sent me a larger "filter" about the size of a nickel that plugs into the harness the same way. By the way, this is a factory wiring harness. Aren't these "filters" just diodes? Does this really work to tone down the voltage to the tach yet still give an honest reading. I am totally ignorant on this subject so any education you can offer is most welcome.
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Old 05-29-2006, 12:50 AM
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Doc here,

The Tach Adapter will make it report accurately, Some GM's (and as I remember, Corvette and camero specific) Require them with aftermarket Ignitions..else the just report low, or not at all..

The Buffer/Filter IS required to knock down the high voltage spike to the Calibration Board of the tach, and in the case of 1980-to like 84 or 6 with CCC or CCCC confusers, buffered the reference signal back to the computer.

Without the Buffer/Filter, you WILL damage the Tach, and 80 up, damage the computer..This was not an "ADD ON" GM installed them stock in those years for that specific problem.

In their normal habitat, they resided on the left side (passenger side) of the manifold, and had 2 white wires (case of an 80 + with CCC computer, or one white with just a GM Tach) in from the body, and one out to the HEI "Tach" plug. Looks like a Noise cap on a coil, about 1/4 diameter and 2 or 3 inches long.

The Buffer/Filter consists of two resistors in series, totaling around 17 K Ohms, and a 0.1 Bypass cap to ground from the first resistor, and a 0.1 Bypass cap to ground on the second resistor (output) to Shape and buffer the wave.

If you ran any length of time without a buffer , You may have damaged the calibration board..(in case it still won't work after both are installed) If that's the case, and you can't find a suitable stock replacement , but wish to keep the "Stock" look, let me know , I'll tell you how to merge an aftermarket Tach calibration board to your stock meter head. (no one will know but you..)

Sounds like you need both the adapter and filter (which I take it you still have..) so see what you have after the adapter is installed and post back.

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Old 05-29-2006, 09:18 AM
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Thanks Doc,

I do have some more questions. Does the same apply for the "OER" brand of restoration tachs? That is what I am currently running. Is the tach adapter you are speaking of a factory item or aftermarket? If you know of a good source for one I would appreciate the info.

by the way, the car has only been run a couple of times for five minutes as it has open headers. Want to get this tach situation taken care of, tune the car, and then off to the muffler shop.
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Old 05-29-2006, 04:23 PM
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Doc here,

The Tach Adapter is aftermarket only, GM HEI never required them, only the filter / Buffer.

The Best source for the adapter, is your Ignition manufacturer, They usually carry a selection of adapters for the System to A Stock Tach..

A few minutes run time shouldn't have hurt the Tach..So you should be good there. My Experience With Corvette Tach's ( about the same as Camero's of that vintage) That when the filter is bypassed, on a Daily Driver, the Calibration Card lasts about a week..Then not only dies, but causes all kinds of drivability issues.. (shorting through the calibration card.)

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Old 05-30-2006, 07:40 AM
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Thanks Doc. I will be on the horn to MSD and will keep you posted.
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Old 05-30-2006, 12:03 PM
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I am awaiting an answer from MSD but their site does not show a adapter for a HEI distributor alone. All they offer is one that plugs into their boxes. What did the factory do when they installed a tach. I have looked over my wiring diagram schematics and can find nothing. Did they put it all on the back of the tach?
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Old 05-30-2006, 03:37 PM
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Doc here,

Wait..I'm confused here..I thought you said you were running a MSD 6 AL .. Correct or No..?

If not, and is the "Ready to Run" HEI, you are correct, no adapter is availble..Or needed, just be sure the tach is hooked to the - side of the coil.

If that's the case, you have a problem with the tach.

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Old 05-30-2006, 03:48 PM
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I am running a HEI with an INTERNAL Msd coil and module. Some other sources are telling me it getting the right combo of components in the "filter" to make it run right. I hope so.

If not, I have a new in the box Sun tach from about twenty years ago. Perhaps you can tell me how to put the calibration board from that one on to my reproduction tach and do a "stealth" conversion. I don't care how I get it to work as long as the thing works.
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Old 05-31-2006, 10:41 AM
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Called MSD and they are sending two new filters of the same type. They figure the filter I had just went bad. I also asked for a schematic of this filter so I can share it on the site.

As an aside, he told me that Autometer tachs will run with MSD equipment without filters.
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Old 05-31-2006, 04:18 PM
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Doc here,

KEWL, on the filters..Should be two resistors in series and two bypass caps in parallel.

As far as Aftermarket Tach systems, almost all will play with aftermarket Ignitions AND stock HEI , with no Ill effects..Only the early FACTORY Tach's were the problem Children (Even with Stock HEI)..and required filters.

Keep us updated!

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Old 06-22-2006, 08:54 PM
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Update

Well, after three hours wasted splicing and soldering connecting terminals to what MSD sent me, and then finally finding time to actually work on my car, the tach still pegs to the right as soon as the key is on.

I tried to post a couple of pics but the files are too big!

Doc! Help! I would like to e-mail you the pics of these filters and get your opinion. I am ready to switch the circuit board of this OER tach with an autometer tach's board if you are still offering your instructions.
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Old 06-23-2006, 01:27 AM
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Doc here,

Sent via PM...

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