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Why Doesn't My Tachometer Work?
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NOTICE:
This article pertains to point-triggered magneto equipped engines that are using an electrically operated tachometer. If you are not using a Vertex or Mallory magneto and/or are not using an electrically operated tachometer [if your tachometer is mechanical and is driven by a cable], then this article is not for you.
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The answer to your problem is that your electronic tachometer was not actually designed to be used with a magneto and because of that it is unable to cope with being bombarded with the extremely powerful electromagnetic pulses emanating from the magneto and the ignition wires. These pulses, in the form of radio waves, are so intense that they are actually drowning out the tiny electronic pulses that your tachometer is trying to read and convert into engine RPM that it can display accurately on a dial and therefore you are getting inaccurate, scattered readings on your tachometer, epically at high RPM. Remember, the faster that you spin a magneto, the more power it creates, and the more pronounced the problem would be. That may be why your tachometer works at idle, but becomes erratic at higher engine speeds.
The way that you can know with certainty that your tachometer was not designed to be used exclusively with a magneto is that it needs 12-volt battery power to operate it. Electronic tachometers that are designed to be used with magnetos have only a single wire, which is connected to the magneto and draw their power from the magneto and do not need 12 volt battery power to operate. This type of tachometer was once available in the market place but manufactures quit making them because they could offer standard battery operated tachometers much more cheaply in that they didn't need to manufacture and stock an entirely separate line of tachometers just for magneto equipped cars.
In order to adapt a standard tachometer to a magneto equipped engine they simply added a transducer, which they call a magneto signal converter, to the system so that it can count the electronic pulses from the magneto and convert them into electronic information that can be displayed on a standard battery operated tachometer. This transducer may be a separate piece or it may be incorporated into the body of the tachometer, but it is still there and it is causing your problems. If, in your application, it is a separate piece, almost certainly, you have mounted the magneto signal converter, near the magneto, probably on the firewall, and it is being soaked in electromagnetism causing it to malfunction or fail completely. If it is built into the tachometer body itself, it may still be close enough to the mag and the ignition wires to be affected by the electromagnetic field coming from them. But even if it is not, the small wires that are routed through the engine area are absolutely being affected.
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Now that you know what the problem is, what can you do about it?
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First, let's look at what you absolutely do not want to do about it.
Often, when describing this problem to others, you will get a standard piece of advice: change the spark plug wires to suppression type wire. Don't do it! Suppression ignition wire will defeat the whole magneto system. If you could see the difference between the spark which is delivered to the spark plug using solid core wire versus suppression wire, you would understand. If you spin a magneto on a test bench with solid wire the spark produced is whitish blue like a bolt of lightning with a bluish corona surrounding it. With suppression wire the spark produced is dramatically weaker appearing red with a purplish red corona surrounding it. It's not a little different; it's a lot different. While it is true that you will stop the problem of the interference from the ignition wires bombarding your tachometer and keeping it from working, you will also stop the engine from running well because you will have greatly reduced the intensity of the spark at the spark plug gap. You will have also put such a load on the magneto coil that its lifespan will be greatly reduced. No mater what anyone tells you; don't install suppression wires on a point-triggered magneto.
So, what options does that leave for solving the problem.
Well, first of all, you could of course eliminate the problem by using a mechanical tachometer. Mechanical tachometers are driven by a cable, which attaches to the body of the magneto. If you have a Vertex or Mallory magneto without the tachometer drive, Don Zig Magnetos can add one to your magneto so that you can use a mechanical tach.
However, if you absolutely have to have an electrical tachometer, because you are using it as a memory device or must have it because you are using it in conjunction with other electronic equipment that sets shift points etcetera, then you are going to have to shield the magneto signal converter (or relocate it), the tachometer, and, perhaps most importantly, all the wires going to and from them, from the electronic field created by the magneto and the solid core ignition wires. This can be accomplished by bundling the wires, where possible, and routing them though shielded conduit and then grounding the conduit. In this way the electromagnetism is carried to ground and eliminated as a source of trouble.
Commercial braided shielding is available at good electronic stores and can be used to shroud the magneto signal converter, the tachometer, and its wires. As a temporary makeshift solution, even a simple aluminum foil shield will help suppress the radiation & noise. In extreme cases, shrouding the ignition wires themselves in grounded braiding may be required to eliminate the problem.
An example of a factory engineered shielding system can be seen on early sixties Corvettes. On those cars, the ignition wires were solid wire Packard 440 which, like all solid core wires, were surrounded by an electromagnetic field. Because the distributor was very close to the dash mounted radio and because the car had a fiberglass, rather than a steel firewall [which would have shielded the radio], there was a terrific radio interference problem, which the engineers had to overcome.
The solution that they developed was to create a formed sheet metal shield which fit over the distributor and the ignition wires and which was grounded to the engine block. This shielding solved the problem completely.
Now that you understand the problem, you can rest assured that you can solve it. It may take some persistence and ingenuity, but it can be solved. Every day piston engine equipped aircraft fly around without having any problem. Every aircraft piston engine has two point-triggered magnetos that give off loads of electromagnetic pulsation. They don't have a problem because all of the radios and sophisticated electronic avionics on board the aircraft are shielded. If they can do it, you can do it.
For a complete technical discussion of electromagnetic interference and how to shield against it: visit the Alpha Wire web site.
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