Tachometer wiring help

[Thenewb]

Ok, here's one for all you electrically inclined people. I want to do the wiring for my JLO aircraft engine, which I intend to put on an air boat or an air sled (iv almost completed the air sled) , but Iv got a few questions before I start splicing. 4 wires come out from the stator plate. 2 yellow, both of which should be hot, the black ground used to shut off the engine when grounded, and a blue one that runs up to the first coil which is then wired in series to the other coil so they both go off at the same time since it is a simultaneously firing opposed engine. Now, here come the questions. I have an old Teleflex marine tachometer that has the setting for the appropriate amount of poles/pulses for my engine which is 4 pole. There are three terminals on the gauge, one is ground, one is ignition, and the other is send. The ground obviously gets grounded, but the ignition requires a DC power source, and the send requires a AC power source. SO For the DC could I pass one of my powered yellow wires through a rectifier/regulator to turn the AC current into crude DC to power both my lights AND supply the required DC power for the ignition terminal on the tachometer? Or would I have to use a battery to provide a more steady current for the ignition terminal and use the DC from the regulator/rectifier to power my lights? Secondly, what wire should I attach the "send" terminal on my tachometer to? Could I hook it to the other yellow wire without any kind of regulator to provide the AC input and get a reading of my RPM? Or should I split the blue wire going to the coil and hook that to the terminal? I look forward to your responses.

 

[Silvertip]

Marine tachometers operate from the number of pules from the stator as you apparently are aware. The calibration dial on the back of the tach normally has 4P, 6P, 8P, 10P and 12P. The "P" in that setting is for the number of pulses, not the number of poles on the stator although there is an obvious direct correlation. That means an 8-pole stator produces 4 AC pulses/engine rev. You have a 4 pole stator so it produces only 2 pulses per engine rev. A pulse is a modified sine wave that goes positive for 180 degrees and then negative for 180 degrees. Therefore the number of pulses is 1/2 the number poles for the stator. If you set the tach for 4P, the reading you get would be roughly 1/2 what the engine is actually turning. My thought would be to use a "Tiny Tach" which is standard equipment many types of yard/construction equipment,.

 

[Thenewb]

Yeah, this one has the appropriate 2 pulse setting. So I found an old sheet regarding these tachs, and it says that these tachs won't work on magneto based ignition systems, but I don't see why not. It says that the send terminal hooks to one of the two yellow wires that come off the alternator on the way to the regulator. My stator plate had two yellow wires that come off and then can go to a regulator and is a 12 volt system just the same as an alternator and producing a AC current same as an alternator, so I see absolutely no reason it wouldn't function. Am I missing something?

 

[gm280]

Obviously you would get a much better supply voltage using a rectifier and regulator verses pulsed DC. So the regulator would be the better choice there. As for the pulses needed at the tach to support the internal circuitry, you have to have pulses applied. So what I would try is getting a Tiny Tach and setting it up to read the engine and then use one of the yellow wires to the signal input at the tach and then set the rear switch settings to mimic the Tiny Tach readings. Then there is no misinterpretations with the correct RPMs. JMHO!

 

[Thenewb]

I think I'm just gona use a battery to power the ignition terminal, at least just for testings sake. Do you guys see any reason that it wouldn't be able to pull a reading from one of the yellow wires? I just think its odd that the old paper would say it "will not operate on engines with magneto or lighting coil ignition systems" Or maybe that means something else and I'm thinking of the wrong thing.

 

[Silvertip]

Any electrical device that requires an "input pulse" is designed with a certain waveform in mind. This tach may not have an input circuit that can recognize the approximately 35 volt ac pulses from the JLO magneto. Those engines (I had one on a snowmobile) had no regulator unless the engine was electric start. To avoid the dim headlights at idle and slow speeds that got brighter with engine speed, I built a full wave rectifier into a medication bottle and epoxied it. Added a motorcycle battery and my sled now had nice bright lights all the time. The AC power provided by a car alternator before rectification is a much higher output. In fact a car alternator conversion was used years ago to run a 120 volt electric chain saw or other such device. Just because you have a tach doesn't mean it can be used. Again -- Tiny Tachs are inexpensive and better suited to your application as the sense wire simply wraps around a plug wire.

 

[Thenewb]

I know the tiny tachs would be easier and work well, but Im building this craft mostly for looks, and I really hate how those little digital tachs look hahaha. I suppose what ill have to do is try it out and just see if it works.I would think this tach would stand a chance to pick it up because the document I found says it will work on 6hp 2 strokes (with an alternator) and I just can imagine those put out THAT high of voltage, although Im sure you know more than I.

 

[Silvertip]

The two stroke I'm talking about was a 40 HP JLO in-line twin. Unregulated and unloaded charging coil output was about 35 volts. When loaded the voltage dropped to 13.5 volts at mid/high rpm. For test purposes use a small motorcycle battery or equivalent to supply power and ground to the tach. Ground should connect to both the battery negative and the engine block. I'm not familiar with the engine you have but presence of two yellow wires apparently means the magneto and charge coils are separate although they could possibly be wound on the same cores. So what you may actually have is a two poles for ignition and two for charging. Regardless, if a document you have says the tach won't work on your engine then it apparently won't work. Why it won't work requires investigation and since we can't see your engine I think I've gone as far as I go with this.