Tach electrical isolation, tips and tricks?

Discussion in '4-Stroke Engines' started by bigoilbob, May 31, 2012.

  1. bigoilbob

    bigoilbob Member

    This must be a topic of general interest. Mini tachs will go wild if the wire from the tach is not properly isolated from the frame metal. Not just from direct contact, but even the proximity. Mine is particularly long (~11') because I have a recumbent tandem with a GX35 way in the back. I have tried running it inside a slim rubber tube, which is in turn in my frame. Any other ideas? I would really like a tach to monitor my Nuvinci Developers Kit app.

    Thanks all;

    Bob D.

  2. Happy Valley

    Happy Valley Active Member

    A 2-wire tach is best, one lead to the plug and the other a ground to the engine.
    I've found the inexpensive 1 wire tachs are unstable.
  3. bigoilbob

    bigoilbob Member

    Thx Happy Valley

    I'll look myself, but any specific recommendations? Just a (probably needless) reminder, my GX35 is single cylinder, 4 stroke.
  4. Happy Valley

    Happy Valley Active Member

  5. MikeJ

    MikeJ Member

    Hi Bob and All -

    I have some long-winded input for you. That pesky rotating magnetic field flying around the engine as the magneto turns affected me the same way as it did you and a few other writers who want to place electronic sensors on their motorized bikes. I had a SenDEC Tach that worked fine on my HT engine, but became useless on my Honda 4-stroke in-frame engine.

    The answer to my problem was a six-foot length of RG-58 coax cable from Radio Shack for $8. 12-foot cables are available for less than $13.

    Without getting too boring with details, I built for myself a light-pulse-sensing circuit and fed the output pulses of that into an analog designed-for-automobiles tachometer (a relatively inexpensive one). My first build had the needle swinging wildly all over the tachometer face. (The circuit and tachometer run off a 12 volt sealed lead acid battery. Both have a positive lead, a negative ground lead, and a third wire, the pulse signal wire, out of the sensor circuit going into the tachometer.)

    I think the rotating magnetic field from the rotating magneto was playing havoc with creating unwanted signals going into the tach; the magnetos are not shielded. To minimize the unwanted signals on the sensor wire, I bought a six-foot length of RG-58 cable. I soldered ground wires to both ends of the braided sheath and soldered the sensor wires to the center lead. The sensor signal now cannot be affected by the rotating magnetic field. I did not bother shielding the positive power wire; I thought it was unnecessary for reasons I can explain in detail if someone is interested.

    The radio frequency characteristics of the coax cable seems to have cleaned up digital pulses out of my sensor as well as keep interference off of the sensor wire. I kept unshielded sensor wire as short as I could. Six feet of coax may be unnecessarily long. When I soldered the components together and saw a huge initial improvement, I just kept it and tie-wrapped the excess cable to the bike frame.

    I wonder if you used a length of coax cable center for your tach signal around the spark plug wire and grounded both ends of the braided sheath to your engine, that would be useful. You would probably have to solder a short stub of tach cable on to the other end of the coax center. If you try that, let us know what the results are. (An afterthought: grounding only at the engine may be the only one needed; try both options and let us know.)

    Today, I took the bike and analog tach circuit to the road. It works almost perfectly. The needle still swings back and forth across a 200 rpm range, but considering the tach mechanics were NOT made for the severely vibrating handlebars of an engined bicycle, I can overlook that. I limit my engine speed to 4000 rpm indicated.

    That mechanical tach and the sensing circuit both take a severe beating when I ride. The handlebars are anything but rock solid steady. On the way back today, almost home, the tach needle stopped working 60 miles into the ride (I rode 65 miles today). Analysis showed that a solder joint wire broke loose from the solder. Physical vibration broke the copper ground wire from the sensor circuit. I disassembled and re-soldered the ground back in place. Vibration will be the death of the sensor circuit; I expect this to happen again.

    The tachometer needle now also reads 300 rpm higher than when brand-new. I used florescent lights to calibrate the tach at 3600 rpm. The needle reads 3900 rpm. I can mentally compensate for the higher reading. This tach circuit is an inexpensive answer for my "I want one" mentality. A dead-on accurate analog tachometer circuit is out of my financial realm. If I needed one, somebody else would have to pay for it.

    Attached should be some photos of the sensor and the tach mounting. As the crankshaft turns, pulses are read by the photocell (LED and photocell and all other parts are also from Radio Shack.) It was a bit tricky getting circuit resistance values just right. Physically mounting the circuit took me a few days to work that out. The circuit container is a camera film canister. (When was the last time you saw one of those?) A slot in the lid keeps out some unwanted light; too much solid-intensity light affects the circuit output. The tachometer mount is a piece of wall paneling that I covered with electrical tape. This was quick and easy. An expensive mount would not have been any better.

    The first photo shows the circuit canister mounted next to the MaxTorque clutch.

    Te second photo show the broken ground wire I had to repair.

    The third photo shows the circuit I had to assemble on the very small circuit board.

    The fourth photo shows the size of the circuit next to a common house key.

    The last photo shows the analog tachometer mounted in place. Also seen are the solid-state speedometer computer, two headlights and the marine horn.

    Attached Files:

    Last edited: Jul 29, 2012
  6. Old Bob

    Old Bob Member

    I use an OPPAMA PET-2000DXR Tach connected to the ground lug on the coil not the high tension lead. It triggers perfectly and does not have high voltage issues.There is only a couple volts on the ground.

    The Sendec unit can be connected the same way as outline in Figure 4 here:


    Try this and see if it works better.
  7. MikeJ

    MikeJ Member

    Hi Bob -

    I removed my optical tach from my bike. It was a cheap experiment that I always wanted to try. It did not live up to my expectations. The entire circuit is still in one piece in a box on a shelf. It still works; I will use it for something else in the future.

    I read the literature on your PET-2000DXR and the SecDEC tachs again. I read someplace to not wrap the wire around the frame too much. So I dug up my old SenDEC tach and loosely draped it on the frame from handlebar to spark plug wire and wrapped the pickup wire around the spark plug wire four times. Then I took my bike out for another 25 miles. Running my engine on both tachs, they both tracked rpm changes and values in sync. In the past, the SenDEC wire was tightly wrapped around the frame. It did not work. Today, it was almost sloppily thrown on and the tach seemed to work just fine; there was no bouncing from high reading to low reading to high reading and so on. Live and learn.....

    Though any tach is optional eye candy to me, I will keep the SenDEC in front of me as long as it looks like it is working. Thanks for pointing out your sources.

    Last edited: Jul 29, 2012
  8. bigoilbob

    bigoilbob Member

    Thanks to all, appreciate the time and effort. I think I'll try the Sendek. I caught some work in Saudi (in Ramedan no less!) but I'll be home in less than 2 weeks and will change out then. I had one on my paramotor, but it seems to have timed out. Not tach smart - do they just put a "life of the tach" lithium battery in them, and you throw it all the way when it gives up?
  9. loquin

    loquin Active Member

    Also, I would recommend, if you are using coax cable for the Tach signal, that you do NOT ground both ends of the coax cable shield. If you do, you introduce a ground loop into your wiring, which can introduce a completely different source of signal noise.

    Possibly, just routing the sensor cable away from the flywheel at 90 degrees (as far as possible) before routing it towards the tach electronics would help.
  10. Old Bob

    Old Bob Member

    Unfortunatly thats the case. but if it was me I would take it apart and see if I could find battery for it.Its already not working you aren't going to hurt it.
  11. MikeJ

    MikeJ Member

    Loquin -

    I thought about that ground loop issue as I was writing. I need to try this for myself, now that I have almost six feet of RG-58 laying in a box someplace. Routing that initial length of spark plug wire could be significant. I remember keeping the sensor wire short and perpendicular to the spark plug wire on my first tach'd 2-stroke. On my Honda 4-stroke, I can clearly see exposed flywheel and magneto next to the cylinder.

    If the SenDEC works, I may start looking for an upscale tach, maybe one with half-inch high numbers, maybe an LED graphing bar (more rpm equals more bars). Maybe some supplier will sell me "just one". They would rather sell hundreds in a single sale......

  12. Old Bob

    Old Bob Member

    If you hook to the ground lug on the coil the problem is non existant.

    Just stay away from the Trail Tech brand, they don't work correctly no matter what you do.
  13. MikeJ

    MikeJ Member

    Old Bob -

    I will look into your recommendation; obviously it works for you. You have one model of engine and I have a different one. And your tachometer make and model are different. The coil is probably in the same relative position; I have to look. If it does not require too much dissassembly, I will try your way after I try the spark plug wrap. Every engine has an easy-to-get-to spark plug wire. I will try the easy way first. . . .
  14. Old Bob

    Old Bob Member

    All Honda GX 35 engines have the same coil,in the same position... I've used the Sendec in the past and the manufacturer says it works... One cap screw and the red cover is off, crimp a spade connector on the end of the wire slide a double male piggy back connctor on the ground lug slide the tach spade connector on and slide your kill switch wire onto it.
  15. MikeJ

    MikeJ Member

    Hi Old Bob -

    I believe most of the wiring connection you just described has already been done for me. I did not state so earlier in this thread, but my engine is the Honda GXH-160, a 163 cc industrial engine. The factory-installed kill switch is on the outside of the starter housing with a wire from it heading into the engine to the coil. If your tach spade connector and the kill switch wire meet at the same point electrically, I have the same point readily available to me in the form of an external wire with exposed connector. I don't have to open my engine. For testing, I can solder an alligator clip to the SenDEC wire and clip it to the exposed connector. I will try that. If that works, I can make it more permanent with a better-fitting connector.

    Thanks for writing,
  16. MikeJ

    MikeJ Member

    To sum up my research for tachometers, here is a list of options:

    Don't use mechanical tachs from the auto store; engine vibration can tear them and sender circuitry to pieces.
    Use only solid-state tachs that utilize inductive spark plug wire pickup. Larger numerals are more expensive (of course).
    Try attaching to a point directly off the coil as described in detail in a previous post.
    Make a shielded signal wire with RG-58 cable from Radio Shack or old CB antenna; extend the copper braid to the engine on one end only.
    Purchase a tach with signal wire and ground wire (little more expensive)
    Use a layer or two of electrical tape to become the signal wire insulation if necessary, maybe liquid insulation.
    Keep signal lead short as possible, coming off of spark plug wire perpendicular to wire.
    Use a resistor spark plug if one is made for your engine; is supposed to minimize interference problems.
    One writer from long ago described how he used toroid chokes around the signal wire to suppress interference.

    I found the Koso North America Mini Tachometer interesting (replaceable battery, shielded lead). Read the .pdf file.
    Go to www. denniskirk. com/ mini-tachometer.p408812.prd (spaces are intentional).

    Post "Lessons Learned" for others to read. . . . .
  17. MikeJ

    MikeJ Member

    Hey Old Bob -

    IT WORKED! I did as you said and the SenDEC tach readout is rock solid with engine RPM. I checked readout against the engagement speed of the MaxTorque clutch, and they are right beside each other. For others with a 4-cycle engine (or Honda clone) that has an oil level sensor and/or kill switch: Tap into the oil level sensor wire (if you have one) or the kill switch wire. Low level voltage magneto pulses exist there while the engine is running. Feed those pulses into your SenDEC wire. You will be physically away from the spark plug wire and magneto. Those pulses are strong enough to activate the SenDEC. I took the lazy way out: I took a cheap alligator-end test jumper wire, stuffed the snipped-off end into the SenDEC and clipped the other end to the jack where the oil level sensor plugged into. Installed in less than five minutes to riding configuration.

    Thank you, Old Bob. Your recommendation was good and worked the very first time!