Tach electrical isolation, tips and tricks?

[MikeJ]

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......

MikeJ

 

[Old Bob]

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.

 

[MikeJ]

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. . . .

 

[Old Bob]

You have one model of engine and I have a different one

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.

 

[MikeJ]

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,
MikeJ

 

[MikeJ]

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. . . . .

 

[MikeJ]

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!

MikeJ