Trying to connect a Tachometer

[tootallofwa]

I have an old automotive tach with settings for 4/6/8 cylinders. It has two leads, one for ground and one for the positive side of the coil. I have the positive side connected to one of my coil wires at the cut out switch and the other grounded. I have set it to 4 cyl and it works, kind of...

Now a 4 cyl 4 stroke coil, fires 4 times for every 4 strokes (2 revolutions.) My 2 cyl 2 stroke with 2 coils each firing 1 time for every 2 strokes (1 revolution.) So I figure the meter expects (and displays) 4 impulses for 2 revolutions (or 2 impulses for 1 revolution) and reading only one coil, my engine is delivering 1 impulse for 1 revolution.

That mean that it reads 1/2 as fast as my engine is turning. I think it would read correctly if I could incorporate the impulse of the other coil. (2 impulses for 1 revolution)

I think first, for it to work at all, there must be a diode to keep it from grounding the coil. Second if I connect it to both coils, that will connect the coils and not work at all. If each coil were isolated from the other with diodes, it would work.

Can anyone suggest an appropriate diode(s), I could use to connect the meter to both coils?
For those who don't know what a diode does... It works like a one way valve allowing power to flow one way and not the other. But I don't know how they are rated or what kind of voltage or current is on the point side of the coil. It can't be much if a 16 gauge wire can handle it.

 

[Moody Blue]

Re: Trying to connect a Tachometer

Why not just install the proper tach designed to work with a stator based system.

Let us know how you make out.

 

[tootallofwa]

Re: Trying to connect a Tachometer

Why not just install the proper tach designed to work with a stator based system.

Let us know how you make out.

Gee... Could it be, they cost 200 bucks???

 

[Silvertip]

Re: Trying to connect a Tachometer

Don't know where you are shopping for tachs but they are readily available for well under $100 and generally in the 70 - 80 buck range. Your engine may also have an ignition system that uses the "wasted spark" design where the coil fires at the power stroke and also at the bottom of the stroke. You didn't mention what engine you have.

 

[Moody Blue]

Re: Trying to connect a Tachometer

Gee... Could it be, they cost 200 bucks???

Not true at all. Where do you shop? Take a look on ebay. They are sold all the time brand new for under $40. I've bought two in the last two years and paid less than $40 including shipping from the USA to Canada. Both are good name brand units.

 

[tootallofwa]

Re: Trying to connect a Tachometer

I was looking at a inductive tach. couldn't find any that didn't require the generator stuff.

Turns out I was reading this one wrong anyway and I realize my diode idea won't work. The only thing it really shows me is it's going faster or slower, but it's so low on the scale I'm not getting enough detail. I thought it was reading 50% but turns out to be only 5% or RPM I expected.

I have some good ideas sometime... but when they're bad... They're really bad!!!

I still don't believe anyone sells 2 stroke tachs for $40.
I did look, I guess I don't know what to search for. It sure isn't "2 stroke tach".

 

[tootallofwa]

Re: Trying to connect a Tachometer

Don't know where you are shopping for tachs but they are readily available for well under $100 and generally in the 70 - 80 buck range. Your engine may also have an ignition system that uses the "wasted spark" design where the coil fires at the power stroke and also at the bottom of the stroke. You didn't mention what engine you have.

Not that it should matter... I said it was a 2 cyl 2 stroke...
but it's a 62' 28hp Johnson. And I'm not looking for a dash gauge. I was simply trying to get this OLD piece of test equipment to give me some more use.
It's a circa 68' dwell/tach for tuning cars, but since they don't use points It's just obsolete.

 

[tootallofwa]

Re: Trying to connect a Tachometer

But thanks for looking anyway

 

[Overnighter20]

Re: Trying to connect a Tachometer

I bought a teleflex Sterling Tach and Speedo for my 200 johnson 2 stroke for 50 bucks buy it now, brand new in the plastic,,,,,,,,plus shipping. Then found matching tilt, temp, fuel, and volt gages to match. Around 200 for the entire set.

 

[Silvertip]

Re: Trying to connect a Tachometer

Took about 15 seconds to fiind this one right here on iBoats:
http://www.iboats.com/Tach-7K-RPM-T...8261881--**********.249427932--view_id.191141

Then there is the Tiny Tach which is an inductive type but no generator is needed. About 40 bucks.

The reason I asked what engine is because there is a difference in ignition systems and the waveforms they produce as well as the number of pulses provided. You have a magneto ignition system not a battery ignition system. Some automotive tachs cannot recognize the waveforms from a non-automotive ignition system. A two cylinder two stroke provides the same number of ignition pulses/rev as a four cylinder four stroke. But since you are looking at only one cylinder you should be seeing 1/2 the actual engine RPM. Since the motor idles at about 700 rpm the tach should show 350. If it doesn't the tach simply cannot read the waveform.

 

[Moody Blue]

Re: Trying to connect a Tachometer

I still don't believe anyone sells 2 stroke tachs for $40.
I did look, I guess I don't know what to search for. It sure isn't "2 stroke tach".

I just took a minute and checked ebay. There are more than 10 listed right now for under $30. Some prices are under $10. Without a doubt many of them will sell at around the $40 range by the time the auctions end.

For the record, I bought my first tach for $19 +shipping and the last one for $29 +shipping. Both brand new name brands in the package.

Not saying the good deals are always around but you've got to shop around unless you've got deep pockets.

 

[Silvertip]

Re: Trying to connect a Tachometer

Great Lakes Skipper (greatlakesskipper.com) has loads of tachometers. Here is just one:

http://greatlakesskipper.com/produc...d-inboard-outboard-boat-tachometer-gauge.html

You can search their site for others. Some more, some less expensive.