More electrickery stuff - Tacho

[warren3200gt]

as you may have gathered, electrickery is not my strong point! How do you wire up an electrical tacho?
The tacho I have has 3 wires. Red - Battery live, Black - Earth, blue - coil negative.
As its four cylinder and 2 coil a pulse should be every revolution ie 1:1, 1 rev = one pulse.
The rev counter has the option to set it to 4,6 or 8 cylinder, but if I set it to 4 it seems to read 50% revs.
Could I join the blue wire to both coil negs? or would that cock up the sparks? if it wouldn't would it then read correct?

[martinz1000r]

You don't say where the tacho is from. I have Scitsu race tacho on my Z and the blue wire takes it's read from the HT side of the coil as is taped to one of the HT leads. Might help others better answer your question.

[warren3200gt]

Its a LED bar tacho, similar to F1 type on steering wheels, first several are green, then orange, then red, then bang.
I know the type you mention, I'm using one similar to check the the LED one's readings. The LED one reads 600 ish RPM yet the HT pick up one reads 1100 ish RPM.
That's what makes me think the LED one is only reading 50% revs.

[ZedHead]

Could I join the black wire to both coil negs?

No...From what you say

Red - Switched 12v
Black - Chassis Earth
Blue - Coil pulse wire

[warren3200gt]

Hhmmm, that's what I suspected. So if I did connect the blue to both coils, is there an "electrical device" which I could put in line between the coil neg and the tacho on each coil wire to the tacho so the current would only pass one way? (Diode?)
Effectively separating the coil negs as far as the bike/ignition is concerned but allowing a pulse from both negs to go to the tacho?

Just noticed I put "black"wire to both coil negs in the above post but I meant blue (edited now)

[ZedHead]

Just noticed I put "black"wire to both coil negs in the above post but I meant blue

That is what got me confused

I guess you could possible feed through two diodes (one from each coil and anode pointing to the coil) this would certainly double up the tacho puls frequency.

Personally, I would go back to the manufacturers to find out why it is only representing 50% Also don't disregard martinz1000r, he could be right that the blue wire "picks up" from the HT side.

When all fails read the instructions

[Coose]

I'd say it's for a car with a single coil, not a twin coil bike....

[warren3200gt]

I'd say it's for a car with a single coil, not a twin coil bike....

Coose,
you could well be right, its been kicking around the garage for at least 10 yrs.

Zedhead,
I doubt the manufacturer is still around even if I knew who it was and one things for sure I don't have the destructions any more.

But the diode theory would work then?

[PAULJAC47]

Just a thought ,Zeds use wasted spark so fire at btdc as well as tdc so you get two pulses per rev?

[needaz1100r]

Just a thought ,Zeds use wasted spark so fire at btdc as well as tdc so you get two pulses per rev?

Are you sure about that?

[zed1015]

Just a thought ,Zeds use wasted spark so fire at btdc as well as tdc so you get two pulses per rev?

Are you sure about that?

One pulse per rev.
Wasted spark is close to TDC on the exhaust stroke.
Your tacho will be for a distributor ign which runs at half speed hence the 50% reading.
Running feeds with inline diodes from both coils to double the signal should do the trick.

[PAULJAC47]

Stand corrected then always thought fired twice per cycle hence wasted spark!

[zed1015]

Stand corrected then always thought fired twice per cycle hence wasted spark!

Yes! Twice per cycle.
Two revs per cycle on four stroke.
Wasted spark on the exhaust stroke.

[needaz1100r]

Stand corrected then always thought fired twice per cycle hence wasted spark!

The whole "ignition" fires twice per crankcase revolution, once for cylinders 1 and 4 and once for cylinders 2 and 3.

The spark that is "wasted" is cylinder 4 on exhaust, whilst cylinder 1 is on compression. When cylinder 4 is on compression, cylinder 1 fires as well and that spark is "wasted" as cylinder 1 is on exhaust.

The same is true for cylinders 2 and 3.

On a 4 cylinder car, the single coil fires once per cylinder, per two revolutions of the crankshaft, and that spark is directed to the correct cylinder by the distributer

That equates to firing twice per crank revolution, each Z coil fires once per crank revolution, hence the 50% reading. The tacho on the 4 cylinder setting is dividing pulses x 2 to get revolutions.

Bearing in mind a car revs typically much lower than a zed couldn't you just change the notation on the display. Where it says 2000, change it to 1000, then at 4000 on the tach the zed will be doing 8000?

[warren3200gt]

Mark, I could do that but it seems to be a sealed unit, or if it isn't there isn't a obvious way in to the markings.
The twin diode route is the way I think I'll go onceI know which diodes I need?