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wiring in a tach?

  • Thread starter Thread starter Anonymous
  • Start date Start date
if i got a regular four cylinder tach it would work on my engine.....oh and whoever said u cant connect the tach to both coils was wrong...i have these two little things that allow current to flow only one way.. if you connected one to each of the coils ground and the other ends to the tach it would only send out and theres no way they would interfere with each other or a signal go where its not supposed to be.
 
bboxer872002 said:
if i got a regular four cylinder tach it would work on my engine.....oh and whoever said u cant connect the tach to both coils was wrong...i have these two little things that allow current to flow only one way.. if you connected one to each of the coils ground and the other ends to the tach it would only send out and theres no way they would interfere with each other or a signal go where its not supposed to be.
Click to expand...
What we said was that you cannot connect both coils directly to the tach. You didn't. You have a device between the tach and coils. We weren't wrong.
 
if i got a regular four cylinder tach it would work on my engine.....oh and whoever said u cant connect the tach to both coils was wrong...i have these two little things that allow current to flow only one way.. if you connected one to each of the coils ground and the other ends to the tach it would only send out and theres no way they would interfere with each other or a signal go where its not supposed to be.
Click to expand...

What you should do is go get a four cylinder tach and connect it with the two little things (They are called Diodes, By the way) and report back to us.

Since I only have an MSEE, you are probably more qualified on this subject than I, but let me try to explain what will happen.

If you splice the banded end (cathode) of the diodes together, and connect the other ends of the diodes (anode) to the same terminal as the igniter on both coils, the voltage setting on the primary windings (+12VDC) from both coils will be connected to the tach input. This is not a problem. The problem is that the coils are triggered (Pulled Low) at separate times. This leaves the +12VDC from the off side coil on the tach input, therefore, no signal for the tach to read.

If you splice the Anodes of the diodes together and connect the Cathodes to the coils, you will have 0VDC at all times on the tach input, therefore, no signal for the tach to read.

There are plenty of good books available that will help you with Basic Electronic Theory!
 
gtsg01 said:
if i got a regular four cylinder tach it would work on my engine.....oh and whoever said u cant connect the tach to both coils was wrong...i have these two little things that allow current to flow only one way.. if you connected one to each of the coils ground and the other ends to the tach it would only send out and theres no way they would interfere with each other or a signal go where its not supposed to be.
Click to expand...

What you should do is go get a four cylinder tach and connect it with the two little things (They are called Diodes, By the way) and report back to us.

Since I only have an MSEE, you are probably more qualified on this subject than I, but let me try to explain what will happen.

If you splice the banded end (cathode) of the diodes together, and connect the other ends of the diodes (anode) to the same terminal as the igniter on both coils, the voltage setting on the primary windings (+12VDC) from both coils will be connected to the tach input. This is not a problem. The problem is that the coils are triggered (Pulled Low) at separate times. This leaves the +12VDC from the off side coil on the tach input, therefore, no signal for the tach to read.

If you splice the Anodes of the diodes together and connect the Cathodes to the coils, you will have 0VDC at all times on the tach input, therefore, no signal for the tach to read.

There are plenty of good books available that will help you with Basic Electronic Theory!
Click to expand...
You outrank me (only a BSEE). I wasn't going to speculate what he was using, but if indeed he is talking about diodes, then you are correct oh wise master. I worked it out on paper (it's late) and what you said cannot be disputed. :wink:

Do me a favor and go to my 80 1100E mods thread and look at my last post and see if it makes sense to you. It's always good to get a second opinion.
 
HMmmm makes sence well anyways i was just trying to prove that you could connect the two tegether and they wouldnt interfere....so what your saying is if the tach is connected to the two and one ignites it wont read that because the others still giving a constant 12vDc........
 
bboxer872002 said:
HMmmm makes sence well anyways i was just trying to prove that you could connect the two tegether and they wouldnt interfere....so what your saying is if the tach is connected to the two and one ignites it wont read that because the others still giving a constant 12vDc........
Click to expand...
If you are talking about diodes they conduct when a forward bias is applied (anode to cathode). If you have both diodes with the anode tied to the negative side of the coils and the cathode to the tach, you are forward biasing them. In this configuration you essentially have an OR gate. It makes no difference if the negative side of the coil goes to ground (fires) since the other coil is at 12 volts and is now applying that to the tach. The tach will always see 12 volts.

I won't go into the reversal of the diodes. There isn't any chance of them doing what you want.

The circuit that you want is an AND gate, not an OR. A low on either side will produce a low on the output and the pulse is seen for both coils. You can make one of these with a few transistors, or even use a logic chip with some surrounding protection and buffers.

The easiest way is to just use the inductive coupling that was highlighted earlier.
 
Busafied said:
Checkout this tach..
Got the link from one of my other boards...

http://www.gpt.it/power rpm english.htm
Click to expand...
Personally, I'm not a fan of digital readouts for tachs. If it had anaolg readout it would be so much nicer.

Designing in the circuitry to read various combinations of coils and cylinders is not difficult, it just requires forethought.
 
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