Tachometer signal?
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Laughing Gas
Joined: Nov 2010
Posts: 924
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From: Johns Creek GA
Tachometer signal?
I've recently decided to get back into fiddling with electronics, and picked up an Arduino to play with. While trying to come up with some project ideas to work on, it hit me that I could probably build a digital tachometer for my truck.
Given that I've got a 300 engine with the Duraspark II ignition, what exactly is the signal coming off the tach wire attached to the coil? Am I correct in guessing that it is one pulse per cylinder firing? If so, at what voltage?
I realize I could probably figure some of that out on my own if I went out and put my meter on the truck. But, (a) my meter is analog, and thus not optimal for pulsed signals, and (b) it's cold outside.
Given that I've got a 300 engine with the Duraspark II ignition, what exactly is the signal coming off the tach wire attached to the coil? Am I correct in guessing that it is one pulse per cylinder firing? If so, at what voltage?
I realize I could probably figure some of that out on my own if I went out and put my meter on the truck. But, (a) my meter is analog, and thus not optimal for pulsed signals, and (b) it's cold outside.
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Laughing Gas
Joined: Nov 2010
Posts: 924
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From: Johns Creek GA
In my truck (which does not have a tach in the instrument cluster), there is a green wire that comes off of the coil that ends in an unused plug. When I am using my tach/dwell meter, I attach one lead to this wire and the other to ground. Is this the same wire you are referring to?
pedant
Joined: Jan 2008
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From: EXTREME southwest CT
No Chris.
By "disconnecting" interrupting the flow of electrons in the primary side of the coil, the field collapses and induces a high voltage spike in the secondary windings.
Here is the schematic;
Dorsai, are you saying the green wire does not go to your DSII module?
By "disconnecting" interrupting the flow of electrons in the primary side of the coil, the field collapses and induces a high voltage spike in the secondary windings.
Here is the schematic;
Dorsai, are you saying the green wire does not go to your DSII module?
pedant
Joined: Jan 2008
Posts: 23,576
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From: EXTREME southwest CT
A points style ignition system fires when the points open, right?
The transistorized ignition module operates the same way, it just uses a tiny electrical pulse from the reluctor pickup to switch the transistors gate and disconnect the coil from ground.
The transistorized ignition module operates the same way, it just uses a tiny electrical pulse from the reluctor pickup to switch the transistors gate and disconnect the coil from ground.
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Fleet Mechanic
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Yes, that is correct.
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Laughing Gas
Joined: Nov 2010
Posts: 924
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From: Johns Creek GA
I knew someone was going to make me go outside in the cold...
I just went out and looked more closely at how the ignition is wired; it is exactly like that (very helpful!) diagram you posted. What isn't shown on the diagram, though, is the green wire that I mentioned above; I am guessing from both the color as well as the fact that the green wire to the coil is labeled 'tach' in your diagram, that this wire is spliced into the green wire that runs from the ignition module to the coil.
So from this as well as your later comment, am I correct in assuming that (a) this wire pulses once per coil firing (well, technically, it would have voltage between firings, but for my purposes I don't think this distinction matters), and (b) that these pulses will be 12v at start, and 7-8v during run?
So from this as well as your later comment, am I correct in assuming that (a) this wire pulses once per coil firing (well, technically, it would have voltage between firings, but for my purposes I don't think this distinction matters), and (b) that these pulses will be 12v at start, and 7-8v during run?
Post Fiend
Joined: Jul 2004
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From: Northern California
(b) that these pulses will be 12v at start, and 7-8v during run?
The starter takes a lot of "power" to operate the engine. With the starter cranking the Voltage at the Coil evens out. What is more correct is that you have unresisted voltage at start, and resisted voltage in run.
Just to make something more clear...
Chris:
It's correct to say if the tach lead is "totally disconnected" that the coil won't fire. Same thing with it being "totally connected" the coil won't fire.
Ardwrkintrk:
It's also correct to say that the coil has to build up charge, and the act of the tach signal being disconnected is what sends the stored charge to the distributor.
It has to reconnect, and disconnect for every firing. Connect builds the charge, disconnect releases it. This is what increases the 12v to 20,000v aproximate.
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Laughing Gas
Joined: Nov 2010
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From: Johns Creek GA
I'm thinking that if I build my circuit to account for a peak voltage around 14v, I should be good under normal conditions. Right?
pedant
Joined: Jan 2008
Posts: 23,576
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From: EXTREME southwest CT
The coil is a simple transformer.
It is the ratio of primary to secondary windings that steps up the voltage.
The coil must have a certain(enough) time to saturate before the circuit is broken again. (dwell)
The coil creates a magnetic field.
Breaking the circuit collapses the primary field and induces a charge in the secondary winding.
The only thing that remains connected is a condenser. (capacitor)
This quells reverberations in the circuit.
FTE Legend
Joined: Jul 2010
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From: Northeast, OK
Your actual voltage will be far in excess of 14v. If you want I'll put my scope on this afternoon and see what it really is. But, when the module turns off, meaning let's the voltage go up to whatever it wants to go to, it'll go way above battery voltage due to the voltage induced while the secondary discharges.
Typically I put a zener diode in the input to the tach to protect it. Something like a 20 volt zener would do it, and it takes the induced voltage to ground. But, having said that, I've not worked on a DS-II so don't know for sure what the voltage looks like.
Typically I put a zener diode in the input to the tach to protect it. Something like a 20 volt zener would do it, and it takes the induced voltage to ground. But, having said that, I've not worked on a DS-II so don't know for sure what the voltage looks like.
pedant
Joined: Jan 2008
Posts: 23,576
Likes: 40
From: EXTREME southwest CT
OK, that makes sense. So in theory at start I could see voltage as high as whatever the battery puts out (~13.6v), dropping to that minus whatever the voltage drop caused by the starter is, and then 7-8v in run.
I'm thinking that if I build my circuit to account for a peak voltage around 14v, I should be good under normal conditions. Right?
I'm thinking that if I build my circuit to account for a peak voltage around 14v, I should be good under normal conditions. Right?
Most components in a nominal 12V vehicle are designed to operate at charging voltage.