Elec gurus - How exactly does the IGT signal behave

[Hen]

Gents, I'm curious about how exactly the IGT signal behaves.

I understand that it controls the ignitors which in turn fire the coil (let's for the moment only consider a single coil and dizzy setup). But when does it go high and low? My guess is as follows, but I am open to corrections.

When the ECU calculates the dwell time should start the IGT pin pulls to ground. It then stays low until the time of ignition when it stops pulling to ground and goes to an open circuit. Then waits for the next dwell time.

Also does anyone have a measurement of how much current the ECU sinks through the IGT pin?

Thanks
Hen

[MWP]

When the ECU calculates the dwell time should start the IGT pin pulls to ground. It then stays low until the time of ignition when it stops pulling to ground and goes to an open circuit. Then waits for the next dwell time.

Yup, that is correct...

[dencio1976]

additional question to this..

does IGT vary from rpm increase or will dwell time have to compensate for an efficient spark?

then again... drawn from the diagram... IGT acts only as a switching current for the power transistor. so i guess it's a fixed value.

so then.. typically, what is dwell time/dwell angle on idling on about 850rpms? on a 4cylinder engine like a 4AGE ST for example?

then if it's 40degrees on 850rpms example... then it'd be .00784sec that IGT would stay on for a computed efficient spark

[oldcorollas]

http://www.autoshop101.com/forms/h23.pdf
remember this?

[Hen]

Thanks for the link. So it looks like it's in fact a bit different.

IGT causes a spark when it goes high. And the dwell time is controlled internally in the ignitor.

[Mos]

Dwell is not always controlled by the ignitor. In fact in most modern engines dwell is controlled by the ECU and the multiple output ignitors are dumb (eg 1JZ-GTE, 2JZ-GTE, 1G-FE).

No info on current - why do you need to know? I'd expect it to be in the milliamps order of magnitude. Output drivers on an Autronic SM4 for the ignition outputs are 1A max (if that info is of any use) and that is made to cater for a lot of possibilities.

Mos.

[Hen]

This all revolves around building an interceptor for a 20V so you can run stock ECU and twin (or four) coils, making RWD installation far easier. I know you can buy them, but I want to make one myself.

I plan on intercepting the IGT signal and directing it to one ignitor or the other depending which cylinders should be sparking. And so an idea of current flows would be handy. I'd also guess mA, and it most likely won't be an issue for the components.

Hen

[takai]

It can be as much as an amp or so, depending on the setup. For a stock car though i would say in the mA, but the Haltechs and a lot of AM ecus have the tendency to blow the grounds off Nissan ignitors, which have the most pissy ground wire ever.

[The Witzl]

Mos,

Perhaps you should come around, and we hook up my dual-trace CRO to the 20V and see if the ignitor is dumb.

I remember some tried this before with smallport 4age or a 20V, and found that the IGT signal set the dwell, and not the ignitor - but i can remember who, where or when.

Personally i see no reason why the dwell would be set by the ignitor and not the ECU on any post 1988 toyota EFI car.

[Hen]

Mos,

Perhaps you should come around, and we hook up my dual-trace CRO to the 20V and see if the ignitor is dumb.

I was hoping to do this too (aside from getting Mos to provide the CRO). But I just need a willing car donor who doesn't mind me fiddling with their wiring.

[takai]

20v uses a dumb ignitor. I remember that from way back in the day. Same ignitor as the 16v Smallport i think as well. Or at least they are crosscompatible.

[Hen]

Dumb ignitors FTW. Except I just discovered Bosch 008s aren't dumb. So I'll have to find another type of cheap, easy, suitable ignitors.

Hen

[The Witzl]

Microtech X4 ignition box - your cheap source of reliable 4-channel dumb ignitorism.

Takai - i remember it from way back too.

[takai]

Yeah, Bosch 124s or Microtech X4, which is your ideal. Then you only have to build a switching device.

[Mos]

It can be as much as an amp or so, depending on the setup. For a stock car though i would say in the mA, but the Haltechs and a lot of AM ecus have the tendency to blow the grounds off Nissan ignitors, which have the most pissy ground wire ever.

I *suspect* that the reason they blow the grounds is that people run them in wasted spark and the ground connection was never designed to handle current from two coils at once. I vaguely remember discussing this very issue with Linden and others - if there was a problem with the Nissan ignitors they'd be blowing in factory applications also.

And... unless you're changing the voltage that you're supplying, there simply isn't any way to supply too much current on the IGT line. So I have to challenge that notion Chris .. The Autronic can supply *up to* 1A, but if the ignitor has a 10kohm input impedance/resistance there is no way for the Autronic to supply more current than IGT voltage/input impedance.

In the case of the 1G-FE COP ignitors, IGT is a 4V active high signal (ie 0V in idle state, goes to 4V to charge, then drops to 0V to discharge). The pulse width of IGT is controlled by the ECU.
The IGF signal is a fixed pulse width, and a fixed time after the IGT goes high. IGF is normally at 5V and drops to around 0.6V for the active low signal.



No biggy to scope the signals off a 20V if someone can supply a working 20V

Mos.