Bench top Engine Simulator for the P01/P59 Pcm

[antus]

The typical value of a pull up in digital circuits is 10k, so your more than double that, no problem. Its usually used on inputs, not outputs to give a default logic level (rather than floating) which can be pulled the other way with a strong signal. Im not sure why its needed for this output, but I would be interested to understand.

[Gampy]

I would be happy understanding half of what y'all have said ...

Well Antus, as much as I'd like to explain it to ya, it's not gonna happen, that would mean I understood!

I do have enough grasp to understand the logic level, but to manipulate it is another story, but I'm gaining, one bite at a time.

My logic for grabbing the 22k was this, after @pman92's comment I did a search and read 1k - 10k was typical, the 22k was the first one I grabbed that was a bigger number and the way I understand it, that was less of a direct connection, so I felt safer.

Thank you all very much.

[MudDuck514]

[edit]
My apologies, I wrote the voltage and resistors wrong ...

The Injector LED hookup is, Cathode to Injector Signal, Anode to +5v thru a 200 Ohm resistor. (Red, Black, Brown, Gold)

So, Correctly,
The Injector LED hookup is, Cathode to Injector Signal, Anode to +12v thru a 100 Ohm resistor. (Brown, Black, Brown, Gold)

As soon as the Pcm gets +B the led glows dim, turn ignition on, it dims a bit more.

When it gets a signal it does go full bright ... However it always remains dimly lit.

Would that indicate a pullup/dn resistor is needed?

Thanks

Hi all,

Gampy, I wonder if the injector line is pulsing. Can you capture video of the LED and slow it WAY down in playback to check?

Mike

[Gampy]

As slow as I could go was half speed, it doesn't look like it's pulsing ...

[pman92]

An LED takes very little current flow to light up, say 0.01 amps. An injector coil will flow much more current, say 1 amp once saturated.
A small current flow of 10 milliamps (which would light an LED) through an injector coil would not have much effect against the return spring of an injector pintle, the injector would remain closed.

The PCM output is an "open collector" output, IE it switches the earth side of the injector to earth to turn it on.

When the injector is off, there will be battery voltage at the output pin (earth side of injector coil), because the voltage comes through the injector. So the injector will have battery voltage both sides.

There is circuitry within the PCM that draws a small current from this pin, maybe a voltage divider that cuts the voltage down to below 5v, which is then fed into an ADC of the processor to "monitor" the injector voltage. By monitoring the voltage it knows when an injector is open circuit/disconnected and can throw a trouble code. Or maybe the small current draw is part of the injector driver itself. Or maybe it is part of a circuit to suppress the high voltage inductive spike of an injector switching off, which may otherwise damage the injector driver.
I'm not entirely sure of exactly what, but the fact your LED glows slightly is proof of this small current draw.

The older Delco PCM's behave exactly the same with just a LED on an injector output pin.

LED's have a "forward voltage", that is the voltage drop across the LED. They also need a certain voltage potential over them before any current at all will start flowing.
By fitting a high value pull up resistor, your creating the same voltage both sides of the LED so it will not flow current and light up.
The small current flow the PCM is drawing is now going via the resistor and not the LED so it doesn't light up.

If you were to fit a very high impedance resistor instead, say a few megaohms, and the current draw of the PCM (say 5 milliamps) was enough to create a high enough voltage drop over that resistor (also the voltage drop over the LED), then the LED would again start to light up.

[pman92]

Also you can safely use a resistor as low as may be expected from the resistance of an injector.

I can guarantee you could safely use a 10-15 ohm resistor, but it would get hot and the PCM would also get warm. But the PCM would have no trouble driving it (it is designed to drive a 10-15ohm injector coil).

You could probably also use a 100k resistor and it would prevent the LED glowing.

The 22k you have will work perfect though

[pman92]

I can't guarantee this and wouldn't try it, but you could maybe also short the injector output pin directly to battery voltage with no resistor.
There is probably current limiting circuitry with the PCM.

Normally if this happened in service (injector wires short together) it would blow the fuse when the PCM switches on the injector

[antus]

Good details, thanks!

[MudDuck514]

As slow as I could go was half speed, it doesn't look like it's pulsing ...

Hi all.

Gampy, thanks. Leads me to believe it is glowing due to leakage current through the injector driver, EXACTLY what PMAN was describing!

Mike

[Gampy]

Whoa, my freaking heads going to explode!

pman92, excellent explanation, I'm still chewing on it, but I do understand more then less of it. That's huge to me!
Yea, it's going to take a few days chewing on it but I assure I will read it more then a few times, I already have!

Thank you all for doing what you do!