What does an injector signal look like?

[mhopeng]

I'm connecting MPGuino to a 1990 Toyota 4Runner. My ECM connector pinout is:

E01 No10 STA Vf NSW
E02 No20 IGt E1 n/c

where the pins are described as:
No10: "Injector"
No20: "Injector"
E01: "Engine Ground"
E02: "Engine Ground"

I borrowed an oscilloscope to look at the No10 and E01 signals. The No10 signal was a high pulse (> 12 V), while the E01 signal was a small negative pulse, more like 0.5 V. Both pulse frequencies correspond to changing engine rpm (pressing the gas while sitting in the driveway).

The shorthand directions for finding the Injector signal call for finding the signal which is *not* 12 V, so I connected the E01 signal to the MPGuino, and it doesn't work (no injector pulses are recorded). So the obvious thing to do is try the other pin, but I thought I would check here first, because its a pretty high voltage.

So, in general, what does the MPGuino expect the injector signal to look like? What is the safe input voltage to an input pin on the MPGuino board? The "Injector" signal seems to be a pretty high voltage spike, higher than the ATmega datasheet allows (maybe this what the zener diodes at the input pins are for?).

Given the choice between the two signals in the picture, which one is most likely the correct INJ signal?

Thanks!!

[Flood_of_SYNs]

You need the Zener diodes in place to drain off extra voltage to the ground line.

As I understand it the Zener acts like an overflow valve, turning those high volt injector pulses into low voltage pulses that the ATMEGA chip can safely handle.

Either [No10: "Injector"] or [No20: "Injector"] is what you want to tap into.

Ideally you want to use the car ground and not the ground on the ATMEGA board.



Here is a simple page I made, mainly as a reference for myself but I was hoping it would help others.

How to build an MPGuino - EcoModder

I created the image on that page because all the other ones I found where kinda confusing.

[dcb]

mh (nice little rotary btw ) ,

1. Make sure you measure/tap by the injector for starters, I frequently have problems at the ECU for unknown reasons.

2. Some older cars (and who knows what else) have a positively switched circuit. So that you can locate one (of two) injector leads that will give you an "inverted" gph reading. I added a "inj trig" setting to the setup recently that you can set to 1 to see if that fixes things. If that is what is going on then the first picture looks right.

the atmega has a large resistor in series with the signals and a zener to clamp it at or below 5 volts, it is meant for a 14+ volt injector system. The atmega needs at least 2.5 volts to register an "on" signal, so 0 to 0.5 won't even register.

[mhopeng]

These replies are very helpful, thanks!! That all makes a lot of sense. I will report back after I have a chance to try the No10 signal.

PS - Rotary engines are the engines of the future!

[mhopeng]

Tapped the No10 signal, and the MPGuino is working like a champ. Will post pics and calibration info when complete. Thanks for the advice!

[winker]

I found this very useful

Automotive Scope Applications