DIY LTCC or similar system for LT1s

[spfautsch]

I wonder if you could just cheat and leave the ignition module hooked up (but not to the coil)

I've put 50+ hours into this project, if this electrical issue takes another 20 it's worth it simply to not have taken the easy route. This is the kind of "solution" I'd go with in an apocalypse, or if disabling the DTC wasn't an option and I wasn't opposed to using wire nuts and electrical tape on my pride and joy.

As I've mentioned about four times, my testing has shown a 470 ohm resistor from the EST line to ground gives enough load to prevent the DTC. If you look closely at the Bailey module in the upper right quandrant of the PCB there are two 1k ohm 1/2 watt resistors (thinking they're wired in parallel for net 500 ohms) very close to a 14 pin dip IC (maybe an op-amp or Schmitt trigger). But if you're going to test with a potentiometer to confirm this I would suggest you try one in the neighborhood of 1k-2k because I was getting the code with a 1k resistor to ground. A 10k pot is going to give you a very small range of adjustment once you hit the threshold. Also, you're probably going to get the code almost all the time if you connect the pot to +5v because the PCM has it's own pullup on this pin.

What happens when the ignition line is switched on is the PCM pulls the EST line low. You can confirm this with a TTL safe test light (led + current limiting resistor) by connecting the positive lead to +12v and negative to the EST circuit. When key is on the led will light solid, and pulse off when cranking starts. When it pulses off what is happening is the n-channel output transistor in the PCM is being switched off and the pin is allowed to float (presumably there is an internal pullup resistor) towards +5v. I believe the voltage threshold mentioned in the DTC description is only measured by the PCM when the line is allowed to float. When I take out the 470 ohm resistor that's simulating the load of the ignition module, I suspect the line rises up to +5v almost instantaneously after the EST transistor switches off.

What I need to figure out is how to amplify this signal on the EST side of the 470 ohm resistor to a clean TTL one that the Atmega can see the falling (or if inverted, the rising) edge of. EDIT: Unfortunately I can't see the signal to quantify what I have to work with here. The optocoupler (essentially just an optically isolated transistor) was the closest, but some of my attempts with the comparator were very sketchy at best. The 7414 Schmitt trigger also seems to be a bust.

going to be zero degrees F tomorrow, so don't hold your breath, as I am working out in the open. Can't spare the shop room at work, too busy. But I will try sometime.

Let me do some more experimenting in my garage. If I can't nail it down I might ask you to do some work with the scope, but let's wait until the weather is a little less stupid.

What I really need is a capture scope of my own. Unfortunately my toy budget was completely annihilated in 2017. I'm going to try kur4o's eehack definition to see what voltage the PCM is measuring here, but I have the feeling the sampling rate won't be fast enough to be useful.