Feature request: remote voltage sensing

[elaw]

Hey, have you guys ever considered allowing a spare analog input to be used to read Batt V instead of the internal divider?

My car, and I have to think there are probably others like it, has the MS box powered from the circuit that originally powered the factory ECU, but the injectors/idle valve/etc. powered from a separate feed coming from the battery. And since the circuits are separate and each have their own loads and drops, the voltage at the Megasquirt's power pin is not necessarily the same as the voltage feeding the engine hardware. And the voltage feeding the engine hardware is what's important for things like determining injector deadtime and correcting the idle valve PWM value.

So it would be really cool if an unused analog input could be used (with a divider network) to read the voltage that's actually driving the injectors and such.

I realize that's not quite as easy as it sounds because the supply for the injectors etc. is usually controlled by a relay. I can think of two solutions to that: 1) read the voltage at a point before the relay or 2) have the code switch between the internal and external source based on a condition. Maybe when the fuel pump is energized? Or when the external input reads > 0 or > some percentage of the internal input?

[tpsretard2]

There is over 1 volt difference between the ECU supply and the injector supply on my car.

[DaveEFI]

Not really quite sure just why the internal divider network isn't reading the correct voltage? After all, there should be no high current flowing within MS, except on the ground side.
My car was built using a single high current feed direct from the battery to supply all the injection (other than fuel pump and ignition) so I kept it the same. The ignition switch just switches the relay controlling that.

[elaw]

It's not that the divider is not reading the correct voltage, it's that it's reading the voltage on the wrong circuit!

In my car, the injectors etc. are on one circuit, with wiring I installed myself directly to the battery. The MS ECU is on a separate circuit: the one that originally powered the OEM ECU, which is fed from a different place and is shared with other loads. So depending on circumstances the voltage at the the Megasquirt ECU could be different from what the injectors and other engine hardware are seeing. And since the voltage at the Megasquirt ECU is used to correct things like injector deadtime, the voltage at the ECU being different from that at the injectors could result in the wrong correction being applied.

[dontz125]

Lift the top leg of R3 (at the top left of the DIP40 socket); run it to a jumper to a spare pin on the DB37. Run a wire from that DB37 pin to the voltage source of your injectors. It may be easier to remove R3 and replace it with a new 50k resistor inline with the jumper.

For a V3.57 board, remove R3 and replace it with a 50k inline resistor as described above. It can be connected at the lower R3 pad, or the right pin of the boot jumper.

The wire can be fairly light, as the 50k Ohm resistance -> current << 1mA.

[elaw]

I thought about that, but if the MS senses 0V at startup won't it go into bootloader mode or something?

I know that would not happen if I put the sense wire before the relay feeding the injectors, but I'd prefer to have it after.

[dontz125]

Good point. I can't think of a way around the boot loader problem; I was going to suggest a 10k pull-up to 5v, but decided that would do really weird things to the ADC reading. Short of some really complex relay / transistor riggins, you might have to take it from upstream of the relay.

[Matt Cramer]

I thought about that, but if the MS senses 0V at startup won't it go into bootloader mode or something?

I know that would not happen if I put the sense wire before the relay feeding the injectors, but I'd prefer to have it after.

MS1 would go into bootloader mode. MS2 and MS3 does not, as I've seen firsthand with some oddball damaged circuit boards.

[dontz125]

*facepalm* Of course, since the MS2 and MS3 cards have their own boot jumpers, that have nothing to do with H1 on the board ...

(Where's a [head bonk] smiley when you need it?)

[elaw]

Doh! I forgot about that too.

Still, an option in the firmware would be a lot easier than taking the Megasquirt apart and modifying it...

[Reverant]

That would probably go inline with the request to be able to assign all the "hardcoded" analog inputs (IAT/CLT/TPS/V) to whichever analogue input you want.