Battery correction. THe correct procedure?

[HidRo]

So, I've discussed with prof315 about voltages and dead times, but one thing was left to answer.
So I guess it's good to ask it here, and so, someone else might change their calibration in the correct way.

So, I see the following voltages while running:
~14V at the battery
~13V at the injectors
~13.5V in MS (after running the car for a while, I see the voltage will drop to 12.9 usually.)

So, to what voltage should the MS be calibrated to?
Voltage at the battery?
Voltage at the injectors?

Thank you!

[Greg G]

I'm interested in the answer as well. I noticed all my MLV logs show alternator/battery voltages averaging around 11.9-12.5v at idle, but checking at the battery with a multi tester, I see ~13.5, with the car idling.

[prof315]

In my opinion it's most important to set the battery calibration so that MS and the injectors see the same voltage. This way the injector deadtimes will be correct which can become VERY important when using big injectors on small (Forced Induction) motors.

[HidRo]

Is MS aware of the possible difference (normal?) between the battery and Injectors?

[racingmini_mtl]

Is MS aware of the possible difference (normal?) between the battery and Injectors?

MS only knows the battery voltage it reports in the log.

Jean

[HidRo]

So, if MS "reads" 14, does it take in consideration that the injectors will have a lower voltage? Or not really, and we should, just like prof said, calibrate to the voltage the injectors "see" so the openning time is spot on?

[racingmini_mtl]

As I said, the MS only knows the battery voltage it reports in the log. If that's not what the injectors and coils see then you have to set the parameters accordingly.

Jean

[HidRo]

Prof315 is the winner!
Set the voltage that we see in TS to the same we see at the injectors, so we have the same open time correction, etc.

[TTC]

Does this mean you probe at the injector with a meter and increase the battery compensation till the meter reads the same as the reading in TS?

or

I'm told you can take the afr reading with as little power load as you can, then turn on all your lights heater ect and adjust the battery comp untill it matches the previous afr.

[robs]

Does this mean you probe at the injector with a meter and increase the battery compensation till the meter reads the same as the reading in TS?

or

I'm told you can take the afr reading with as little power load as you can, then turn on all your lights heater ect and adjust the battery comp untill it matches the previous afr.

The first thing to get right is that the voltage at the power wire to the MS must correspond to the voltage at the injectors and coil(s).

This isn't (or wasn't, at least) stressed enough in the MegaManual, but just like the IAT and CLT wires, the power wire is a sensor input to the MS, it isn't just the power source. The MS watches the voltage at its power supply and applies voltage compensations to dwell and injector open time, just like it compensates for air or coolant temperature. If the MS isn't seeing the same voltage as the coil(s) and injectors, these compensations are going to be meaningless and no amount of tweaking compensation values is going to fix it; you'd probably be best off to turn off compensations altogether.

Easiest way is to feed all from one relay (as per the standard wiring diagram), with sufficient wire gauge to avoid voltage drop. My first wire-up reused the previous ECU's power wire which didn't power the injectors. There was all sorts of noise on that wire and the MS was making bogus compensations all over the place. Everything was much more stable after rewiring the MS through the relay.

Have fun,

Rob.

[pmbrunelle]

One could always run a wire (thin gauge is fine since we're driving a 60K resistance) from the the live side of the injectors, as close to the injectors as possible, to the voltage divider inside the MegaSquirt for a more accurate reading of voltage across the injectors.

Of course, MS also uses voltage for dwell adjustments, but that's far less critical IMO.

Anyway, all that said, if you really have significant voltage drops in your car, you should really just fix the electrical problems rather than trying to band-aid around them.

When a car's electrical system is in good working order, the benefits of an external voltage sense wire should be minimal.

I have a question, and I didn't find the answer in the manuals. The injector dead time, for what voltage is this supposed to be measured at? 14.5 V?

[racingmini_mtl]

The injector dead time, for what voltage is this supposed to be measured at? 14.5 V?

13.2V

[TTC]

I dont want to fix something that isnt broken so im curiou as to what kind of symptoms you would see if the ecu and inj voltage are different. Car seems to run fine, makes good power. I had some issues with voltage drop that would make me drop PW randomly but that is now sorted out. Turned out when I was tuning the battery would run down and the ecu would see 11v. This did wonky things to the MS.

Currently I seem to have no problems with my electrical. I am planning on running a 10g wire from the battery to a painless fusebox/relay to power ecu, fuel pump and ignition all seperatly. I figured that wasn't a terrible idea.

[robs]

I think there's not much need to get in too deep with this. As long as your MS takes its power from the same place as the injectors, and that you have used cable that's up to the job, there's not much more to be done.
Given this, if the BattV log is swinging wildly, you can address it electrically (bigger battery, etc.) or learn to live with it, tweaking the compensation values to try to keep things stable.

Have fun,

Rob.