MegaSquirt and an OBC.

[DaveEFI]

There must be a few here who have fitted a MegaSquirt (with injector peak hold drivers) to a Rangie etc which has an OBC only to find the MPG readout no longer works properly. I found this with my SD1.

The reason is the Lucas injection simply grounds the injectors to open them. The OBC measures the time one of them is open and calculates fuel use from that. The MS holds the injector open with a series of pulses (PWM) since less current is needed to hold it open than to open it. These pulses are at a frequency too high for the injector to react to - so it just stays open. These pulses confuse the OBC which is designed for a simple DC signal.

I tried a few methods of converting these pulses to a suitable signal (just for the OBC) without success. Then discovered someone had addressed the same problem but for other reasons.

JayCar do a kit to convert peak hold signals for their injector open time counter. It works by measuring the current in the positive feed to the injector and produces a ground 'output' for all the time current flows. Which is of course the time the injector is open. It uses a very low value resistor in series with one injector (low enough not to have any effect on injector performance) and amplifies the tiny voltage developed across that.

http://www.jaycarelectronics.co.uk/prod ... rm=KEYWORD

It's a very easy kit to build as these things go.

[daxtojeiro]

hmm, why are you using a peak and hold circuit?? The V3 board has PWM built into it.
Also, why aren't you using the resistor box thats used on the flapper setups, you dont need the compexity of any of that if you use the original resistors, I assume this is for the flapper injectors as the hotwire are all hi-impedance so non of that is needed either,
Phil

[DaveEFI]

hmm, why are you using a peak and hold circuit?? The V3 board has PWM built into it.
Also, why aren't you using the resistor box thats used on the flapper setups, you dont need the compexity of any of that if you use the original resistors, I assume this is for the flapper injectors as the hotwire are all hi-impedance so non of that is needed either,
Phil

Low resistance injectors with series resistors to limit the current don't perform so well as direct drive using PWM to limit the peak current. The resistor alters the response of the injector to the drive signal. See the MS manual for a full explanation. But if you're into Hi-Fi, it's the same sort of reason as a good power amplifier has a very low output impedance to drive a loudspeaker correctly. It makes the speaker cone assembly follow the original waveform more closely.

MS recommend getting rid of the resistors, so I just followed their advice.

I suppose I could have changed to the later injectors - but why spend money that wasn't needed? I have several sets of the standard ones - including a refurbished set which I fitted with the MS. The later units also have a different fuel rail.

[daxtojeiro]

Ceers Dave,
I have read a few as I wrote most of the manuals (MSExtra)

Theres no measurable gains from driving them directly. Generally, lo-impedance injectors are used when massive flow rates are needed for highly boosted engines as they can give a very small increase in control (resolution) but the performance of the flapper injector is such that the gains of PWM are null and void due to their design, age, general condition, etc. The only gain you have is in noise, which is an issue in HiFi too, well it was when I was designing valve amps a few years ago. We always recommend fitting resistors to lo-impedance units, better still always buy hi-impedance devices, indeed the MS3 (latest MS) can not fire lo-impedance injectors without resistors,
cheers,
Phil

[DaveEFI]

Ceers Dave,
I have read a few as I wrote most of the manuals (MSExtra)

Theres no measurable gains from driving them directly. Generally, lo-impedance injectors are used when massive flow rates are needed for highly boosted engines as they can give a very small increase in control (resolution) but the performance of the flapper injector is such that the gains of PWM are null and void due to their design, age, general condition, etc. The only gain you have is in noise, which is an issue in HiFi too, well it was when I was designing valve amps a few years ago. We always recommend fitting resistors to lo-impedance units, better still always buy hi-impedance devices, indeed the MS3 (latest MS) can not fire lo-impedance injectors without resistors,
cheers,
Phil

Interesting. Wish the MegaManual was altered to make this clear. It is quite specific about the benefits of deleting the resistors and using their current limiting setup - otherwise why bother designing/including it? I must admit their explanation made sense to me - otherwise I'd have kept the existing resistors.


I made up a new loom eliminating the resistors and wiring and since everything works ok I'm not going to change it right now. I do have a hotwire system lying around with high impedance injectors - but dunno the condition of them. However, I'll keep this in mind if any future mods involve the loom. But thanks very much for the info.