I am thinking about building a MegaSquirt DIY fuel injection computer. I would like to build a set of cables so I could swap between computers and retain the existing wiring. Will have to add a couple because MegaSquirt is a SD system. But the wiring doesn't seem to be much of an issue. What I am worried about is HEI timing. MegaSquirt controls fuel only, they do have some extra software that controls timing but requires an MSD box, which I don't want to invest in. I am wonder if I could piggy-back they Megasquirt into the system, controlling fuel delivery while retaining the stock computer for timing.
I figured if I just removed the injector control lines from the current computer and moved those to the MegaSquirt, then just tap into the rest of the sensors. So both computers would run, just the stock computer wouldn't be sending the injectors any signals. Seems simple enought, but is is too simple. Will it work? Would I mess up the signals by having both computers hooked to the same wires and trying to read the signals?

I'm just getting started with fuel injection programming and should be getting some good experances with MegaSquirt in the next couple months as I am helping my friend convert his VW Cabriolet to electronic fuel injection as he has pushed his stock Boshe system to the 200HP limit!
Hope you guys have some good ideas.

 

Nobody has an ideas? :eek:
I find that hard to believe

 

Ahh, that should be easy to do. Get a GM wiring diagram to start with. You should be able to share the TPS, MAP signals. Of course you will also need all the power and grounds active on the GM ECM. Make sure that the grounds for both ECMs are at the same point, this will assure that they are all at the same potential. All you need to do is to bring back the sensor outputs to the GM ECM. The TPS connection is at Pin B and the MAP is also Pin B. If you have a MAF sensor just leave it in the circuit.

As for coolant temp input, you can either install a second sensor so that both systems has its own temp input. Or you can lie to the GM ECM by installing a resistor across the two wires. A 300 ohm resistor would be adequate plus or minus 10%. If you have a MAT (Manifold air temp) sensor you can use a 1000 ohm resistor in its place or just leave it connected if your new EFI system don't require a MAT sensor.

That will fool the ECM into thinking the engine is warm. Also you may want to consider a second O2 sensor so that the GM ECM thinks its getting fuel feed back this however isn't really necessary other than to keep it from generating a DTC code. Other than that it should work fine.

Does the Mega Squirt system require a spark reference to fire the injectors? Most EFI systems does require this.

I don't know what year car the car you are working on is. But it you have a 85 to 91 then you can just parallel the SPARK REFERENCE from the GM ECM. That circuit comes from PIN B of the GM Ignition HEI module 4 way connector in an early large diameter HEI distributor. It is typically a purple wire with a white strip. I can't remember what pin location it was with the later small diameter distributors with a remote coil But I do think its probably the same color. That is one thing GM is good about and thats standardizing on its color codes.


[Modified by tjwong, 12:35 AM 12/13/2003]

 

Thanks for the ideas. MegaSquirt doesn't handle spark at all, this is why I want to retain the GM ECM. BTW, this is on a 85 corvette. MegaSquirt works on MAP, so I will be running wires just for that. I would rather just tap into the existing sensors, that way I am not running new wires or "fooling" anything with fake data. I already have the 85 shop manual so should have all the wiring diagrams I need. The only thing I'm worried about is the ECMs reading sensors incorrectly if two units are monitoring the signals.

 

Whats the advantage to using the mega?

 

You will not have any problems sharing the single TPS signal or the spark reference signal if needed. As long as you make double sure that the grounds are at the same potential then it will work fine. I have piggy backed several systems to GM controls and never had a problem. From what I understand of the MegaSquirt all you need for spark refernce is a signal from the TACH lead of your HEI distributor. So in the end the only thing you need is the TPS signal. You can just use the existing GM temperature sensors in the engine now and wire in the resistors as I described earlier.