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[Murray]

This subject has been discussed at length in other threads and I've searched high and low but would appreciate some clarification.Correct me if I'm wrong but the "fuel pump mod" that Peter Florance has devised is required because the Megasquirt connection #37 provides a ground for the fuel pump and what is actually required is +12v to supply the coil of the L-Jet fuel pump relay is that correct ?
The circuit he has devised changes connector #37 to a +12 V output on the DB37- correct ?
The reason I'm asking is that I am building a V3 MS II board and the fuel pump drive circuit is different from the board which Peter Florance modified. (Anybody know why it was changed ?)
Has anybody simply used an external relay to address this issue ? It would be very simple to use the MS ground on pin # 37 to pull in a relay coil powered from 12V external and supply + 12 through the relay contact to the fuel pump relay wouldn't it ?

[Steve R]

Hi Murray,

The fuel pump mod does indeed just change the polarity of the signal.

The mod sends 12v to the combi relay for the pump, so providing either 0v or 12v is sent at all appropriate times (running, engine stopped but ignition on, cranking, just plain off) I don't see why you couldn't do this externally via other means.

However my 2c is I'd stick with the board mod and leave the loom wiring alone, partly as it would be hard to retro fit L/jet whilst you are getting the feel for MS, partly as it's probably easier anyway. Lets face it once you've built and have MS running you will actually be pretty darned good at soldering and figuring such circuit board related things out!

[Murray]

Steve and largely due to your excellent write-up the assembly process has been a pleasure.I'm only hesitating on the pump mod because the tried and true mod which all you guys have used successfully is not directly applicable to the V3 board,but I'm sure I can figure it out eventually.

[Murray]

I'm in the final stages of understanding this fuel pump circuit and would appreciate some clarification.
In this circuit, transistor Q2 provides ground for the pump when the processor provides voltage through FP-1 and R16.What is the function of transistor Q19 ? The zenor diode D4 is there simply to protect against voltage spikes from the pump circuit ?
Once I can fully understand this circuit I'll feel better about modding it to provide +12V to the pump.
Thanks