Hardware/Circuit design v.fuelpumpoutput

[Six_Shooter]

Hello.

I'm getting into my first full fledged MS2 install. I've been using Delco ECMs for years in conversions and decided to get an MS2 to contrast and compare to the Delco stuff I've been using. This will be going into my already running 1973 Datsun 240Z, that has a turbo GM LX9, and previously had a turbo Nissan 2.8L I6 also running off the Delco.

Anyway, I have most of the mods done for my applications, still need to set up boost control (not worried about that), and want to modify the fuel pump relay drive circuit.

In my application, the fuel pump relay is driven with a positive from the ECM (for safety/crash reasons), and would like to do the same with the MS2, using the proto area, etc. I'm not worried about how to go about that, I have several different PNP transistors or P-channel MOSFETs I can use for the final drive portion. I'd rather not simply use an external relay, to invert the signal (like most people seem to do) because it's overkill IMO to have a 30A relay driving a 150mA circuit driving the coil of another relay. ;)

My question pertains to the Fuel pump drive circuit (and IDL circuit) of the v.3.0 board. I see that a ZXT450 NPN transistor (Q2) is used as the actual drive transistor for the fuel pump output, but that there is also a 2N3904 (Q19) connected across the base and emitter of the ZXT450. What purpose does this addition of the 2N3904 provide? Is it just to pull the base of the ZXT450 harder to ground for holding reasons? Take some of the load off the MS uC pin?
This circuit intrigued me, but I have not been able to find anything online about this layout, so I figured I'd ask here to see if there's any insight to it.

I also looked at the v.2.2 board schematics and see that on that board it was simply a 2N2222 transistor for the fuel pump replay drive circuit, so that has to be a reason for the change that I'm not familiar with.

My plan in modifying the Fuel pump drive circuit is to eliminate the existing circuit as it is and build on on the proto area to provide the positive signal out that I need

Thanks.

[Matt Cramer]

The extra transistor is for current limiting. If the current through the fuel pump output gets too high, the input on the second trigger triggers and shuts the output off.

[DaveEFI]

You don't need to use a 30 amp (normal auto relay) to change from a ground output to a +12v one. A much smaller (and cheaper) 1 amp or so relay with a 12v coil will do - lots of varieties from Digkey etc. Small enough to be wrapped in as part of the loom - to save modifying the MS.

[Six_Shooter]

The extra transistor is for current limiting. If the current through the fuel pump output gets too high, the input on the second trigger triggers and shuts the output off.

Interesting. I might have to build this on a breadboard and do some practical testing.

Now that I look at it with the current limiting safety in mind it does make some sense. Thank you.

[Six_Shooter]

You don't need to use a 30 amp (normal auto relay) to change from a ground output to a +12v one. A much smaller (and cheaper) 1 amp or so relay with a 12v coil will do - lots of varieties from Digkey etc. Small enough to be wrapped in as part of the loom - to save modifying the MS.

Yes, I have a bunch of small relays, but would prefer to do the modification internally. I get to apply some skills I've learn at school (Electronics Engineering), and I don't have things attached to the harness or taped into it, call it OCD, but I don't like parts just simply spliced into harnesses. ;)

[WillExoIX]

You don't need to use a 30 amp (normal auto relay) to change from a ground output to a +12v one. A much smaller (and cheaper) 1 amp or so relay with a 12v coil will do - lots of varieties from Digkey etc. Small enough to be wrapped in as part of the loom - to save modifying the MS.

Yes, I have a bunch of small relays, but would prefer to do the modification internally. I get to apply some skills I've learn at school (Electronics Engineering), and I don't have things attached to the harness or taped into it, call it OCD, but I don't like parts just simply spliced into harnesses. ;)

A 1a relay should be small enough to fit into the MS case IIRC. Ive seen some pretty small ones before. Then you can just run the output to the regular FP output wire and separate it from the original circuit so nothing is external or in your loom.

[DaveEFI]

Plenty relays have the same pin spacing as electronic components - so could be fitted in the proto area. There are DIL types that fit a DIL socket if you prefer.