I'm trying to do the fuel pump hot wire modification as described on Darth's site here --> www.gmtuners.com/fiero/hot_wire.htm


I understand everything except step 8:

quote 8) Now all that needs to be done is wiring up the ECM/PCM's fuel pump relay output control to the Fiero wiring. Simply connect this circuit to the existing Fiero tan/wht (or grey) fuel pump power wire going thru terminal L of the C203 connector. NOTE: DISCONNECT/DO NOT USE the oil pressure fuel pump backup switch when wiring the fuel pump circuit in this way. To do this, simply cut or disconnect the tan/wht or grey wire coming from the oil pressure sender connector.

I'm looking at the service manual schematics and am confused as to what he describing or if this step is even needed. Does this step mean connect the dkGrn/white wire coming from the ECM to the tan/white wire for the fuel pump at C203, therefore bypassing the original fuel pump relay? And if that's the case, why is disconnecting the wire from the oil pressure switch important?

NO

Doing that will blow your ECM. The fuel pump RELAY output of the ECM can not support enough power output to run the fuel pump. It is only robust enough to run the relay.

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Darth's instructions are to install a new relay near the battery, connect the switched power input of the relay directly from the power distribution post, extend the ECM's relay control output to the relay, and then run the switched power output of the relay directly to the fuel pump.

Not sure if that will answer your question. If not please ask and I can be more specific.

[This message has been edited by phonedawgz (edited 06-15-2012).]

So do I need a 5 terminal relay?

Right now I have a 4 terminal...

Sorry for the poor paint skills but THIS is how I interpret the instructions.


Please correct me if I'm wrong. I still don't understand step 8 because the only wire that was changed was the fuel pump power wire at one place and there are no other terminals to go to.

quote Originally posted by ConvictedRedneck: I'm looking at the service manual schematics and am confused as to what he describing or if this step is even needed. Does this step mean connect the dkGrn/white wire coming from the ECM to the tan/white wire for the fuel pump at C203, therefore bypassing the original fuel pump relay? And if that's the case, why is disconnecting the wire from the oil pressure switch important?

I find it odd to disagree with Phonedawgz (so I'm probably wrong ) but I think you have the right idea actually. The tan/white wire originally supplied power for the fuel pump but after following the other steps only functions to activate the new relay. As such, connecting the ECM's DkGrn/Wht wire as you described will allow it to maintain its original function.

As for the oil pressure switch (OPS), originally the ECM relay output and the OPS are not hardwired together. The OPS provided a backup power source for the fuel pump in the instance that that the fuel pump relay wasn't functioning properly. Using the new design, if you were to splice together the ECM's relay output with the OPS wiring then you would be sending 12V to the ECM's FPR output (which originally sees a ground) when the OPS was active. To retain its backup functionality, you could wire the OPS to the new wire that feeds power to the fuel pump at the three wire connector (30). I don't think there are any detriments to doing this, but I could be wrong. Alternatively, you could skip the final step altogether but it would leave you with two fuel pump relays and an overly complex fuel pump circuit. I can't foresee any other problems doing this, but again, I may be wrong.

[This message has been edited by GraterFang (edited 06-16-2012).]

Yeah, that's what I was thinking. The relay output wouldn't be running the pump. It would actually be connected to terminal 86 in my drawing and therefore only control the switching of the relay.

OK - Well I have to admit I didn't understand Darth's instructions fully.

I do agree with what GraterFang states.

I thought the question that was asked was "Is it ok to connect the relay control output of the ECM directly to the fuel pump?" Of course the answer to that is always no.
Also it is not ok to connect any other voltage producing source to the ECM relay control output.

Thank you GraterFang for correcting my mistake.

Glad we're all on the same page now. Thanks both of you for the help. I should be able to get around to doing this tonight, so I'll update on how it goes.


Quick question, what amperage relay and fuse should I be using?

[This message has been edited by ConvictedRedneck (edited 06-16-2012).]

20A is ok, but 30 seems to be the norm.

Thank you, 30A is what I had!

Got to something I wanted to run by the experts...

When I did my 7730 swap, I spliced the "fuel pump circuit voltage signal" wire from GE13 into the tan/white wire at C203 per Darth's instructions.

Now doing the hotwire mod, I'm supposed to splice the dkGrn/white wire into the tan/white wire at C203 as well. Are there going to be conflicting signals on the same wire or is it ok to have both wire spliced into tan/white@C203?