What's up guys. Sooo...gonna be a lot of these similar questions probably coming up in the next few weeks. I just purchased the Holley HP EFI kit for my 89 Iroc. ....and being from 89, the engine harness and chassis harness are all integrated. I went ahead and peeled away all the split loom tubing and tape, and separated as much of the TPI harness as possible, and as of now, it's just zip tied together up and out of the way.

As far as I know, everything else except the ECM (and obviously the engine) should work right? ...and that leads me to my question...

....Holley has a non-pigtailed lead for an electronic fuel pump wired into the harness. Will my current existing fuel pump set up work as it is currently wired up? Factory wiring??? ...and simply NOT hook up the Holley pump lead??? That's my understanding, but electrical always messes me up. Figured I'd ask.

thanks,
Chris

 

In stock configuration , there are two ways your fuel pump can be activated . One way is the oil pressure switch and the other is the relay that is (in your case , was) controlled by the ECM . To retain the same 2 second prime functionality of your old ECM your going to have to hook up the new ECM's fuel pump drive circuit as per it's manufacturer's instructions .

Otherwise , it's likely to need to crank over long enough to build oil pressure before the pump comes on and pressurizes the rail . This will take a longer cranking time than most would find acceptable , depending of course how fast the engine builds oil pressure while cranking .

Bottom line = Hook up the new ECM's pump circuit .....

 

OK cool. That all makes sense. ....so I just find the wire (tan and white I think) coming from the fuel pump relay and going the old ECM, right? ...and connect the new ECM to that? That simple? ....and all the physical pigtails/wiring/connections between the relay and the actual pump stay the same?

 

Looking at the wiring diagram from austin third gen I see that to activate the fuel pump relay the original ECM puts +12V power to a dark green with white tracer wire . It is that dark green/white wire your going to want you new ECM to put power to so that the relay switches the pump on .

I have no schematic for your new ECM , and thus have no way of knowing the "logic" of it's fuel pump signal (as in , does it switch the + or the - side of the circuit to activate the relay)

 

OK, so I actually have that schematic up and running on my other monitor as I'm typing this!

...I see the dk green/white coming from A1. When I said tan/white earlier I just saw the pump. ...and from what I understand, although I do NOT have the diagram up, the Holley FP lead is supposed to connect to THAT wire. ..AND, it's green so maybe that makes sense. Either way...I'll obviously double check. ...that green and white wire from ECM to the relay is what I was looking for.

....sooooo if you don't mind, can I ask you another question??? My problem is that I admit, I don't know HOW to actually read that diagram. I mean I understand color obviously, and I understand most of the sensors and relays, but the ECM is sending and receiving all these signals, right? What tells YOU, that the green/white wire is the 12v source to the relay? There are all kinds of different colors coming off that relay and tying into other relays, etc. etc. I can only assume it's the symbols??? Again, I have lots to learn of course. That tan/white junctions off in 4 different directions.

Thank you BTW!!!

 

Hi Chris , sorry for the late response ...

When you look at the symbol for the relay you will see that there is on the right side a symbol of a line with an arrow at the end , facing tword two different possible places for the arrow to point tword . That is the symbol of the "switched contact" , those are the contacts that are actually going to switch the power to the device being controlled (in our case , the fuel pump) . Those contacts are generally rated 20 amps , 40 amps , and so on .

Now on the left side , you will see what looks like a spirally wound line and a jagged line that are connected together with one side connected to your dark green/white wire and the other side connected right to ground (battery -) through the black/white wire . That symbol , the coiled one , represents the relay's coil that when powered will become a magnet and cause the switched contact to activate . The reason for this device is that you can switch a great amount of current (for instance , your starter's solenoid , a type of relay , switches better than 100 amps to the starter motor using only a few amps to activate) with a much smaller amount of current . That squiggly line in parallel with the coil is a resistor , likely to keep the relay's coil from pulling in the contact till sufficent current exists in the coil circuit , to prevent contact chattering during engagement .

Anyway , since one side of the coil goes straight to ground , the other side , the +12V side , has to be the one that gets it's power switched to activate the relay . Both positive side and negative side switching are used on relay coil circuits and in this case they are switching the positive side .

I spent a lifetime as an electronics hobbyist so anytime I can help you with questions about wiring I'll be happy to do so .

 

Yeah...I stared at the schematic for awhile, and it made sense. I also see the oil pressure switch and how it's wired in as well. When I KNOW what it does, I can kinda reverse engineer it and figure it out. ...but when I don't know, that's when I'm in the dark.

...I'm probably going to be wiring in the Racetronix hotwire harness when I do the pump swap, and I know that'll change things a bit.....but for now....I'm just concentrating on the factory wiring and the Holley. I may hit you up again, LOL!!!!!

Thanks for your help.
Chris