Dholland17

I'm looking to mount a Holly external pump but not sure on how to wire it. The cutout above the tank is there and a new sending unit was installed. I see 2 wires going to the sending unit on top of tank. Underneath there is a brown wire in the loom with the other two ending unit wires, but brown wire was cut. I assume this is the original fuel pump hot wire?? The previous owner converted to a mechanical fuel pump so guessing that is why it was cut??

Looking at the attached wiring diagram confuses me. I think the p/o removed the fuel pump relay and oil pressure fuel pump switch. Were those originally on the firewall near the master brake cylinder? I see the fuel pump fuse on the fuse box is still there.
So how do I wire this? Should I just take the cut brown wire and use that for my new fuel pump and ground it? Do I need the relay?
Thanks all!

Dholland17

Help....please.

John in RI

iTrader: (167)

I assume that your car was a "HO" car ?? Here is the factory wiring schematic for the L69 engine (code G) in 85,.... should be pretty much the same as your 83. ( if the car was an HO.)




If your Z28 is an LG4 (code H) then it did not get a factory electric fuel pump. ( Pull the Fuel Pump fuse and check if there is a terminal on each side )

I don't know what the PO did to the car or what wiring is-or-isn't there at this moment, but if you plan to convert to an external pump I would "build" the system as described in the schematic above. Either re-install/repair what was hacked out,..... or build it yourself 'from scratch'.

If the fuel tank sending unit that's in the tank right now has a fuel pump on it and you plan to run an external pump, your advised to remove the in-tank pump. A stock in-tank electric pump should have a 3-wire connector, The PINK wire will be for the fuel gauge, TAN (or TAN/White - maybe GREY) is power to the pump and BLACK is ground.

Dholland17

I very much thank you itrader. I work in RI, Johnston.

So that brown wire is as I suspected and is hot. Assuming it can handle the Holly mighty might demand. I read a wire upgrade is needed?
Is running without the relay a bad idea?

OrangeBird

iTrader: (1)

First things first , the man who replied to you is named John , and not "itrader" .....

Now , with that out of the way , there is one very important detail here that's missing , the detail that will determine what can/should be done with your electric fuel pump plans ;

Does your car still have the original "CCC" carb (Computer Controlled Carb) or has it been changed out to a Holley or some other non ECM controlled carb ?

If the stock ECM controlled carb is still in place with a functioning ECM , all you need to do will be to replace the relay (which is supposed to be near the brake booster) and the oil pressure switch (screwed into the engine block) , and repair any cut/hacked wiring ....

BUT ! If the car does have a non ECM controlled carb , and if so has likely had the ECM hacked out , you might as well scrap the external electric pump idea and stay with the mechanical pump .

Why do I say this ? Because it's both the ECM and the oil pressure switch (in parallel as the wiring diagram shows) that run the electric pump and with no ECM or oil pressure switch there will be nothing to provide power to the electric pump . The idea of the ECM/Oil pressure switch combo is that should the engine stop for any reason the fuel pump stops . Sounds simple enough , yes ? Well the problems arise when in a no ECM situation someone tries to run the pump straight from the ignition or from a toggle switch , bypassing the original circuits , to get the pump to run . Such a "hotwired" pump will not turn off if , for instance , the car is involved in an accident bad enough for fuel lines to be broken and then the pump keeps running , literally "adding fuel to the fire"

So , I await your answer , ECM present and functioning ? Great ! fix the original wiring and run your electric pump . ECM & CCC carb gone ? Bummer ! Skip the electric pump and go with the mechanical .....

PS , a mechanical pump is good up till 500 or so HP , and there are damned few of these cars actually putting out an honest 500 HP .......

Dholland17

Thanks Orange Bird, and John

The computer was hacked out. Is that why the fuel pump wire has no power? The sending unit wire does??

I bought a 1996-1999 Vortec 350 engine that does NOT have the fuel pump hole provisioned so can not run the mechanical pump.
There is a hol alreay cut in on top of fuel tank area as the po cleaned the tank and dropped in a new sending unit. It looks new on the outside but I have not pulled the sending unit to look inside.

So, I need to run the electric fuel pump. Can't decide to how wire it and where to mount it. Rear frame rail looks best.
Can I just run it to an ign on hot wire for now and ground it?
Should that wire come off of the fuse box? The original fuel pump wire is still in the rear but it looks thin. I'm thinkind I should run a new wire but from where?? I see an IGN slot open on the fuse box, and the original fuel pump fuse (dual legs) that I can't get a voltage reading on at all.

I have Dyno Don headers coming in a few weeks and am trying to get this engine ready to drop in. Fuek pump has me scratching my head tho. I know it isn't that difficult I just can't determine where to wire it in. How do I tap into the fuse box panel to power this pump?

John in RI

iTrader: (167)

Wire it properly and save yourself from a possible car-becue. Seriously; The schematic above is a pretty basic system to build. If you want to increase the wire gauge on the pump's power wire you will not harm anything. The Purple wire shown on the schematic is the same exact circuit as the PURPLE wire on the auto shifter Neutral Safety Switch - OR - The PURPLE wire on the Clutch pedal switch. EVERYTHING else that you need top know to wire this up is shown on that diagram.

The oil pressure cut-off isn't an accessory "nice to have",....... in VERY REAL TERMS: it's something that could very well could keep you from burning to death.

Dholland17

Thanks again John.
The pink wire for the fuel sending unit, should that be 12v?
Mine works but is reading as 2.5V??
-Dave

John in RI

iTrader: (167)

No sweat bud !!

The PINK wire leads to the fuel gauge. It's readings will depend on how much gas is in the tank.

Dholland17

So I just tested it again. It reads 9V if I disconnect the pink from the sending unit top of tank. When it's connected to the sending unit it reads just over 2V.
Does that sound right?

John in RI

iTrader: (167)

I'd need to duplicate your steps in order to give you an answer for you to compare to,..... I just can't help with that right now. I'm sure the shop manual will have some type of "flowchart" testing steps you could perform to verify the gas gauge circuits..........

Basics = ( in a functional set-up, key turned to RUN ) If you disconnect the plug at the sending unit,......... the gas gauge needle will travel to the top of the scale; plug it back in and it should return to the level indicating the amount of fuel in the tank.