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1975 280 z Fuel Pump has no power


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Hi there,

I am having trouble with my 1975 280Z. I just bought this car and am trying to understand this beauty. I saved it from going to the junker. (:

Here are my observations followed by my questions.:

1). The original wire to the fuel pump does not have any power. The previous owner ran a wire directly from this rectagular white block in the engine compartment next to the coil, it seems, to the fuel pump. I don't what that block is called. It is rectangular, white, and has a wire from the coil going to it and other wires attached as well. When I turn the ignition to "On" the fuel pump comes on.

2). The fuel pump RELAY clicks when I turn the ignition on.

3). The engine turns but does not start. Sometimes there is backfire from the intake.

Questions:

A.) Should the fuel pump turn on when I turn the key to the "ON" position or should the fuel pump turn on when I turn the key to the "START" position?

B.) Why doesn't the original wire to the fuel pump (by the fuel tank) get any power?

C.) Does the fuel Relay have anything to do with the wire at the fuel pump not getting any power?

D.) Does AFM have anything to do with the wire at the fuel pump not getting any power?

E.) Where does the wire from the fuel pump lead to? Does it go to fuel relay, ECU, ????

Somebody please help.

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Sounds like it might be a combination of things. I have a 78, and the fuel relaying system is different. Let me check the manual real quick.

(5 min later) OK I'm TERRIBLE at explaining these things, but here's a link to the best EFI manual made in my opinion for the 280z's

http://www.atlanticz.ca/zclub/techtips/efisystem/280zfuelinjectionbook.pdf

page 23 talks about the fuel injection/fuel pump relays. If you follow the schematic, I'm sure you'll be able to figure out why it's not getting power.

Mine was bypassed for some apparent reason, but I hooked it back up normal and it works fine. (but i bypass it as an anti-theft measure)

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On early 280s there is a switch in the AFM that prevents the fuel pump relay from sending power to the fuel pump unless the engine is "running". Air flow through the AFM, even during cranking, is what trips this switch. On later 280s there was an oil pressure switch (part of the oil pressure sending unit), but the early 280s did not have the oil pressure switch.

So, with the key in the "on" position you should get voltage to the fuel pump IF you rotate the AFM flap slightly by hand. You can do this by removing the front AFM boot and pressing the flap open slightly (stick a rag in it to hold it open). You could also remove the black cover from the AFM and rotate the counterweight that is connected to the flap. If you still don't get voltage to the fuel pump while the flap is rotated then you've got other problems to find.

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Thank You. I will start with the AFM to see if there is a wire that is connected to the switch in the AFM. Is this switch for the "potentiometer".. ?

I looked at the AFM this morning, before I read your helpful comments and don't remember seeing any wires going to the AFM..hmm. maybe its at the bottom??

....so, basicly if the AFM was working properly including the switch, then the fuel pump would turn on ONLY when I cranked the engine. The fuel pump would not turn on if I turned the key to the "ON" position.. am I correct?

Thanks.

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I really don't know how the 75 EFI is set up as I don't have a 75 FSM. Not sure if it's the same as a 76 which I do have an FSM for. I do know that the 78 is different from all of them in the way it activates the fuel pump, in that is doesn't have fuel pump contacts in the AFM, so my experience isn't of much help. But, contrary to what m4xwellmurd3r says, I think that best EFI manual made for the 280z's would be the FSM for the particular year 280z that you own.

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Thank You. I will start with the AFM to see if there is a wire that is connected to the switch in the AFM. Is this switch for the "potentiometer".. ?

I looked at the AFM this morning, before I read your helpful comments and don't remember seeing any wires going to the AFM..hmm. maybe its at the bottom??

....so, basicly if the AFM was working properly including the switch, then the fuel pump would turn on ONLY when I cranked the engine. The fuel pump would not turn on if I turned the key to the "ON" position.. am I correct?

Thanks.

The AFM connector is indeed on the bottom. The switch is separate from the air flow potentiometer, although it is part of the same mechanism. If you remove the black cover from the side of the AFM you will see a set of sliding contacts at the bottom as well as another pair of contacts near the left side for the fuel pump circuit. There is a picture of it on Blue's tech tips here: http://www.atlanticz.ca/zclub/techtips/afm/afm1.jpg

You are correct that the fuel pump will only turn on when the engine is cranking or running... just turning the key to "ON" or "CRANK" is not enough to send juice to the pump... the AFM switch must also activate the fuel pump relay.

I'm in the same boat as sblake01. I also own a 76, so I'm not POSITIVE that the 75 and 76 are the same... but I ASSUME they are the same since my Datsun service manual says it covers 75-76 models.

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Looks like I learned something today. Turning the key to the "START" position should be enough to send juice to the pump. Here is a snippet of text from the service manual...

When the ignition switch is turned to the START position for cranking operation the fuel pump is actuated irrespective of the position of the air flow meter contact point. After starting the engine (the ignition switch is ON), the air flow meter contact turns ON through rotation of the engine thereby actuating the fuel pump. If engine stalls for some reason, the air flow meter contact is turned OFF, and the puel pump is stopped, though the ignition switch remains in the ON position. In this manner fuel supply is cut off for safety purposes when the engine accidentally stops during driving.

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I looked at the AFM and there are two sets of wires coming in. The first set plugs into the AFM at the bottom and the second is a single black wire. That wire was not hooked to anything.!!

Is that for ground? Maybe that is why the switch did not send signal to fuel pump?? Where should that wire connect to?

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ok.. so I got a test light, electrical connection cleaner and I pulled the plug out of the AFM unit, cleaned it and turned the ignition on.

I used a simple test light to touch each of the pins to see which pins "light up".

I looked at the wiring diagram and only the pins labeled "W39" and "W9" are the ones that light up. The rest of the pins don't.

Should the rest of the pins light up if I crank the engine?? or should all the pins be lit up?

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  • 1 month later...

I also have a 1975 280z coupe, and I am having the exact same problem as you are with your 280z. The rectangle box under the coil would be called a ballast resistor

1) Do your injectors pulse when you turn the ignition key to the on position, as it would to prime each of the cylnders?

2) Did you check to make sure the EFI fuseable link is in good working order. It is the wire circled in the picture. http://i296.photobucket.com/albums/mm195/hybridzmaster/EFIharness.jpg

3. I was told to read this thing called the EFI Bible, created for the 1975 280z it has everything you need to know about the EFI System. except the amp rating for the EFI harness. It also has the diagnostic tests you can perform on the ecu to see if each part of the efi system is functioning correctly here is the link. READ THIS IT IS VERY IMPORTANT http://www.atlanticz.ca/zclub/techtips/efisystem/280zfuelinjectionbook.pdf

I hope this helps you out. read this memorize this, and print it out if you have to.

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I guess when you have an old car, surprises should be expected. I could kick myself just about now. I hope someone will see this post if they are doing what I did.

So... there are two wires at the fuel pump. Per the wiring diagram, one is green and the other one is black. When I looked underneath my car, I saw two wires but one was green and the other was red. .. by the way, this is after I fixed the AFM so that the fuel pump would turn on when I cranked the engine.. . Thanks fellas.

I connected the green wire to the negative (-) side of the pump and the "red" wire to the positive (+) side of the pump and wallah!!, the pump would turn on when I cranked the engine. BUT, there was no fuel flowing out of the pump.

I could not understand why the color of the wiring on the car was different from the wiring diagram. A bulb went on in my brain and I decided to test the wires using a test light. Upon doing so, the green wire lit up and the red wire did not.

Here is the bummer part. The wires at the fuel pump are original. The sleeve on the red wire was originally black. This was the ground wire. Over the years, the black coating wore off to expose the red sleave underneath. So when I was taking the "red" wire, I was actually connecting the ground wire to the + side of the pump and the green to the - side of the pump. This was causing the pump to work but in reverse?

...so the lesson is to use a test light to test the wires before connecting. This was a frustrating experience.. the AFM, the fuel pump, the wiring.... but I want to thank you guys for your input. Now I have a fuel pump that works as it should. Thanks!!!

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