Confusing Fuel Pump

[10cjennings]

While driving my 73 240z began to sputter and die, s if it was running out of gas. I checked and the fuel pump (it's an electric inline pump) was definitely not turning on. Towed the car home, lifted the back end up and after the key was in the ignition turned to where the fuel pump SHOULD work for 2-3 minutes it all of a sudden appeared to have turned the pump on (it was making it's standard noise), but still, no fuel was getting to the carbs. I hooked a Voltmeter up to the fuel pump wires and even though the pump would make noise, and I even saw a small spark when I hooked up the voltmeter it read no volts (I know it was working, read 12V on my car battery). So I pulled the fuel line directly off the pump, ran the pump and it is definitely not pumping gas through its self. I pulled the pump off the car, attached it to another car battery, and then it pumped fuel. So basically the pump works, the fuel lines/filter aren't clogged, and it appears as if the pump is not getting power (though it makes its "whirring" noise). So what I need to do is work my way backwards up the electrical system tryng to find the bad connection but I have no idea where to start. It's a '73 so I'm pretty sure there isn't a fuse for the fuel pump in the fuse box, and I have no idea what wiring behind the dash correlates to the pump. Any ideas on what I should be looking for to identify the fuel pump electrical parts, or other things I should check to narrow down the options?

 

 

Thanks,

 

Chris J.

[JSM]

Chris, I sent you the electrical diagrams for the 240z in PM.

 

In the meantime, put the pump back in. Run a new wire from your Battery to the pump. See if it runs okay this way first. If it does you know it is wiring, which is completely bizzare to me at this point.

 

I would then put the old wires back on and check the pump again.

 

Also did you physically check all the fuses with an Ohm meter and not just by sight?

Edited May 8, 2010 by JSM

[JSM]

Here is the factory FSM scanned that you can download:

 

http://www.xenons30.com/files/FSM/1973%20240z%20FSM.zip

 

Look at page BE-5 when you open the ZIP file. BE Body Electrical.pdf

 

Check also the colors on the wires to the pump from the car. On page BE-4 (Diagream 4) it shows the fuel sending unit wire as Yellow "Y" going to the gauge and I think the Green "G" is what is powering the pump. You may need to get some rubbing alcohol and rag to rub down the wire to determine the true color.

 

The Original 73 Pump was mechanical. So the wire being used to power the pump I believe was / is for something else. That was how it was hooked up when I had the car originally.

 

If all else fails you could run a new relay to the pump if that wire is definitely not getting power.

Edited May 8, 2010 by JSM

[10cjennings]

Well, I hooked up the pump to a car battery, and it worked. Put it back into the car, hooked up the wires, and it works (and I'm now getting the correct reading from the voltmeter on the pump wires), so the electrical problem has solved itself (for the time being). I'm sure w.e. it is will resurface again so I'm going to try and figure out which wires correlate to the pump so when it does I'll know where to look.

[JSM]

Good idea. I would run a new wire and add a relay.

[JSM]

Another thought is that when I replaced the pump the connection wasn't solid for some reason. Cut off all (car and pump) the connections and replace with new ones. I didn't have any new ones at the time and reused the old ones from the pump.

[JSM]

Any updates?

[Zmanco]

The fuse for the electric fuel pump on the 73 is an inline one attached on the side of the main fuse box. Open the lid on the console and look around as it may be "floating" after all these years. The contacts on the inline fuse holder and/or the fuse could very well be oxidized after all these years. Mine were.

 

Also, not sure if this is relevant to the issue you are seeing with the pump, but you should NOT see a spark when connecting a voltmeter to 12 volts. Something else is wrong. For example, are you sure you had it set to measure volts and not amps? Did you have the leads plugged into the correct sockets? Even a cheap volt meter has a high enough internal resistance that it will not cause a spark when properly set to measure voltage.