280zx Won't Start - Low Fuel Pump Voltage

[mazikowski]

I have a non-turbo '81 280zx that has decided it doesn't want to start. To give a little background it had been running just fine but had some teeth missing on the flywheel. It drove into the garage completely under its own power (after a push-start, lol) where I placed it on stands and replaced the fly and clutch. I am also replacing the rear calipers, for what it's worth.

 

Anyway, after I got the transmission and everything back in I wanted to start it and make sure nothing was binding or making weird noises. The starter works and the engine turns just fine but won't start. I checked spark at the distributor and I'm getting a solid arc between the cap and the wire... enough to jump through the rubber dipped handles on my pliers. Next thing I checked was the fuel pump. When the ignition is turned to the ON position I can hear the pump try to spin but it makes more of a dull thud sound and that's it. I pulled it off and used some test leads to hook it straight to the battery and it ran just fine, spewing what fuel was in it all over the garage. I plugged it back in without the fuel lines attached and, again, all I get is the dull thud. Using a multimeter to check the voltage at the fuel pump plug (green and black wires) I only see 7-8 volts at the connector while cranking.

 

Can anybody think of a reason for the low voltage at the connector? I am considering just wiring the pump up to the ignition through a new relay, but would rather fix it proper if I can.

[Tony D]

What is the voltage at the battery when cranking? It drops, and that combined with wire drop and it can easily drop that much if the battery is weak

[mazikowski]

The battery reads about 12.5v just sitting and the voltmeter in the dash shows it drop to about 10.5-11v when cranking... I don't know if I exactly trust the accuracy of that meter, though, so I will try to check it with a multimeter tonight. I just don't know what could have happened between driving it in the garage and now. Worst case, I do what I was considering originally and run some new wires to the pump.

[mazikowski]

Checked the voltage at the battery while cranking and it shows 10.5 just like the voltmeter. I wasn't able to work on it this weekend, but it looks like I will just be replacing the wiring back to the connector.

[supernova_6969]

one thing you can try is plugging the pump straight to the battery for a test (with a fuse).  if the pump works, you know the issue is the original wiring. 

 

if the pump does not work any better, you know something is wrong with the pump. 

 

maybe the pump has a hard time pumping when under load, but without load (say, on your bench to test it), it runs fine..

 

I think another thing you could do is test the load (amperage) that is being used by the pump.  if it's straining, I believe you'd get a lot of amps.  don't quote me on this though, and look up the numbers you're supposed to get online.