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84 vette crank no start

Old 05-13-2018, 11:31 PM
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HPaddic3d
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Default 84 vette crank no start

Hi there. I posted about this issue a while ago and after doing some more research and implementing some changes I feel better about asking for help on this.

Problem: Vette cranks but won't start.

Source: on the 3 pin connector to the wiring harness side of the fuel pump connection there is a blue wire which provides power to the fuel pump. The other 2 are the red wire which provides the float sensor power and the ground wire. The red wire recieves 12 volts strong at the pump and that sensor works. The blue wire for the fuel pump on the WIRING HARNESS not on the fuel pump gets no voltage whatsoever regardless of the key being turned on or not.

What has been replaced: fuel sending unit entirely, fuel pump, and fuel pump relay.

As i said. The fuel pump is not receiving power but the float sensor is. Since this car is pre-VATS I wouldn't imagine that being a problem. I am looking for advice before i go pulling the whole wiring harness in the car out for nothing. Thanks guys!
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Old 05-14-2018, 08:22 AM
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Originally Posted by HPaddic3d View Post
Hi there. I posted about this issue a while ago and after doing some more research and implementing some changes I feel better about asking for help on this.

Problem: Vette cranks but won't start.

Source: on the 3 pin connector to the wiring harness side of the fuel pump connection there is a blue wire which provides power to the fuel pump. The other 2 are the red wire which provides the float sensor power and the ground wire. The red wire recieves 12 volts strong at the pump and that sensor works. The blue wire for the fuel pump on the WIRING HARNESS not on the fuel pump gets no voltage whatsoever regardless of the key being turned on or not.

What has been replaced: fuel sending unit entirely, fuel pump, and fuel pump relay.

As i said. The fuel pump is not receiving power but the float sensor is. Since this car is pre-VATS I wouldn't imagine that being a problem. I am looking for advice before i go pulling the whole wiring harness in the car out for nothing. Thanks guys!
Bumping for Monday crew.
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Old 05-14-2018, 01:42 PM
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Originally Posted by HPaddic3d View Post
Bumping for Monday crew.
There's 12V at the pump, but it isn't pumping/there's no pressure? I've had a bad pump out of the box on my girlfriend's 86, if there's power at the top of the tank and yet it isn't pumping, either the sending unit wiring is bad, or the pump is bad.

Edit: just re-read your post, so you don't have power at the pump. Then I'd check fuses, then the ECM pin that commands pump on.

Last edited by TheBlaster9001; 05-14-2018 at 01:44 PM.
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Old 05-14-2018, 01:50 PM
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Originally Posted by HPaddic3d View Post
Bumping for Monday crew.
W/O my tech stuff handy, there is a FP testing 'port' on the ALDL plug under the dash, but I don't recall which pin. Jumping it should activate the pump, but the relays are frequently the problem. Swapping the identical relay from another (like O/D) is a quick test of it.

U have a FSM?
I doubt the entire harness will ever need pulling.

Last edited by whalepirot; 05-14-2018 at 02:13 PM.
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Old 05-14-2018, 02:00 PM
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Originally Posted by TheBlaster9001 View Post
There's 12V at the pump, but it isn't pumping/there's no pressure? I've had a bad pump out of the box on my girlfriend's 86, if there's power at the top of the tank and yet it isn't pumping, either the sending unit wiring is bad, or the pump is bad.

Edit: just re-read your post, so you don't have power at the pump. Then I'd check fuses, then the ECM pin that commands pump on.
allow me to clarify. There is not 12v at the pump. In fact there is not 12v at the wiring harness connection to the sending unit. It is not the sending unit as i have tested the pump and unit and both are newly replaced and work. There is 12v going to the fuel float sensor which is the wire next to the fuel pump's power supply wire.

Last edited by HPaddic3d; 05-14-2018 at 02:01 PM.
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Old 05-14-2018, 02:07 PM
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Originally Posted by HPaddic3d View Post
allow me to clarify. There is not 12v at the pump. In fact there is not 12v at the wiring harness connection to the sending unit. It is not the sending unit as i have tested the pump and unit and both are newly replaced and work. There is 12v going to the fuel float sensor which is the wire next to the fuel pump's power supply wire.
Gotcha. Refer to my edit, as well as the other commenter's post. You should be able to jumper the ALDL connector to force the pump on, that way you can test the circuit. Then you can check what I mentioned at the fuse panel and the ECM 'turn on' signal that goes to the FP relay.
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Old 05-14-2018, 03:12 PM
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Thanks guys. Ill run these fixes and get back to ya.
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Old 05-14-2018, 03:39 PM
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'84 FSM:


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Old 05-15-2018, 11:20 PM
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So im going to hand out sticks for my own flogging. I was testing power at each of the circuit's points and got to the fuse box. The fuse for the fuel pump was shot . replaced it and the car fired right up. Sorry to put you all through the hassle for nothing lol.
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Old 05-16-2018, 10:58 AM
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Originally Posted by HPaddic3d View Post
So im going to hand out sticks for my own flogging. I was testing power at each of the circuit's points and got to the fuse box. The fuse for the fuel pump was shot . replaced it and the car fired right up. Sorry to put you all through the hassle for nothing lol.
Hey, at least its fixed! Fuses are funny, because you check them every time since they are easy to check, but that one time you don't, its a fuse.
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Old 05-17-2018, 12:13 PM
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Originally Posted by HPaddic3d View Post
So im going to hand out sticks for my own flogging.
A core lesson with all new cars. KISS strands for....

Similarly, a vacuum gauge is a great diagnostic tool!
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Old 05-17-2018, 05:32 PM
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By the way on the fuel pump wiring harness at the fuel pump does indeed have 3 Wires.

One is the RED wire which supplies the pump with 12 volt power to turn on. This wire is a little bigger than the purple.

Two is the Purple wire in between the other two, this wire goes to the fuel gauge. This particular wire is smaller than the other 2, it feels like 14-16 awg as not much is needed for the level sensor.

Three is a Black wire which supplies ground to the whole mess back there.

I have never seen power going to a fuel level sensor. I believe that it is resistance that it measures.
When testing my fuel pump I started by powering my connector back there. I had to apply 12 volts at the red wire and then ground the black to get my pump to wake up.

Check your Fuse panel near the passengers end of the dashboard for power at every fuse. I experienced corrosion on this fuse block where they looked okay but had no power or current behind them. I would also check the stud behind the battery in front of the drivers door where many of the fusible links get their power from.

Like people have said before, get a FSM if you don't have one already. On my year (1988) they had two books to make up the manual. The one that people forget to order is the "Electrical Supplement".

Glad to hear that your car is running, now you can go get some smiles per miles again!
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