Fuel Pump Not Powering On - 89 Comanche 4.0L

[AvianFlu]

On 9/1/2021 at 11:41 AM, Ωhm said:

The Fuel Pump/Sending Unit is mounted to the tank on a rubber gasket, so the FP/S unit itself must find vehicle ground. If your good here, then I'm not sure where to go since Fuel Pump runs when connected to a battery.

 

If disconnected (fuel level gauge), should have no affect.

 

Tough call. I won't spend your money, but seems like vehicle wiring checks OK.

Here is what I have after swapping new pump, no change directly, but I did confirm that it works.

 

I tested with a battery directly, I hooked up negative to C139_B & B+ to C139_C. Pump ran.

Pump does not run with negative connected to C139_A (ground).

 

I had someone else turn to key to ON while I held C139_B on a ground, and held an alligator clip between the connections on C139_C, and the pump ran. So essentially, the pump just runs when the fuel gauge sender is grounded. I know all of the wires inside the pump are correct, should I just cut off C139 and connect the wires myself?

[Ωhm]

Two things here:

1.Is there an open circuit between C139_A on the fuel pump side of connector and the fuel pump unit itself? If so, my suggestion would be to buy a new fuel pump housing unit with sender.

2.Using C139_B for your path to ground, you are now using the sending unit resistor as a ground path/heating element. C139_B would normally be a very low current flow circuit used for operating the fuel gauge, now you are running fuel pump motor current through this circuit. IDK, swapping wires that go inside the fuel tank may not be a good thing.

[AvianFlu]

26 minutes ago, Ωhm said:

Two things here:

1.Is there an open circuit between C139_A on the fuel pump side of connector and the fuel pump unit itself? If so, my suggestion would be to buy a new fuel pump housing unit with sender.

2.Using C139_B for your path to ground, you are now using the sending unit resistor as a ground path/heating element. C139_B would normally be a very low current flow circuit used for operating the fuel gauge, now you are running fuel pump motor current through this circuit. IDK, swapping wires that go inside the fuel tank may not be a good thing.

I wasn't planning on swapping out C139_B for a ground, I'm just sharing what I found.

C139_A is as it should be. I haven't messed with it outside of the testing.

 

I could buy a new housing unit, but is there no way for me to fix this?

 

[Ωhm]

1 hour ago, AvianFlu said:

I wasn't planning on swapping out C139_B for a ground, I'm just sharing what I found.

Understood

 

1 hour ago, AvianFlu said:

C139_A is as it should be.

During ENGING CRANK and C139 connected, fuel pump should run using C139_C for battery voltage (power) and C139_A as path to vehicle ground then back to the battery negative (ground).

[AvianFlu]

57 minutes ago, Ωhm said:

Understood

 

During ENGING CRANK and C139 connected, fuel pump should run using C139_C for battery voltage (power) and C139_A as path to vehicle ground then back to the battery negative (ground).

I guess C139_B is the problem then?

I tested C139_A and get 0 ohms on both ends when backprobing then touching a good ground. Testing B+ on C139_C gets 12v.

According to these tests, it should be working, right?

 

What can I do from this point to further isolate the problem?

[Ωhm]

59 minutes ago, AvianFlu said:

I guess C139_B is the problem then?

Now I'm guessing. Between C139_A connector (fuel pump side) and the negative terminal on the fuel pump motor itself, has a open circuit. Fuel Pump motor is finding ground using C139_B.

 

1 hour ago, AvianFlu said:

What can I do from this point to further isolate the problem?

On the fuel pump motor side check for continuity between C139_A and C139_B. Should read fuel level within tank.

88ohms=FULL

0ohm=EMPTY

[AvianFlu]

39 minutes ago, Ωhm said:

Now I'm guessing. Between C139_A connector (fuel pump side) and the negative terminal on the fuel pump motor itself, has a open circuit. Fuel Pump motor is finding ground using C139_B.

 

On the fuel pump motor side check for continuity between C139_A and C139_B. Should read fuel level within tank.

88ohms=FULL

0ohm=EMPTY

Any easy way for me to test the open circuit?
 

On testing continuity between C139_A & B, looks like I'm getting 25 ohms, which seems about accurate to the amount in the tank.

[Ωhm]

1 hour ago, AvianFlu said:

Any easy way for me to test the open circuit?

I think you just did with that 25ohms reading. I'm at a lost here. I can't see what I'm missing. Anyone else. Pls chime in.

[AvianFlu]

2 hours ago, Ωhm said:

I think you just did with that 25ohms reading. I'm at a lost here. I can't see what I'm missing. Anyone else. Pls chime in.

Does C139_A have to ground onto the fuel pump assembly? I grounded it onto the frame. I know that it's supposed to ground onto the pump assembly originally.

I'm going to need to solder a new connection onto the assembly to ground it on there.

[Ωhm]

On 9/6/2021 at 6:32 PM, AvianFlu said:

Does C139_A have to ground onto the fuel pump assembly? I grounded it onto the frame. I know that it's supposed to ground onto the pump assembly originally.

Fuel Tank with straps are not the best of grounds.