Fuel Relay Voltage?

[jb99]

So my problem is the fuel pump not running at all, and I'm chasing down electrical gremlins. I've been reading a lot of other threads with helpful ideas, but haven't found this answer yet... What is the correct voltage reading for the orange wire on the fuel relay? I'm getting 12v with everything off, but if I turn the key on, it's dropping to 2v. I'm assuming it should stay 12v?

I'm also only getting 8.5v at the fuel sending unit, which I believe should also be 12v. Can anyone confirm? And yes, I will be double checking & cleaning all grounds.

 

[Erik the Awful]

With the circuit de-energized all the current flows through your meter and you read 12v. When you turn the key on there is an alternate path and the current through your meter drops. "Theoretically" it's a parallel circuit and you should still read 12v, but it doesn't work that way in practice.

Have you checked your grounds? If they're grungy, frayed, loose, etc., you'll need to fix them first.

 

[jb99]

I've cleaned every ground I could find, most of them looked ok but I cleaned them anyway. I'm now getting 2.5v with the truck off, which seems to be a step backwards. I also checked the 20A fuse, it's good but has no power running to it. Should that be hot at all times?

 

[Schurkey]

WHAT VEHICLE?

 

[jb99]

WHAT VEHICLE?

Sorry - '94 K1500 with the 5.7 TBI.

Planning to check the ECM B fuse, as well as the fusible links on the junction box. I cleaned and inspected them already, but maybe I missed something.

 

[jb99]

I have continuity on the orange cable from the fuel relay side to the ECM side, but I still do not have 12v.
-If I jump 12v to the fuel pump directly, the pump works.
-If I jump 12v to terminal G, the pump works.
-If I jump 12v to #920 in the first diagram I've attached, the pump works.

So the question is, why am I not getting 12v to #490 (labeled red/white, but mine is orange)? Is the first connection on the junction box the "fusible link f" that's mentioned in the second diagram I've attached?

 

[Donald Mitchell]

I would pull the wires of that distribution block and wire brush those terminals.

 

[jb99]

Just took them all off & cleaned it for a second time; no change. Based on the "Fuel Pump Wiring" diagram, I would expect there to be continuity between the orange wire (#440) and one of the terminals on the junction block. I couldn't find any, so now I'm wondering if one of the fusible links is bad, despite not visually seeing any damage.

 

[Schurkey]

I have continuity on the orange cable from the fuel relay side to the ECM side, but I still do not have 12v.

The orange cable (440) from the fuel pump relay doesn't go to the ECM. It goes through a fusible link to "hot at all times" system voltage, at the power center over the A/C evap. If you don't have power there, the pump won't run.

-If I jump 12v to the fuel pump directly, the pump works.
-If I jump 12v to terminal G, the pump works.
-If I jump 12v to #920 in the first diagram I've attached, the pump works.

So the question is, why am I not getting 12v to #490 (labeled red/white, but mine is orange)? Is the first connection on the junction box the "fusible link f" that's mentioned in the second diagram I've attached?

The question is NOT why you don't have 12V on 490. 490 is red in one illustration, red/white in another. Orange is 440. The question is why you don't have 12V on 920. I'm guessing you don't have power on 920 because you have no power on 440.

490 goes to terminal G on the ALDL, and you have power there when you jump it. That's the only time you'd have power there. Nothing wrong.

Do you have power on orange 440?

Just took them all off & cleaned it for a second time; no change. Based on the "Fuel Pump Wiring" diagram, I would expect there to be continuity between the orange wire (#440) and one of the terminals on the junction block. I couldn't find any, so now I'm wondering if one of the fusible links is bad, despite not visually seeing any damage.

Yup. That's where I'd be looking. Trace the orange wire 440 from fuel pump relay to Splice 114, check for power. If no power, check for power at the stud that 440 connects to. If there's power at the stud (there better be!) but not at the splice...the fusible link popped or there's some other wire damage.

 

[Erik the Awful]

I'm wondering if one of the fusible links is bad, despite not visually seeing any damage.

Fusible links don't always show when they're blown. Basically they're a too-thin wire with an extra-thick layer of insulation, designed for the wire to safely melt without melting through the insulation. Sometimes they actually work that way and you can't tell they've blown. They're a bad idea that lasted too long.