No voltage to fuel pump

[VIKING_MECHANIC]

I was working on my 89 on Saturday trying to figure out why is wasn't starting(unrelated to this - i think). I adjusted the valves yet again and when I had my buddy crank the engine, I noticed there was no fuel coming out the injectors.

Okay, here we go.
First thing I checked was the ignition fuse. Not blown. So I moved on. I listened for the pump as my buddy KOEO, nothing coming from the pump. I grabbed my power probe and checked the body side of harness. I had ground, but only 8.5v. Hmm not good. I also checked to see if the pump was working, which is was. Although that doesn't rule out a pressure issue with the pump itself.

I checked the relay next. I got 12.4v and ground at the connector. Honestly not sure how to test the relay itself, as from what I understand, you have to test the relay with it connected and I'm not sure how to do that. I then next checked the fuse and there was no voltage at all on either side of the connector. I took apart the harness and starting looking for broken wires but everything I could see was in good shape. At this point I stopped as I wasn't sure which direction to go in.

Correct me if I'm wrong, but the fuel pump module was only in the 25 and 3500s with a 5.7 right? I've read that it regulates voltage to the pump, but only in the trucks that have them.

I also have my oil pressure sender disconnected for the last several months and haven't had this issue.

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[Schurkey]

I was working on my 89 on Saturday... ...I noticed there was no fuel coming out the injectors.

I listened for the pump as my buddy KOEO, nothing coming from the pump.

The pump wouldn't provide a 2-second "prime"?

I grabbed my power probe and checked the body side of harness.

WHERE? WHICH HARNESS?

Is this the fuel pump harness near the tank?

I had ground, but only 8.5v.

Pump running, or not running? Harness disconnected?

If the pump is running, 8.5 volts signifies significant voltage drop in the system.

HOW MUCH voltage on the ground side, with the pump running?

I also checked to see if the pump was working, which is was. Although that doesn't rule out a pressure issue with the pump itself.

So the pump runs, but was it providing pressure?

I checked the relay next. I got 12.4v and ground at the connector.

WHICH PINS?

Honestly not sure how to test the relay itself, as from what I understand, you have to test the relay with it connected and I'm not sure how to do that.

Fastest way is to swap in a known-good relay. Otherwise you're taking voltage drop readings across the relay connections. There's no good access to do that. I guess there are special-tools for testing relays, but I don't have 'em.

I then next checked the fuse and there was no voltage at all on either side of the connector. I took apart the harness and starting looking for broken wires but everything I could see was in good shape.

Always check the fuses/circuit breakers/fusible links first when there's a suspected electrical problem.

I have no idea how you can have power downstream, even allows the pump to run, but there's no power at the fuse. Makes no sense.

Get the service manual set for your vehicle from the links in the sticky section. Verify the wire harness schematic. Find out what supplies power to the fuel pump fuse.

Correct me if I'm wrong, but the fuel pump module was only in the 25 and 3500s with a 5.7 right? I've read that it regulates voltage to the pump, but only in the trucks that have them.

The "Hot Fuel Module"? Far as I know, that's a big-block only item.

I also have my oil pressure sender disconnected for the last several months and haven't had this issue.

You'd want to fix that, too. Takes about half the amperage load off the relay contacts when it's working properly.

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I hope the plug wires on #5 and #7 aren't routed like the plug wires on this side--bundled together with a tyrap.

That's no way to route plug wires.

 

[Erik the Awful]

Fastest way is to swap in a known-good relay. Otherwise you're taking voltage drop readings across the relay connections. There's no good access to do that. I guess there are special-tools for testing relays, but I don't have 'em.

With the fuel pump running, put your meter leads on the load terminals of the relay (terminals 30 and 87). If you read anything more than a tenth of a voltage, that's corrosion on the relay contacts that's preventing you from getting full voltage at the pump, causing low fuel pressure.

 

[Schurkey]

^^^ Yes. But how do you get to those terminals?

 

[Erik the Awful]

Pull the relay back about 1/16" and stick your probes in there.

 

[VIKING_MECHANIC]

Okay, so it's been about a month or so and I figured out why the 89 wouldn't start and when I discovered why, I felt kinda retarded.

While I was waiting on the LCA for my C1500, my buddy was helping me with testing the injectors again. With KOEO, I had 12v at the injectors, but not ground. Kool something is shorted somewhere. I was going to stop there, but on a whim I decided to check the fuses under the dash again.

Pulled the Injectors fuse, not blown. Pulled the IGN fuse, BLOWN. Popped in a fresh fuse, turned the key and I could here the fuel pump. Truck started and ran and didn't die when I gave it throttle. No idea why the IGN fuse blew as I didn't find any obvious shorts.