Hello
After a few years of faithful service my pickup decided it wouldn't start. As per the thread at bottom I've:
-tested the B+ to ground for power and it's 12V+.
-tested FP to ground and got 248 Ohms, much higher than the 3.4 Ohm they got.

The thread says that would indicate a broken wire to the pump. However when I check the current at the pump (Key On and jumping FP to B+), I get 12V+ (often much higher, like 14v+). The break could be intermittent, but I've done it back to back several times and it's always the same.

As I understand it there are two coils in the COR circuit, could this indicate that the start coil is good but the run coil bad?

There are no error codes showing.

Thanks for any assistance.

Thread: https://www.yotatech.com/forums/f116...-quits-312747/

 

248 ohms wouldn't be a broken wire (that would be infinite ohms). You could have a "green-crusty" connection somewhere; you'd still get battery voltage at the pump because your multimeter pulls (basically) no current, so there's no voltage drop.

It's also not the COR, because Fp is direct to the fuel pump bypassing the COR.

More likely you've got a crummy brush or something in the fuel pump. With your multimeter measuring voltage at the pump to ground, jumper Fp to B+ (key-on). You should hear the fuel pump run, but in your case it (presumably) doesn't run. Do you have >11v at the fuel pump? Then you're not dropping much voltage in the wire to the pump, and all the current is going into your (non-running) pump. Or does it drop to single-digit voltage? There's a good chance you're dropping too much in the wire (green-crusties). It that happens, get a decent piece of wire (18ga would probably be enough, but plain-ole 12 or 14 ga house wire would be even better) to run battery to the fuel pump. (Test it only briefly; you don't have a fuse in that wire and burning up a pump in the gas tank could ruin your day). If the pump runs fine then, the problem is in the wire. If it doesn't run then, replace the pump.

 

Thanks. I should have mentioned that I replaced the pump. I'll check the numbers again, but sometimes the voltage at the tank would be higher than at the battery, which didn't make a lot of sense to me.

Another strange thing was that it would measure 12v+ from the wiring harness connection at the pump, but it wouldn't run a pump from there (I had the old pump, still good, and another pump that I'd bench tested and was fine, neither would run).Is that something to do with enough voltage but not enough amperage?

Thank again.

 

Nor to me. That's almost certainly "measurement error."

Your voltage-drop is current x resistance. Say your crusty wire has a measly 2 ohms resistance. Your fancy digital multimeter pulls micro-amps, so with 2 ohms your voltage reading would be a handful of micro-volts low. Too low a difference to show up on your 12v scale. My fuel pump pulls about 4.3 amps. With 2 ohms that's an 8.6 volt drop, leaving only 4 volts, which isn't enough to turn the pump. (I'm simplifying the computation here, but you get the idea).

The easiest test, as I described, is to run a piece of wire straight from the a battery to the fuel pump (either the one in the tank or the old one on your bench). If that runs the pump, you've got a high-resistance connection somewhere between Fp and the fuel tank (which does NOT include the COR).

 

Granet, THIS:
Free, simple, easy, advise from the expert.

 

Often the ground for the tank/fuel pump corrodes badly over the years.

Don't forget, when checking a circuit, that the ground is just as important as the hot. You think you have good voltage available to run the fuel pump, but it doesn't run, check the ground for the pump. I bet it's got corrosion built up someplace, preventing the full current carrying capability of the circuit. Bad ground, the pump won't run, no matter HOW good the input voltage to it is. Even with a wire directly from the battery to the pump, if the ground isn't any good, it won't run.

Hope this might help somewhat...
​​​​​​​Pat☺

 

Thanks very much for the input. A couple of things:

-both the pump and the pump mounting bracket are new, so grounds are good.

-in the truck the pump is hard to jump a wire to. I can pull the tank, but since I know I have 248 Ohm resistance at B+ to ground I'm following that first.

-I connected an old pump to the fuel pump connection at the tank (I can connect to the harness side as I have an extra tank side connector) and the pump runs with Fp-B+ jumped.

-Previously I had re-installed the old pump and pump bracket back in and it fired right up, but was spraying fuel from the metal tube at the top of the tank, hence the new mount. My guess at the time was that I'd jiggled a connection that had been corroded.

-I tested the new fuel pump in water. I've since learned that's stupid and can seize the pump. It was a 20 second test, but the water stays in there. My bad.

My current guess is I may have two problems:
-a bad connection somewhere in the circuit, creating the 248 Ohm resistance at B+ to Ground. The strange thing here is that I'm showing the resistance, but I'm also able to run/turn over (I haven't tested pressure) an old fuel pump at the tank.

-possibly a seized fuel pump from testing it in water

I'm going to chase down the 248 Ohm resistance and go from there.

Any other thoughts let me know, thank you!

 

Edited as the meter was faulty. Results with new meter below.

 

Oh, yeah, that's right. When components are new, one never has any reason to suspect that the part might be bad from the store, or that it was installed incorrectly, or that some other part broke during the installation. The 30 seconds it would take to check continuity-to-ground would be much better spent asking questions on the internet.

2ToyGuy raises a good point. The fuel pump is not grounded through the mount (like a starter) or the tank; it has it's own ground wire. If you have 248 (or 2) ohms in the ground wire, the fuel pump won't run. If you're testing the power lead with a fuel pump to a good ground, you'll miss the problem.

There's no way your battery has an 18v charge (I assume the engine is not running; a dead voltage regulator can let the alternator to get that high, and it will fry the battery). So everything you do with that meter is suspect. You might be able to work around it, but for the money there is just no point. https://www.harborfreight.com/7-func...g_q=multimeter Get a known-good meter.

 

Checked with new meter and got:
Battery: 12.37 v
Harness at pump: 12.1 v, plugged in extra fuel pump and works well
B+ to Ground: 247 Ohm (that's with and without the fuel pump connected to the harness)

I bench tested the fuel pump/bracket before installing, everything worked fine. However a few days later an older pump that had tested fine wouldn't turn, knocked it a few times and is now working. I suspect the water is jamming the pumps (?), so pulling the tank again. But the 247Ohm indicates there's still a problem somewhere.

 

Pulled out new pump and the connection was a bit dodgy, fixed that, put back in and fired right up.

Runs fine and have restarted several times, but B+ to Ground is still showing 247 Ohms. Should that be possible?

Thanks.

 

I assume you mean FP to ground. (B+ to ground is a broken ohmmeter. Or at least a blown fuse in the meter.)

Are you still using the meter that gave the odd voltage readings? if you put the two leads together, do you get less than 0.3 ohms?

While FP is "straight" to the fuel pump, so is pin 1 of the COR (there's a splice between the two). So I guess you could have a bad length of wire from FP to that splice, while the wire from the COR to the splice (and on to the pump) is fine. You could check that by jumpering FP to B+ with key-on, and listen for the pump. If there really is 248 ohms between FP and ground, the 50ma or so you can push through it won't run the pump.

But if fixing the connection seems to have resolved things, at some point you might want to stop looking this gift horse in the mouth.

 

Scope thanks for pointing that out, I was testing Ohms on B+ to Ground, I got the image and text from the other thread mixed up. I'll check FP to Ground in the morning with the new Meter.

Ya I'm very happy it's running, I've just been looking further so that I don't miss something and get stranded again. My best guess so far is that it was a crusty connector that got sorted when I removed/reinstalled the tank.

Thanks for all the help!