flatbed

I've had an on going issue of EFI 15amp fuse blowing, I suspect fuel pump's blue hot wire is shorting through tank to chassis. I unplugged fuel pump connection and tested continuity to ground on both wires on pump side of harness. The blue hot wire going into pump reads 5 ohm, and the white/red ground wire going into pump reads .5 ohm. Should pump ground wire be isolated from tank/chassis? I assume it's normal for pump ground wire to have continuity through fuel pump bracket mounting bolts not just harness.
Is 5 ohm on the hot wire just resistance through pump motor winding before reaching ground?

Paul22RE

The main reason the EFI fuse blows on these trucks is an 02 sensor wire melted and grounding.

scope103

I don't know the "exact" resistance of the fuel pump motor (and reading 5 ohms with even a good multimeter is hard to get accuracy better than a handful of ohms), but the resistance should be very low. So your 5 ohms sounds like the pump, not a short.

And I think Paul22RE is correct; I'd check the O2 sensor (give the harness the wiggle test) before messing with the fuel pump.

13Swords

Just another thought, but it could also be the COR (Circuit Opening Relay) contacts being pitted or corroded, causing excess resistance that is blowing the fuse. It basically acts as your fuel pump relay. scope103 talks about it's location & functions here:
https://www.yotatech.com/forums/f116...cation-300489/

Info: Toyota Part # 8591035010. Anywhere from $90-some to $130. Wells & Standard make them too & cost less, good quality though: https://www.rockauto.com/en/catalog/...ing+relay,3380

scope103

How would increased resistance blow a fuse?

2ToyGuy

Increased resistance, increased current flow to make the fuel pump function. It will draw more current to function. Too high a current flow, bye-bye fuse!
Pat☺

scope103

Hmmm. Sounds like you've forgotten what you learned with slot cars.

Ohm's law gives us I = V/R (I is current, V is voltage). The voltage "to" the fuel pump is battery voltage, which doesn't change with the condition of the pump or COR (it goes up by about 10% when the alternator is running, but that's true for every part of the vehicle.) So when the total resistance of the system (fuel pump and COR contacts in series) goes up, the current goes down. Never up.

Say the fuel pump is 6 ohms, and battery is 12 volts. Without any excess resistance floating around in the system, the fuel pump pulls 12/6 = 2 amps (this ignores back-EMF produced by the DC motor). Now, let's say that bad COR contacts add 2 ohms. Since they're in series with the pump, resistances add, the battery is now looking at 8 ohms. 12/8 = 1.5 amps. If that isn't enough current for the fuel pump to "function," well, that's just too bad! It can't "draw" any more! (Isolating the components, you get a voltage drop across the 2 ohms of IR = 3 volts. The pump "sees" 12-3 = 9 volts. All the fuel pump can "draw" is that 9 volts divided by its 6 ohms = 1.5 amps)

What do slot cars have to do with fuel pumps? The slot car has a DC motor, and the controller is just a rheostat (variable resistance). At "full throttle" the rheostat is set at zero ohms. As the rheostat moves toward "slow," it's just adding resistance in series with the motor of the slot car, so the motor slows down. The slot car can't "draw" more current any more than a fuel pump can.

old87yota

Increased resistance means lower current and decreased resistance means higher current. The low resistance/high current would be what blows the fuse.

EDIT: Scope103 beat me to it, but with a much more scientific and detailed explanation!

13Swords

I only said that based on my experience and what I've seen. I've worked on several trucks with corroded COR contacts that kept blowing the fuse. Once the COR was replaced with a new one the fuse didn't blow again. No other service was done to the circuit at all, or to any other system of the truck, just a straight COR replacement.

I admit that there could have been something else going on that I didn't know about, and I certainly would not dispute the facts you outlined in your following post. However, it seemed a reasonable assumption though, that just replacing the COR stopped the fuse from blowing afterwards.
If my assumption is/was wrong, it's wrong, but I was just going by what I've actually experienced.

scope103

This is the problem with the "parts cannon." You replaced a part without any particular reason to, you didn't test the part, and somehow the problem went away. So you thought about it carefully, and concluded "increased contact resistance causes increased current flow." But that conclusion violates the laws of physics.

So why does the "parts cannon" hit the target every once in a long while? Your problem wasn't corroded contacts, but if the COR was in such bad shape that the contacts were (visibly?) corroded, one of the two coils could have been shorted. A shorted coil will blow the EFI fuse as soon as your turn the key or the engine starts sucking air (depending on which of the coils is shorted). If you had tested the COR when you pulled it out, you would have known what was wrong with it, and you could have replaced it with confidence (it's not a cheap part; replacing it on a whim is really false economy).

But it isn't always that easy. Your COR may have been just fine. But to get it out, you've got to pull and push on wiring harnesses and connectors. Maybe you had a short in a wiring harness, and your wiggling it caused it to (alas, temporarily) unshort. Maybe you had some conductive crud in the relay socket, that fell out when you pulled out the COR. Without testing, you have no idea if the part you replaced was the real problem. Then, even though you spent the money on the new COR, the real problem just comes back with a little vibration.

2ToyGuy

The reason corroded contacts can increase current flow is thus: In a series circuit, current is common, voltage is additive. In the original circuit, there was I amount of current flowing through the different components, pushed by E amount of voltage. The same current would be felt wherever you measure it in the circuit. Basically, the current drawn by the fuel pump motor, as the rest are negligable, essentially. Each component would cause E amount of voltage drop, which would increase as you proceed through the circuit. The sum total of all the voltage drops would wind up being the total of the battery voltage. On one side of the COR, for example, you would see a certain amount of voltage. On the other side, when it's new, you would see a very slight amount of voltage decrease, as the contacts are clean. They get old and dirty, that voltage drop would increase. The motor, being the only other component would drop the rest of the voltage, and draw the current to drop the rest of the voltage.
Dirty COR contacts, however, would drop more voltage. The motor, having a certain resistance, would then draw less current, and run slower, but is designed to run at a certain speed, and draw a certain amount of current.. The COR contacts would draw more current, having increased resistance, because the motor resistance + COR resistance would add up to a higher total circuit resistance, but the motor would still try to draw the same current. Total current flow would increase, and blow the fuse.

It sounds counter-intuitive, I realize, and I am lousy at trying to explain it, but I've seen it I don't know how many times in DC motor circuits. A corroded connector, a dirty set of relay contacts, blown fuse. Happens all the time.

I don't mean to be argumentative. I don't. But I've seen it many, many times in radar systems. And yes, a lot of radars have DC motor circuits in them for a lot of reasons.
Just going by personal experience. All I've got left
Pat☺

Co_94_PU

iTrader: (-1)

..
​​​​the startup current of your fuel pump is what may pop your fuse, let me say that again "may", since a simple electric motor like this draws a lot of current when it's not spinning. We call this " stall current " and/or " start up current ". In a normal situation this peak current spikes in a millisecond or so and declines as the rotor spins up typically over another 10 or so milliseconds (depending on the load).. However if your pump is stuck in this nonmoving (stalled) state this peak current blows fuses..


Here is a random (crappy) oscilloscope image of what's going on in what I tried to describe.




..
I think the phenomena you tryed to describe is actually a high resistance plug connector, which generates heat, which turns non conductive plastic into conductive carbon and now you two wire isolated connector is a short circuit across said carbon.

scope103

The reason that "start up current" is higher (in any motor) is the back-EMF. (The motor actually acts as a generator, so as it spins up the back EMF (voltage) reduces the current through it. )

"Start-up current" is really only an issue with electric motors that START under heavy load. (Elevators, certain machine tools, ...) An automotive fuel pump is starting with (basically) no pressure in the line. By the time the motor is doing any work it's up to full speed (full back-EMF). I don't know if a stalled fuel pump would draw more than the 15 amps it would take to blow the fuse, but I doubt it. And I don't think it would have anything to do with increased resistance in the COR.

I suppose the radar sets 2ToyGuy worked on might have had stalled motors that blew fuses. (Radar sets are notorious for having the mechanical bits designed with less than no margin of error.)

Where I come out on all of this is: sure, test the COR, it won't hurt. It could have a short, or I could even be wrong. But don't replace it (or any other part) just on a guess-and-by-golly. There are more likely places to look.

Co_94_PU

iTrader: (-1)

Here is another one...

Geo metro fuel pump prime

Paul22RE

So how do engineers calculate the fuse size in these situations with an initial spike? In your Geo example, is seems a 15A fuse would be cutting it pretty close.

Co_94_PU

iTrader: (-1)

Even the non slow blow fuses take longer than that to heat up.

I²t =nominal melting
Or
A²Sec

Is a rated characteristic you'll find in fuse specifications. Which has to do with how fast you have to get the current below the fuse rating. There are various formulas based on the wave form of the surge/pulse.

Have a look at some of the application guides from fuse manufacturers.
(google: Littelfuse fuseology, for an example)

..
Take for example the data points above, a five ohm winding. I= V/R.. This will pull something in the neighborhood of two amps at a minimum (ignoring the shaft and impeller weight, with no load from actually doing any pumping, ignoring the EMF, and any commutator changes.)

... You could get that fuse to blow by triggering the fuel system prime fast enough, similar to exceeding the FL cool down time by repeatedly activating your starter.

Co_94_PU

iTrader: (-1)

5 ohms doesn't seem to out of line for motor windings.

0-0.5 ohms is fine assuming that is actually the ground wire..

(Not pulling up schematics)

Easy enough to check if your pump or pump wires in the tank maybe involved. Unplug it and jumper the test port to energize the pump wires. If the fuse doesn't open its probably worthwhile to open the tank and inspect further.