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PostPosted: September 6, 2022, 12:48 pm 
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Location: Guadalupe, CA
I discovered an odd unexpected thing during a recent fuel pump failure, and I'm wondering what exactly went wrong..

My car is a BEC, and I'm using most of the donor motorcycle parts in the car, with the inline aftermarket fuel pump being an exception (it's just a generic Jeggs/ Summit 50 PSI off the shelf unit). On a recent trip, the car had been running great for about 200 miles, but then wouldn't restart after a short break. A little trial and error led me to an inoperable electric fuel pump- it was 100% not working; dead. Long story short: The pump was fine, but it had a poor ground wire, so I put a temporary different 'jumper' ground wire on it and it got me home. Once home, I fiddled around with my 'original' ground wire (the one that failed), and I noticed that sometimes (not always) it would engage the pump, but the pump ran noticeably slow. Then I switched over to my 'new' (shorter and thicker gauge wire) ground, and the pump ran noticeably faster (it ran like it should).. So the issue is the ground wire, no doubt, but I'm confused by the wire that failed and why. It apparently offered a weak ground that sort-of worked sometimes? How was it that the bad (perhaps too thin??) wire was able to spin/ground the pump at all? I thought grounds were essentially an all or none thing? It was almost like the bad wire would spin the pump on 'low' speed, and the good/ thick ground wire would spin it at 100% speed.

In short, why did the first wire fail (and how do I avoid this again)? Too small (thin gauge)? Too long a run on too thin a wire? (I did extend the bike's OEM fuel pump wires about +9 feet to reach the inline pump at the rear of the car, and I used thin-gauge wire that matched the bike's wiring to do it).. I'm thinking I'm asking too much from too-small a wire over too long a run(?).. If this is true, then why did the wire work fine for all of those miles, only to fail to spin the pump when I took a short break after driving?

** In the end, I've now installed a short, permanent 10 gauge ground wire at the pump, and it's working great 8)

Thanks for your thoughts.. Wiring is an area of weakness for me, and I just don't want to commit this error again..

--ccrunner

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PostPosted: September 6, 2022, 4:47 pm 
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This article may help you avoid another wiring issue. https://jascoautomotive.com/automotive-wire-amperage-capacity-chart/.
Waltj


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PostPosted: September 6, 2022, 4:49 pm 
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Too thin or long of a wire for the amp load drops too much voltage and will slow the pump operation. It also becomes a heater and can burn through itself and everything in the bundle with it. Here is a chart I labeled to help you figure out acceptable wire length and size for the amp load. There are online calculators also if you want to trust their math. Notice the chart doesn't say positive or negative as each is equally important, but the nice thing about grounds is the chassis allows for short jumpers that can be thinner per the chart.

Your pump should have amp specs.

Because you are extending wires, the voltage at the component will always be less than it would have been originally so the component will not work as well. The way to correct that is to replace the entire wire with one that will not drop more voltage than the original.


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PostPosted: September 6, 2022, 5:35 pm 
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Poor ground connections can cause the most frustrating electrical issues, made worse if the connection are intermittent. Just as bad, maybe worse, is a partial connection in the form of a high-resistance junction. In the latter case, say there's 1 ohm of resistance due to corrosion. If the pump draws one amp, that means (via E = IR) that there's one volt drop across the connection. The pump isn't seeing 12V, it's seeing 11V. Now say a high performance pump is installed that takes 10A (quite common). That same bad connection is now dropping 10V, so instead of seeing 12V, the pump now sees only 2V (12 - 10). Oh, and in the case of high current bad connections, you can test for it by touching the connection. A bad connection will get VERY hot. On a related note, an easy and non-invasive way to test ground connections is to put a volt meter right across the connection point. A good connection will show very little voltage, while a bad one will show a lot.

Regarding wire size, electricity is often compared to water, and in this case, the comparison is apt. If you want to run a certain amount of water through a hose at a given volume, there's several factors: length, diameter, and pressure. In the case of a car, the "pressure" is always 12V, leaving only the length and diameter. If you want to extend a wire but want the same amount of power to run through it, its diameter must be increased. If you replace a small pump with a larger one, its diameter must be increased.

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PostPosted: September 6, 2022, 6:08 pm 
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Well, from what I've learned from you guys, I've used too-small wire in a few parts of the car (tail section), which I will need to go back and redo.. The recent on road breakdown was frustrating, and in hindsight, altogether avoidable if not for my ignorance about the need to increase wire size as the length of the run increases. I feel lucky to have gotten away with it for this long (now knowing what I know).. I'll say this; bikes tend to use small wires, just barely enough to get the job done.. if you're extending a bike harness to fit the shape (length) of a car (BEC), increase the wire size :roll:

All very helpful guys- thank you :cheers:

--ccrunner

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PostPosted: September 6, 2022, 6:40 pm 
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It is also possible that the conductor was broken somewhere between the pump and ground and making bad contact that was either intermittent or high resistance. Either would prevent current from flowing. I had something similar happen in a circuit that kills the pump if oil pressure is lost - it was a short length of wire with no support or strain relief that failed from vibration. Fortunately in that case the fail safe position was pump always running with ignition on.

If the terminal or attachment at the frame were not protected with dielectric grease or similar, also possible that some corrosion kept the electrons from flowing.

10 gauge sounds like it is way overkill for the pump so you should be set. I would have expected somewhere between 16 and 18 gauge.

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PostPosted: September 7, 2022, 8:43 am 
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well covered explanations here. I always think of the ground wire as a resistor with the goal of 0 ohms. Poor ground= high resistance and resulting voltage drop.

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