Fuel gauge or fuel sender causing fuse to blow

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Pucketsport
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Fuel gauge or fuel sender causing fuse to blow

Post by Pucketsport »

I have just finished rebuilding a 57 Moggie and have fitted a new wiring harness. When I turn on the ignition the fuse that supplies power to the green wires blows. When I disconnect the gauge green wire and the green/brown wire the fuse does not blow. Does this mean my gauge is broken or the fuel sender unit is broken and causing a short. Also there doesnt seem to be an earth connection on the tank sender unit, surely this needs to be earthed? Is there a quick and dirty way to check if the gauge or sender are faulty using a multimeter ?
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geoberni
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Re: Fuel gauge or fuel sender causing fuse to blow

Post by geoberni »

Pucketsport wrote: Sun Sep 09, 2018 2:00 pm Is there a quick and dirty way to check if the gauge or sender are faulty using a multimeter ?
Yes, but that depends on your understanding of basic electrics.
I assume you don't want to keep blowing fuses, so the use of a multimeter is a very good idea.
But here is not the place to be giving lessons in using a multimeter. If you've got one, I'd like to think you either know how to use it or have an instruction book. Try searching out a lesson in using one on youtube.

You've already broken one basic rule of fault finding, you've made 2 changes, disconnecting the Green and the Green/Brown, so you don't know which is causing the problem.
Pucketsport wrote: Sun Sep 09, 2018 2:00 pm Also there doesnt seem to be an earth connection on the tank sender unit, surely this needs to be earthed?
No, it's earthed through the bodywork, that's why the wiring diagram shows the resistive element connected to the Sender case.
Sender.JPG
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It could be a shorted out Sender, or it could be a Gauge, or even a damaged G/Br cable.
I'd be using the multimeter to look for a short to earth on the G/Br from the back of the gauge.
If there is one, then disconnect the gauge and see if it goes away, then if it does, check the sender for a short.
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Re: Fuel gauge or fuel sender causing fuse to blow

Post by Pucketsport »

Thanks very much Geoberni. I have a basic knowledge of multimeters/ electronics but clearly no expert. I'll do the checks you suggested.
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Re: Fuel gauge or fuel sender causing fuse to blow

Post by geoberni »

Just realised I made an error here:
I'd be using the multimeter to look for a short to earth on the G/Br from the back of the gauge.
If there is one, then disconnect the gauge and see if it goes away, then if it does, check the sender for a short.
I meant to write:
I'd be using the multimeter to look for a short to earth on the G/Br from the back of the gauge.
If there is one, then disconnect the SENDER and see if it goes away, then if it does, check the sender for a short....
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StillGotMy1stCar
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Re: Fuel gauge or fuel sender causing fuse to blow

Post by StillGotMy1stCar »

If I remember correctly the green wire/s, are piggy backed on the back of the gauge to supply the wiper motor and heater fan, if the wiper motor on yours has the self park facility and is has the green wire connected to terminal 1 instead of terminal 2 it will blow the fuse as the supply will go direct to earth via the self park switch. Might be worth checking, as I think it is the most likely cause.
It won’t be the sender unit but could be the gauge, but check the wiper motor first.

Edit, Just noticed an error in my theory as the wipers would have to be off of park position to blow the fuse but if wired incorrectly the motor would turn until the switch makes contact and then blow the fuse. So definitely needs checking.

Regards John
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Re: Fuel gauge or fuel sender causing fuse to blow

Post by Pucketsport »

Thanks very much John and Geoberni, I will check tonight ( ran out of time last night)
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Re: Fuel gauge or fuel sender causing fuse to blow

Post by Pucketsport »

So I think I may have traced the fault with my limited knowledge of electrics. The green wire that supplies the power to the flasher unit, fuel gauge, wiper motor and brake pressure switch was shorting to earth. Then I checked the wire at the fuse-box which supplies the green wire with power through a 35A fuse. This white wire seems to be shorting to earth which I guess it shouldnt be is that correct ??? so tomorrow I'll continue the investigation.
Thanks all you electric gurus for your patience with my questions which may sound silly to some.
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Re: Fuel gauge or fuel sender causing fuse to blow

Post by geoberni »

Guys

I based my answers on the following data/assumptions:

1957 model, and this is the relevant diagram (N.30)
Pucketsport wrote: Sun Sep 09, 2018 2:00 pm When I disconnect the gauge green wire
I'm assuming that the other Green wire is still connected to the Wiper and indeed the Oil Pressure Circuit, hence I discounted that as part of the problem...
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Re: Fuel gauge or fuel sender causing fuse to blow

Post by StillGotMy1stCar »

Regarding the white wire, if you have a short to earth on that it isn’t protected by a fuse so would quickly burn and melt the insulation and ruin your new loom. If your testing this white wire with your Multimeter on the Ohms setting, on that circuit is the ignition coil (3 Ohms) with the points most likely closed, the fuel pump(4 to 6 Ohms) and charging light (cold filament possible single figure Ohms), all of these on their own or especially added together will give a low Ohms reading of as low as 1 or 2 Ohms and that could be seen as a short but is in fact normal. It can be confusing using a Multimeter to test circuits on cars as most bulbs will read as very close to a short circuit.
I am guessing you have had the battery connected and not had the white wire go up in smoke and just got confused by the Multimeter readings.
Regarding the green wire, what I have done in the past is solder a high wattage bulb say a headlight bulb across a blown fuse and that will light and limit the current through the circuit, saves blowing fuses and burning wires. Doing this the load is shared with this bulb so will glow in accordance with the amount of load. So wipers or brake lights will operate at reduced voltage and the bulb will glow but the fuel gauge should be such a low current that the bulb doesn’t glow. Just operate one circuit at a time, if the indicators are this circuit they will probably just glow and not flash.
Be careful where you place the bulb as you’ll most likely use a halogen bulb and gets very hot.
Hope you understand my rambling on and good luck :wink:
Regards John.
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Re: Fuel gauge or fuel sender causing fuse to blow

Post by Pucketsport »

John thank you so much, your post made a whole lot of sense and I confirmed its not a short but a resistance in the circuit of around 6 Ohms as you pointed out. :D This has helped me a lot as I have now realised that the white wire was not an issue and I was chasing my tail LOL.
Long story short I did find some wires connected incorrectly in the flasher circuit which may have been the reason the fuse blew initially. Thanks to John and Geoberni for all your help, much appreciated :D
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