My fuel tank is positively charged?

[timlewis86]

I don't really know how to phrase it but when I connect my multimeter from an exposed point on my tank to the ground of my battery it shows up with roughly 11.5 volts when the fuel pump is on.  My instincts tell me that is bad and I need to remove the sending unit on the bottom and double check my insulation; bare in mind I've already through my thoroughly cleaned my tank and replaced the internal wires with new 16 gauge wire and to be honest my soldering isn't my best work.


So I've noticed that the tank is insulated from the frame is it supposed to be "positively charged"?


Tim

[Scott_]

Check the integrity of the ground path of the brown wire in the electrical pigtail. It sounds like you may have a bad connection. Not uncommon for these connectors to fail. More rare, but the point of "pass thru"(part of the sending unit bracket) of the wiring to the internal of the tank has failed for some.
The ground wire is connected to the internal of the tank(sending unit bracket) for the purpose of the fuel gauge sending unit/low fuel lights(depends on your bike options)

[BrickMW]

Still on 1st cup of coffee so my theoretical brain may be slightly off with this hypothesis... but, since the terminals of both the fuel sender and fuel pump are exposed in the tank, wouldn't you pick up this "potential" VDC reading with the meter? Your only getting an 11.5VDC reading which means there's a fair amount of resistance between your meter and the source. (like fuel)
What is your OHM reading between the tank and battery neg then to frame ground? They should be similar, and low.

My instincts tell me that is bad

I'm pretty sure if it was bad, you would be blowing fuses..

[Scott_]

The Ohm reading between the tank and the frame/battery should be very, very low, like 0. As the ground wire is connected to the internal of the tank.
If you are reading voltage from the tank body, you are having resistance in the ground circuit that should not be there.

[timlewis86]

When I ground the tank to the frame i blow the fuel pump fuse.  Which wire (of the 4) from the sending unit (bottom of the tank) needs to be grounded?


In addition to that it sounds to me like 5he positive wire leading to the fuel pump is making contact with the tank itself.  The pump seems lower than usual and is a little sluggish when raising rpms from an idle which may be caused by the external additional fuel filter I installed or less than optimum power getting to the pump from increased resistance.


This is my first bmw motorcycle and my biggest question was; should the tank be grounded to the frame and the rubber pads at the front and rear are only there for dampening or do they also serve as insulators from the grounded body?

[Martin]

You have got a short in the tank should not be live and the rubber dampers are just dampers. Probably the power wire that supplies the fuel pump.
Regards Martin

[Scott_]

The wire pigtail:
Green -- +12 for fuel pump
Brown -- Ground for pump, level sending unit, low fuel lamp.
White -- low fuel lamp in cluster
Yellow -- for fuel level gauge

[Elipten]

My wires were severely deteriorated due to interaction with the fuel.  I replaced with new wires that are fuel resistant.  Just cannot use regular wire due to the insulation decaying in fuel.


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