BMW K bikes (Bricks)

You are not connected. Please login or register

View previous topic View next topic Go down  Message [Page 1 of 1]


1Back to top Go down   Fuel Gauge Empty Fuel Gauge on Mon Aug 17, 2015 4:18 pm

Howard64180

Howard64180
active member
active member
OK, I decided I wanted a fuel gauge on my 1989 K100/2.  Last time I had the tank off I fitted the harness for fuel and temp gauges.  Then I was lucky to get a used fuel gauge off Ebay cheaply.  It just needed the wires re-soldering.  Before I go any further, I am sure the wires are connected correctly (searched for and found a clear picture to check).

The gauge is apparently not working on the bike.  On the bench, when I connect a 9v battery across the terminals, positive to green/black, negative to brown, the gauge does not read.  (The following will show it's actually trying to go backwards).  If I then reverse the polarity it swings to full, then if I quickly return it to the original polarity it swings back to zero.  Is this normal?

My fuel tank is roughly 2/3 full.  I don't know if I'm doing this correctly, but measuring the ohms between the yellow/brown wires at the gauge end, with the gauge unplugged and ignition on, gives me infinite resistance (OL).  With the fuel gauge fitted and ignition on, I have 12.2 volts at the green/black wire and just over 7 volts at the yellow.  

Do you think the problem lies with the sender, or with the gauge itself?  Thank you for any input.



Last edited by Howard64180 on Sat Aug 22, 2015 3:51 pm; edited 1 time in total

    

2Back to top Go down   Fuel Gauge Empty Re: Fuel Gauge on Mon Aug 17, 2015 8:30 pm

RicK G

RicK G
VIP
VIP
From your description I would say the gauge is faulty. When the power is removed the needle should return to the zero position.


__________________________________________________
"Man sacrifices his health in order to make money.
Then he sacrifices money to recuperate his health.
And then he is so anxious about the future that he does not enjoy the present; the result being that he does not live in the present or the future; he lives as if he is never going to die, and then dies having never really lived."   Dalai Lama


Bikes 1993 K1100 LT, 1998 K1100 LT, 1993 K75 RT, 1996 K75RT, 1986 K75 GS, 1979 Z1300 Kawasaki X 2 for now
    

3Back to top Go down   Fuel Gauge Empty Re: Fuel Gauge on Tue Aug 18, 2015 3:36 am

charlie99

charlie99
VIP
VIP
a picture of the wiring at the gauge  would help howard

is this possible ?


__________________________________________________
cheezy grin whilst riding, kinda bloke ....oh the joy !!!! ...... ( brick aviator )

'86 K100 RT..#0090401 ..."Gerty" ( Gertrude Von Clickandshift ) --------O%OFuel Gauge Au-log10

'86 k100 rs.. #######..  "Fred " (f(rame) red ) ( Fredrick leichtundschnell ) - -
bits and pieces from many kind friends across the k100 world ...with many thanks ..
    

4Back to top Go down   Fuel Gauge Empty Re: Fuel Gauge on Tue Aug 18, 2015 5:17 am

Howard64180

Howard64180
active member
active member
Thanks for the replies.  Two things I should have mentioned.  

First, the gauge will return to the zero position on its own, but applying power as described above drives it back faster.

Second, a picture won't really help as I've lengthened the wires and they are not colour co-ordinated.  With the gauge on the bike, green/black is connected nearest the rider, brown central (screw terminal), yellow furthest away.  Grey/black goes to the bulb.  It is wired the same as this one: http://www.ebay.co.uk/itm/1989-BMW-K100LT-K100-LT-S468-gas-tank-fuel-level-gauge-motometer-/351258226543?hash=item51c89f2f6f  The wires were attached when I got the gauge, badly.

The gauge also works when either the green/black wire OR the grey/black bulb wire is attached to the battery positive, yellow to negative.  Very confusing.

EDIT:  Does this make any sense?  Perhaps the working part of the gauge is earthed through the yellow wire, and the position of the gauge depends on the resistance through that wire from the sender giving various voltages.  If that wire is open circuit, i.e. not effectively connected, the gauge will not work.  A 9v battery gives a reading of almost full on the gauge.

    

5Back to top Go down   Fuel Gauge Empty Re: Fuel Gauge on Tue Aug 18, 2015 6:54 am

Howard64180

Howard64180
active member
active member
Aha!   From: http://www.aa1car.com/library/fuel_gauge_diagnosis.htm

ANALOG FUEL GAUGES........

The amount of current that flows through the fuel gauge is controlled by the ground circuit provided by the sending unit in the fuel tank. As the fuel level inside the tank goes up and down, the hinged arm that's attached to the float rotates a rheostat. This changes the amount of resistance in the ground circuit which allows more or less current to flow through the gauge.


A shorted sending unit or a short in the wiring between the sending unit and gauge would reduce circuit resistance causing the fuel gauge to read full. And with nothing to slow the amps, the circuit would probably overload and blow a fuse. An open in the sending unit or wiring, on the other hand, would prevent the needle from moving at all and the gauge would read empty.



Looks like a job for winter.  This is very useful, thanks RobMack: http://hrsbstaff.ednet.ns.ca/bvogel/K100/download/electrical/fuel%20sensors/fuel%20level%20sensors.htm
Obviously, first I need to check the wiring further.

    

6Back to top Go down   Fuel Gauge Empty Re: Fuel Gauge on Sun Aug 30, 2015 9:04 am

Howard64180

Howard64180
active member
active member
UPDATE:  The yellow wire from the sender is definitely open circuit.  So as it's Bank Holiday weekend, a good time to stay off the roads, I've stripped the sender out of the tank expecting a clean up of the corrosion would fix it.  No chance, the sender electrical windings are badly messed up.  A secondhand unit will soon be on its way.  So my cheap and easy fuel gauge project turned out not-so-cheap and easy.

    

View previous topic View next topic Back to top  Message [Page 1 of 1]

Permissions in this forum:
You cannot reply to topics in this forum