Can someone provide me with the ohm range of the in tank sending unit? I checked mine and it reads from 110 down to about 85 through about half of the float arm movement and at that point it loses continuity all together. My guess is it is faulty and should read all the way down to 0 and never lose continuity. The fuel gauge has always read from full to about 3/4 tank and despite bending the float arm the range never changes.

from memory jimz i think it goes down to 30 odd ohms .

you may be able to remove the sender (after undoing the fuel pump as well, as the wiring is more or less fixed to the sender unit base )

i have a feeling that it is more or less resembles a wire wound potentiometer . possibly the windings have gathered at one end ? or the wiper has become distorted .

So, the ohm reading should be from approximately 110 down to 30? I had the sending unit out of the tank and there were no obvious problems with the windings. Regarding the wiper: there are actually two wipers. One sweeps across the windings and the other across a plain plate under the windings. The odd thing is the plain plate has a plastic insert of some type about mid-point of the sweep and that is where the ohm reading loses continuity. There are two green wires that provide the reading from the windings. One green wire is attached to the end of the windings and the other green wire is attached to the plain plate.

hmm sounds like the insert shouldnt be there at all ....unless at one time it used to house a track of some kind , is there a noticeable difference in height between the plate and the insulating section ? what im asking is is there a continuity of the plate if the insulatng section was removed ? or is there a chance that there were fingers of the wiper either side of the insulator once apon a time ?

i havent actually pulled one out of the bmw before ....but i guess after seeing similar units many times, they shouldnt be much different erm in theory .

hopefully someone has a detailed pic of a good one somewhere

Actually, the plastic insert is flush with the surrounding metal. Seems it was intended to be there, I'm just not sure why. I put the sending unit back into the tank, otherwise I would take a picture of it.

According to information I have, the resistance should be from 3,4 to 110 ohms.
Haven`t done any test to verify this....one day with plenty of time, and nothing to do.

Here is a picture of one that have seen better days and is pryed open.



Inge K.

inge ....you dont have a pic of the sweeper arm ? i'm looking to see if it has more than one finger for the common contact ...or in fact, if the pivot arm is one of the contacts itself (pivot point being the common contact other wise ) it would be just good to know i guess ..

You can see in Inge's picture that the one plate has a round plastic (red) insert. Notice the arch shaped line on that plate. Although shorter, it's similar to the arch line on the windings. To answer the question regarding the contact arms (sweepers), there are two sweepers attached to float arm. One arm sweeps over the windings and the other sweeps over the plate with the round plastic insert. The varying resistance is sent back to the cable via the two blue (actually green) wires that are soldered to the orange plug. I took my resistance measurements at the solder points while moving the float arm through it's full range.

ah! so the body sweeper is more a constant distance/ stabilizer for the second or active sweeper / across the resistive element ...the pivot being the main constant contact point ?

that explains a lot

jimz the only thing i can think of is... that at some point the sweeper contact across the resistor may have become distorted ...and now not making "good"contact with the resistor body as it sweeps its full movement ...maybe the pivot pin has been bent or something ? (possibly durring an attempted adjustment ) maybe you could clean up the contact windings on the lower half of the resistor with a bit of thinners or something ...there maybe some form of coating ---corrosion across that part, from the now low tension ?

the resistor looks to have a rising rate or angle associated with its length so the sweeper makes more presure on the resistor element as it moves across its length ....could the sweeper arm lost some of its tension ....and now not contacting the whole resistor .

the resitance values seem correct so it sounds like a non contact type issue to me

just some thoughts that may help

I thought possibly the sweep arms were not making good contact when I first took the cover off the sending unit body. I brightened up both contact areas with some fine steelwool and bent the sweep arms so they would provide a more forceful contact. Unfortunately, neither effort changed the outcome.

The resistor does not rise in rate of angle. It is flat across the body of the sending unit.

At this point maybe I should start looking for a replacement sending unit. Hate to approach a problem by switching out parts, but I'm not sure where to go from here.

maybe the pivot is bent after all ....worth another look jimz

can you measure the dimensions of the O ring??? (internal diameterX thickness in mm)

thank you

jimz49 wrote: I took my resistance measurements at the solder points while moving the float arm through it's full range.

Measure between the yellow wire and ground (brown), the plate with the red dot is
for the warning lamp and have nothing to do with the gauge.
The plate for the warning lamp can be adjusted if you wish to (while/if you still have things apart) look for a toothed segment at the bottom.

Inge K.

Inge K. wrote:

jimz49 wrote: I took my resistance measurements at the solder points while moving the float arm through it's full range.

Measure between the yellow wire and ground (brown), the plate with the red dot is
for the warning lamp and have nothing to do with the gauge.
The plate for the warning lamp can be adjusted if you wish to (while/if you still have things apart) look for a toothed segment at the bottom.

Inge K.

Oh Inge you spoil all my fun I was waiting to find out how long it would be before someone worked it out about the plastic bit.
I know its mean and horrible but it is fun

Don't have it apart right now, I've moved on to a slipping clutch issue. Glad someone cleared the air on the plastic bit, couldn't figure out why it would be there. Thanks to Inge for that one.

Rick G wrote:Oh Inge you spoil all my fun

Sorry to ruin your day, Rick........I add on another 24 hours next time.

Inge K.

Okay, got a chance to check the sending unit today and this time got the correct resistance (3-108). Since it isn't the sending unit that's amiss, where should I look now? I have the fuel gauge off the bike. Can I do a bench test of the gauge using the sending unit?

yep shouldnt be a problem jims just need 12 and ground ...tie the guage earth to the meter earth and the output of the guage to the meter ...+ the 12 volts on the right terminal of the guage

9 = fuel guage sender

10 = guage



by the way ...have you checked the 4 pin tank connector plug under the right hand side cover ...the socket part tends to expand around the pins of the plug and intermittent-corroded connections result ...often a sqeeze of the sockets restores good connections

Did the bench test today on the fuel gauge. The gauge reads from about 3/4 full to sightly past full as I move the float arm through it's range. Is this normal for the gauge to fail in this way?

verry odd jims .....what happens when you add a 150 ohm resistor in circuit to earth and the sensor input of the guage ?

kinda sounds like the shunt resistor has changed value ?

you could measure accross the b+ of the guage and the sensor input and see the value (unfortunatly i dont have mine out at the moment for a comparison )

Don`t help you much, but mine also have a rather strange behaviour.

Start from full...as normal down to half....then moves up again to 3/4.....
and then stays there untill the tank is empty.

charlie99 wrote:you could measure accross the b+ of the guage and the sensor input and see the value (unfortunatly i dont have mine out at the moment for a comparison )

This value should be 125 ohms.

might be just as easy to measure the movement of the sender jimz ....just put your meter across the earthy side of the sender unit and the send wire .....you should get a nice sweep from 3 to 110 ohms ......if not you have found the little critter .... check that there is good presure across the wiper as it moves across the windings ..you may have to clean up the wiper contacts with some really fine wet and dry soaked in with wd40 or rp7....only problem with that is that it could leave some grit in the surface ....but paper used after might release it

just some thoughts

just saw the value inge ...thanks

The reading went straight to full, which is odd. According to my resistance readings of the sending unit, empty is the higher resistance. So adding resistance should have made the gauge swing closer to empty.

If you are saying to measure the resistance between the yellow and green wires at the back of the meter, then I'm getting a reading of 221ohms.

wow almost twice as much as i expected ....sounds like some thing bad has happened inside to me

opening the meter has its issues .....like trying to reassemble it ....the hard bit is getting the glass face and surround clipped around the circumference of the meter body ....not an impossible thing ...but awkward for me with shakey hands

charlie99 wrote:...but awkward for me with shakey hands

I always thought this was a Monday morning problem....it`s still Friday......Hooray

jimz i went inside one of the meters a little while ago ....you might gleem some ijnformation in this thread

http://k100rt.aforumfree.com/t2977-anyone-succesfully-been-inside-a-temp-guage

UPDATE:  

Created a complete How-to posting in the How To section of the site.  It includes all the original material plus a description on how to change the sender to be compatible with modern gauges.

Job well done Rob we shall have to get Bert to place that in the downloads section.

great write up rob

did I see somewhere that inge said that the block for the reserve could be moved somehow ? I didn't see how it could be done

In Robs excellent post above he shows the left side contact pic supplies ground to the reserve light and the right side contact pic is nonconductive because its when the fuel is either full or heading down to the near empty point where the light comes on and the sweeper on the windings is only for a gauge if one is connected??? is that correct????

Yes, that is absolutely correct, K75cster.  This conclusion is also substantiated if you examine the K100 schematic diagram.  The four wires from the tank connector are:

Green/White = +12V for pump (pin 15)
Brown = Ground (pin 31)
White = Low fuel indicator (pin W)
Yellow = Fuel level (pin G)

Green/White comes from Fuse 6; ground from the frame connection under the tank.
The white wire goes to pin 7 on the instrument panel.  It is the low fuel indicator function.
The yellow wire goes to the "Connector for Additional Instruments", an Amp 4-way female connector under the right side of the tank.  This is the connector that is used on an RT or LT to connect up the Gas Gauge.

This post I liked. Thank you

Now I think I can fix mine.
Thanks

charlie99 wrote:great write up rob

did I see somewhere that inge said that the block for the reserve could be moved somehow ? I didn't see how it could be done

Charlie, you're absolutely right and I totally missed Inge's comment earlier in this thread.  Thanks for pointing this out.

I've found the toothed wheel about which he spoke and I've updated my description to include this adjustment.

Any help on the 85 style fuel sender unit?

Some info you find in this old thread.

Thank's very much for this, but can you clarify something for me.

I am using a car fuel gauge and looking at your wiring, I only need to attach the yellow wireto one side of the gauge and the brown to the other (without worrying about the light)

You should tap off the Additional Instruments connector  for your automotive fuel gauge.  This connector provides all the signals you'll need for your gauge -- ground, +12V, sender and parking lights.  I don't know your type of gauge, but normally one side of the gauge is to +12V and the other to the sender.

Be aware that the BMW sender works opposite to modern fuel senders.  Your gauge may respond falsely, indicating empty when the tank is full.  Hopefully it can be adjusted to compensate.

I have looked for the additional instruments connector, but I don't appear to have one. This mat  be down to being a ex police bike

Sometimes with the police bikes they are treated very badly when they are stripped of the police gear and it can be a cut and tape festival.

RicK G wrote:Job well done Rob we shall have to get Bert to place that in the downloads section.

Yes very well done Rob.   


I placed your posting (with a link to this thread) in the downloads section. (It's under Electrical -> Fuel sender units) It will be easier for people to retrieve it when needed.

Bert

You're looking for something like this on the right side under the tank.

Nope, definitely not.

In the relay box I found this which has most of the right colours








But nothing like I 'should' have on the right under the tank

What you have ahold of in the first few pictures is the Connector for Special Equipment.  

It's not a useful connector for your gauges.

if your bike didn't come with gauges from the factory then it's probably taped off to the clump of wires and not hanging out in the open.