My 1985rt is equipped with a mechanical fuel sender unit to control the 7L and 4L fuel lights.


Other models are equipped with an electronic sender unit.


Here is the inside of the fuel sender:


I was under the impression that the electronic unit was only for the bikes equipped with a fuel gauge. On the schematic, I cannot see any connection for a gauge. I see only the 7L and 4L dash lights and reading about it, I found the following description. Do I miss something or another type of sender is used for the gauge?


There are two thermistors housed in the fuel sender unit which is located in the fuel tank. The thermistors are positioned at two different levels corresponding to 7 and 4 litres. When the thermistor is exposed to the air in the tank the appropriate light is illuminated.
Each thermistor is supplied with 12 volts that is current limited to 95mA, this current limit is controlled by the BD240A's and there accompanying circuitry. With the applied voltage the thermistors self heat, when they are immersed in fuel the heat is dissipated and the resistance is approximately 600 ohms. While the thermistor is immersed in fuel the inverting input of the op-amp has 12 volts applied to it. This voltage level is above the 4.7 volt reference applied to the non-inverting input of the op-amp and thus the output of the op-amp is below the 0.7 volts needed to turn on the NPN transistor (BD877) which controls the lamp. When the thermistor is above the fuel level and in air, the self heating causes its resistance to drop below 53 ohms, the voltage at the inverting input of the op-amp drops with it to below the 4.7 volt reference level applied to the non-inverting input of the op-amp. This causes the output of the op-amp to swing to its maximum positive level (+12V).

Love to help you Crazy Frog, but after reading that, my brain hurt.

Hi Bert!
Can`t help you out here!
My -88 RS has the sender as listed as no 10 in the scematics.
I`ve got a fuel gauge wich is mounted in the fairing.
My previous -86 RT had both the yellow and red fuel warning lights.
My local tech at the time removed the bulb for the yellow light and told me not to worry,just refuel at about 280 to 300 kms.
I took his advice and run that bike for about 100.000kms.
If the speedo is working correctly I run by the trip meter.
Have not yet had to push the bike to a petrolstation.
It was a close call in the Italian alps though just last summer vacation.......
Cheers,Wikur

Hi Bert,

You're correct with the description you gave about the circuit operation. The opamps are acting more as digital switches than analog amplifiers. The thermisters are reliable electronic fuel level sensors and feed a "go no-go" signal to the circuit. The circuit is not analog and therefore cannot be used to send a continuously varying signal to a gauge.

My 88 K100RS tank came with the sender unit, labeled 10, as shown in the first parts explosion diagram. This unit is electro-mechanical. The float level varies according to the level of the fuel and that float level is converted into a resistance value that, when fed to a gauge, deflects the needle proportional to the level sensed.

If you want a solid state replacement for part 10 then you'll have to look aftermarket for a capacitance based sensor, such as Racor's electronic fuel sender. I am not advocating that you use this particular unit but there probably is one that will fit the tank somewhere out there.

Why would you want to replace the existing sender if it works?

Uhh......
The early 7L/4L types sound more 'electronic' as they use thermistors and op amps
than the later types which use a more common float and variable resistor.
If I understand the diagram I have correctly, the later ones also have a microswitch type affair
which turns on the low fuel light at a given float position.

I like that, its no more complicated than it needs to be
I find the earlier thermistor op amp malarky a lot of
over engineered gubbins that gives a limited and inflexible result

Is it possible to fit a later type sender to an early tank
or is it so tricky that it isnt worth the effort.?

The two thermistor setup doesnt seem to offer much leeway for modding
unless you added more thermistors to have an led/bargraph type affair
but thats probably not worth the time and effort.
I wouldnt bother meself TBH

Thanks for your answers. I am not planning to replace the sender on my bike. I was just curious about the 2 different designs and found the concept of using thermistors really interesting. This concept seem to be over engineered for the function of lighting a bulb.
I much prefer the mechanical one as you can bend the float if it's not accurate or if you want to modify at what level the light will come ON

Hello Guys,

My 1984 K100 RS is also has a fuel gauge on the left pannier and 7L/4L indicator lights on the dashboard. They are NOT working at all since I got the bike till now.

There is a round (4wire) plug attached to the left side of fuel tank. When I checked and removed the fuel pump, there were only 2 wires attached to the pump. I don't know where the other 2 wires go.

I am very happy that Bert puts up this topic, which I can learn about this and repair mine.

Please advice me, where are the thermistors positioned inside the tank?

Thanking you in advance.

Adiwan Djohanli wrote:There is a round (4wire) plug attached to the left side of fuel tank. When I checked and removed the fuel pump, there were only 2 wires attached to the pump. I don't know where the other 2 wires go.

There should only be two wires inside the tank going to the pump. The other two wires connect internally to the variable resistor that is actuated by the fuel level float. You probably won't see these wires as they are contained within the assembly.

Adiwan Djohanli wrote:Please advice me, where are the thermistors positioned inside the tank?

The fuel level sensor assembly is bolted onto the tank. The opening on the tank is located on the right side of the tank near the back. There should be a square plate with four pan-head machine screws that secure the plate to the tank. Your tank may have insulation material glued over the plate, insulation that protects the tank from heat generated by the engine and trapped under the tank. If you look inside the tank through the fill hole, the fuel level float is visible near the front right side of the tank. If you follow the lever arm from the float, you will see the fuel level sensor assembly.

If you have an older K100, the electronic sensor will be located in the sam spot but will just be a long rod protruding from the bottom of the tank.

You can tell from the pictures above which tank you have by observing the mounts for the tank. The earlier mount was a single fastener (part 12) attached to a bridge that spanned across the frame (part 9). The later tanks had two protrusions that fit onto brackets welded to the frame (parts 14 & 15 are securing clips for these protrusions).

Hello Rob,

The tank attached to my bike is the later model (?), the second picture at above this topic.

I did open the tank, cleaned it and also checked the fuel pump. But I do not see anything or any sensor or float inside the tank.

I shall recheck this, or may be the previous owner removed the float / sensor.

Thanks for the advice.

Best regards.

To the best of my knowledge and going on Bert's diagram the 84 model does not have a float and the only electrical connection is the round plug with 4 wires.

The sender unit is in the front of the tank and while not the most accurate device combined with a working odometer gives a reasonable idea of how much fuel is in the tank.

The sender unit is the bar fitted on top of the 4-pin connector, so it is positioned in between the pressure and return fuel lines on the left forward side. As usual people are confusing matters by not understanding the various modifications over the years.
First bikes - two sensors as described in the long pin on top of the fuel pump connector (left front of tank) - speedo fitted with 7(yellow) and 4(red) litre light. Fuel pump connector with two wires for level sensor fitted left front of tank)
Later bikes - takes resistor signal from float arrangement. Float arrangement fitted on square plate fitted centreline in aft part of tank. This arrangement allows for fitting a fuel gauge, the earlier one does not. Speedo fitted with 1 fuel light for 5 litre level alarm. The other light position was initially left empty and was later used for an ABS warning light. Fuel pump connector is with wire coming from the sensor plate aft of the tank.
So the top drawing is a later arrangement, the bottom one the earlier.

Hope that helps.

Thanking you all for the input and advices.

I will check the tank again soon.

Have a nice week end Guys.

Now that I can ride my 100RS, I need to know HOW FAR I can ride before refueling. My yellow and red fuel warning lights are not currently working. How could I confirm if the lights are faulty or the sensor is faulty? I believe I have the electronic sender unit.

What might be a silly question do you know if the bulbs are OK in the instrument cluster?
Regards,
K-BIKE

I will be checking those too, haven't had a chance to pull it apart yet.

If you want to be safe (not empty the tank whilst driving) pull the return hose of the tank, put it in a container and let the bike run on the stand. You'll see if they work or not.

Good call on that. I know they are not working, just need to figure out if it's because of the sensor or the bulbs (which could include burnt out filaments or something else in the circuit).

Good info guys. I want to start commuting on mine and it would be very handy to have the gauge as some days a fair amount of time is spent sitting in traffic. Need to pull my cluster to add some air vents to decrease the condensation and this might be a good time to add the fuel gauge if I can find the parts.

J

The fuel censor of my bike had been removed by previous owner.

I checked inside the tank and it was not there.

How to get a replacement?

Regards.

Here is one on fleabay. International shipping is not indicated but you could ask the seller...

Hi Bert,

Thanks for the information.

Best Regards.

Adiwan, do you have a tank that'll take a float? I've never heard of someone removing the level indicator, so it could be that yours has a float and would take a gauge??

Hi Rene,

I am not sure myself about it. There is no float or censor inside the tank.

I believe the RS had the sensor, not the float

Reason why I'm asking is that the fuel pump wiring was part of the levelpipe. I think it would be hard to remove the pipe without taking out the fuel pump wiring, therefore wondering if anybody fitted a tank with float.

Thanks to "Club_C" & Rene.

I shall keep learning and try to fix this problem step by step. At the same time I am enjoying riding the bike. Bearing in mind, I ride the for my enjoyment.

I will update you and share my finding and experience to benefit all our member in this forum.

Have a nice week end guys.

Adiwan, we have the same bike, just a year apart, so most of it should be the same except for my Sprint fairing.

"Club C",

You can see the picture of my bike on my profile id. I asked Motorad Germany about the bike. They replied that it was exported to Australia/New Zealand and should be RT, but somehow someone converted to RS.

So, being a beginner in BMW K 100 bike, I just learn the bike in this forum.

Cheers.

So then you might want to modify your profile to K100RS modified from an RT. That way stuff like the fuel sensor problem can be more easily solved. If your bike was originally an RT, then I think it should have the float in the fuel tank. Should be able to confirm RT or RS by the serial number.

The bike has been modified according to Motorad. But I do not know this and knot familiar with the differences between the RS and RT.

The chassis number is RS but engine number is for RT originally.

Cheers.

Maybe Bert can help with this, because he has one of each. Would be a good idea to start a new thread for that though, this one is getting all messy.

I fully agree.

I may start a new thread later on. But now, I still need to learn a lot about the bike, put more effort/work and buy / collect parts. Parts are very difficult to get here and expensive to import.

Cheers.

Then if the Chassis is RS and the engine number RT then I suspect a replacement engine has been fitted.
Regards,
K-BIKE

Yes, I strongly suspect this.

Reagards.

Hi Adiwan,
Good to see you on line, when you look in the tank from the fuel fill cap are you positive there is no float with a thin metal arm like a thick stiff shiny wire with a float on one end connected to some stuff down on the floor of the tank? It is item 10 on the front page of this thread.
Regards,
K-BIKE

If your tank has a float it is easy to see by opening the fuel filler cap and using a small torch to light the inside of the tank. Look to the rear of the tank and you will see the sender unit with a thin metal arm comeing out of it. This arm goes forward on the right hand side of the tank with the float on the end of it near the front.

I filled her up, put a petrol can with 4L of fuel in my topbox, ran until it ran out, then used the fuel in the can.
That then gave me an idea how many miles i can go before i ran out and tested both my 7L and 4L lights.

found my 4L light wernt working too.
Would it be any benifit in cleaningthe sender unit in the second diagram? maybe with a very fine sandpaper?

I took off the tank sometime ago. Clean the inside by rinsing with cleam petrol several times.

But I do not see any float or censor. It might be taken off by the previous owner. Not to mentioned the cap was also damage. The lock was tempered and the closing part is gone. Then I have a mechanical shop to "cut" a 7mm thickness aluminium plate to fit the tank and a hole in the centre for a new cap of Honda motorbike.

I will trace the 2 wire from the bottom left tank which should be for the float / censor.

Thanks for the advices.

Cheers.

AAAAAAAAARRRRRRRRGGGGGGGHHHHHHHH!

H**** parts on a BMW!?!

What's the world coming to?

blakey wrote:AAAAAAAAARRRRRRRRGGGGGGGHHHHHHHH!

H**** parts on a BMW!?!

What's the world coming to?

Cheapland?
I use a Harley Davidson oil filter, generic fuel filter, Standart injectors, Yamaha mirrors, generic hidraulic seals, champion sparkplugs and truck turning signals on my K100RS.
Oh the valve shaft seals are FIAT, not very happy wit them found the originals but no time to swap.
Rear shock is progressive 465 and tyres Dunlop D205
To ride around here, You have to adapt to the enviroment.
Best regards
JAP

Hello Men,

I was having a problem with my fuel light staying on. (The red one.) when the tank is full, I am waiting for the tank to go empty to see if the light go's out. I have not seen the yellow come on yet either. they were both working before I grafted white side instrument plug into speedo unit. Lights in speedo unit were expired. After replacing bulbs and cable plug and other minor problems with speedo unit everything is working. Triple checked my wiring to ensure every wire was correctly spliced into the main loom. This bike was stored for about five years I would like to add.

I ended up replacing fuel pump, filter basket, cleaned inside of tank. After reading posts I believe I have the the fuel sensor up at the forward left side of tank. will check went fuel level is low. Have to replace fuel return valve. (one of my other problems that I posted about). My bike is listed as a 1985, but was built in 1984. so when doing electrical work I refer to the prints for a early K100. Thanks for the info men. I will proceed with repair work after reading these posts.

Dino

dino ...check the 4 pin plug from the tank .....sometimes the socket part expands in the connector ......you might have to squeeze the sockets to mate up with the pins on the plug side



hope this helps

I have always been amazed at the way BMW overengineers things, I had a bad turn signal relay of a R750 years ago and thought I could just pop the cover off and clean some contacts and keep the orginal part. It looked like a radio inside!!! I went to the auto parts store and bought a generic 3 prong ts relay and it worked for a long as I had the bike.

In all fairness the turn signal relays on our K's also cancel the signals if we forget based upon distance travelled or very long time which a generic relay will not do. Not sure if the R750 auto cancelled (or was supposed to)
Regards,
K-BIKE

The R75/5, /6 & /7 had fully-manual turn indicator switches, similar to our K bikes but you had to remember to switch them off, or they just kept blinkin' away merrily.

Oh Man Crazy Frog,



I love the repair description that you gave at length. The only thing I wished for, was how to trouble shoot it using a meter? Or a Hammer!

Thank you for the diagrams, the pictures and everything else. Maybe I can fix that troublesome unit now. I will let all of you know.



Dino

these do appear to give problems and expensive to replace , i think i have found the cure , a stick which appears to work well so far

firstle wrote:these do appear to give problems and expensive to replace , i think i have found the cure , a stick which appears to work well so far

That sounds very interesting, could you give some more information? Thank you!

A piece of wood, such as a tree branch, that is used for fuel, cut for lumber, or shaped for a specific purpose (dipping in your tank maybe) , try finding a nice straight part and cut the bark off .