The Fuel Warning Lights seem to be problematic judging by the posts on the forum.

My specific question relates to whether these should stay on after they have been triggered (I think the electrical term is latching) Both lights work, though they go flash on and off when they first light, then eventually stay on.

At this stage I'm not worried about the accuracy as this seems to have been well covered elsewhere.


Thanks

Shug

You will find that these lights have minds of their own. After going through what you are going through I just relied on the trip meter to gauge refueling by distance. The early lights were activated by change of temperature in the tank.....whether there was fuel around the sensor.
Good luck.

Personally, I've found on 2 different 85 K100rs' these fuel lights appear to work actually quite well.
It is normal that as the fuel level begins to slosh around the level of the sensor, the 1st lamp (7L) will come on, go off, come on, go off.....etc till the fuel level is well below the position of the sensor. That flashing seemed to irritate a lot of users..especially at night...doesn't bother me at all, I've grown accustomed to it.

The same for the operation of the 4L lamp as the fuel level gets lower.

My experience shows that the 7L lamp activates at about 130 - 140 miles after full. The 4L lamp comes on at around 170 miles and the bile runs out of fuel at about 221.

BMW did publish a service bulletin to disable the 7L lamp altogether and add a 90 second delay function to the ops of the red 4L lamp to avoid most of the on/offs at the trigger point.

Now you understand how it's designed to operate and what you can do to change it if it makes you antsy.

As a matter of fact, I went through a lot of hoops......to backwards engineer my 87 speedometer so it would operate both lamps as the original did.

Welcome aboard. Regards, Ibjman

they don't latch as you have already gathered

but it might be a good recommendation to slow up the rate of change that the sensors react to ...and then presented to the lamp ..... wouldn't be too hard to do for the technically inclined
just an observation

ainsjac wrote:The early  lights were activated by change of temperature in the tank.....

Yes, have noticed that after re-activating the light on my '85 RT. Temps around here have been in the upper 80s, and after some stop-and-go traffic the tank got noticeably warm. All the time that amber '71' light came on and stayed on all day even though I ridden less than 100mi on a full tank of gas.

I believe that it's normal to see the amber light at about 100 miles......maybe intermittently a slight bit earlier.
I have trained my brain to disregard the intermittents and pay more attn. to later on when the lamp becomes more steadily on.
I also haven't seen a lot of experiences in operational change between when the tank is hot of not so hot.

As I understand it, the thermistors are intentionally heated by the circuit board (probably to a temperature that is "substantially higher" than the fuel ever gets) no matter what the weather.
Even if the fuel were to reach a very high temp. on a hot day, that temperature is still way colder than the ambient temperature of the uncovered sensor. .....So even the hottest fuel, when it covers the sensor will be sufficient to cool the sensor more than enough to extinguish the light.

As a side note: I've learned that the fuel height sensor for the duel light system incorporates a plastic "blocking strip" with 2 very tiny "flow holes" that allow the fuel level an the internal sensors to be a bit more modulated and slightly more resistant to "slosh"
This plastic strip has a tendency to become dislodged (fall off) from the sensor hence allowing the fuel slosh unlimited access to the sensors. If that occurs the action of the fuel lights would be very erratic.

Anyone complaining about excessive activity of the duel fuel light system might want to pull the sensor out of the tank and inspect to see that the thermistors are fully and properly "enclosed" inside the sensor and not directly exposed to the fuel slosh.

I actually considered blocking up the top & bottom "bleed holes" in the sensor assembly and re-forming them even tinier to slow down the action of the fuel level change inside the sensor body, I have not actually tried it.
Regards, Ibj...

Thanks for the feedback.

Looks like I'll need to get used to the flashing lights. I usually use the odometer as my first port of call for how much fuel there is or should I say how far I have left. It's nice to have a gentle reminder to fill up though.

The thought of a heated wire inside the fuel tank just doesn't sound right.

I've tried to look inside the tank to see if I have the Damper Tube in position but as yet to no avail. I need a small mirror or as already suggested strip the part out to have a look.

Thanks again

Shug

I'm not sure about the reference to the "damper tube" you refer to in the tank.

I rather like the lights as they are/were originally....maybe a nostalgia.

At any rate I try to refuel with 40 miles after the red light comes on steadily.

As for the heated parts on the tank.....they are called thermistors....on most models there are actually 2 potentiometers on the fuel light control board where they can be fine tuned.

Our friend Inge once mentioned that they are adjusted to 89 Ma......but I have no idea how one would measure that while making an adjustment.

I assume if a fuel light is coming on too early for instance, that it's theoretically possible to re-adjust it.

I read one person's post around here somewhere that said he actually put a 1000~ resistor potentiometer in the wires to each tank sensor. He then put 4L in an empty tank and adjusted the pot until the 4L lamp just went out......he then added 3 more L to the tank and adjusted the other one till the yellow one just went out..........seems like splitting hairs to me.
I have also heard where some riders have put a lower wattage bulb in especially the yellow one to make it less annoying especially at night.

My feeling is, if you're put off by the warning light(s), you have too little to occupy your "to-do" list. LOL!

LOL I currently have a large essentials list and a much bigger Wish List.

Posted by ibjman

Personally, I've found on 2 different 85 K100rs' these [b class="coloradmin" style="color: rgb(170, 0, 0);"]fuel[/b] lights appear to work actually quite well.
It is normal that as the [b class="coloradmin" style="color: rgb(170, 0, 0);"]fuel[/b] level begins to slosh around the level of the sensor, the 1st lamp (7L) will come on, go off, come on, go off.....etc till the [b class="coloradmin" style="color: rgb(170, 0, 0);"]fuel[/b] level is well below the position of the sensor. That flashing seemed to irritate a lot of users..especially at night...doesn't bother me at all, I've grown accustomed to it.

The same for the operation of the 4L lamp as the [b class="coloradmin" style="color: rgb(170, 0, 0);"]fuel[/b] level gets lower.

My experience shows that the 7L lamp activates at about 130 - 140 miles after full. The 4L lamp comes on at around 170 miles and the bile runs out of [b class="coloradmin" style="color: rgb(170, 0, 0);"]fuel[/b] at about 221

I got used to the 92LT and the 5 litre warning came on and stay on like clockwork about 150 -160 and would be at 15 litres used. However it would run out after about 30 miles even though there was some little amount remaining. A top up would only take maybe 18 litres and not 21 litres.

The 84RT has the 7 and 4 litre lights and the 7 litre light flashes at under 11 litres used. I haven't run  it dry yet to see how far it will go after the red light comes on. Economy checks have revealed a significant improvement on the LT in the order of 15% or more. Seems there is not much one can do but the real question is how much fuel remains in the tank after it has cut out due to lack of fuel. My feeling on the LT was that 2 to 3 litres of it was unusable.

ibjman wrote:.................


I read one person's post around here somewhere that said he actually put a 1000~ resistor potentiometer in the wires to each tank sensor. He then put 4L in an empty tank and adjusted the pot until the 4L lamp just went out......he then added 3 more L to the tank and adjusted the other one till the yellow one just went out..........seems like splitting hairs to me.
I have also heard where some riders have put a lower wattage bulb in especially the yellow one to make it less annoying especially at night.
 

I did the above mod, info HERE not because the fuel lights would flash on and off, but because the lights would be ON STEADILY when there was plenty of fuel left.

Here's a pic of the installed mod.

However I found when setting the pots for 4 litre and 7 litres of fuel, each pot had to be set at a minimum resistance anyway, meaning nothing had been achieved.

ibjman wrote:......................................

BMW did publish a service bulletin to disable the 7L lamp altogether and add a 90 second delay function to the ops of the red 4L lamp to avoid most of the on/offs at the trigger point.

I have recently purchased another brick:D , 1985 K100RT, I've noticed that the 7 litre fuel light is not on even when the 4 litre one is. (I have not yet checked the bulb)
When and why did BMW disable the 7 litre light?

From memory it was the end of 1985 when they changed to the 5lt light and a float in the tank along with different wiring to the tank as well