'93 K75 Instrument Cluster for an 85" K100

[duckytran]

Can someone let know if the fuel level sensor/sender on an 85' K100 is compatible with an instrument cluster off an 93' K75? I believe the fuel level sender is completely different between the 2 models. The K75 use a fuel float type and I believe the older K100 use thermistors type, but I'm not fully certain.


Thanks.

[daveson]

I believe it's compatible with the cluster but not the tank or the wiring harness.
I just put a '91 model k75 tank on an '84 model k100 using the '84 sender because the harness connector was different. The low fuel light came on because the sender was outside of the tank.
The '84 sender is located between the fuel tank hose barbs, you might be able to see it in a photo.
I think the sender change occured in 1985. My bike is from September 1985 and has the newer type sender at the rear.

[billday]

The low fuel light won't work right,  but it's worth it to have a newer cluster.  Use your trip odo to tell you when to stop for gas.

[duckytran]

Thank you to both for your input.

[szabgab]

Can someone let know if the fuel level sensor/sender on an 85' K100 is compatible with an instrument cluster off an 93' K75? I believe the fuel level sender is completely different between the 2 models. The K75 use a fuel float type and I believe the older K100 use thermistors type, but I'm not fully certain.


Thanks.

I have read somewhere,  that you can replace the small PCB that sits on the rear bottom of the speedo gauge inside the cluster,  but I have never tried that,  so take this info with a pinch of salt

[Andy FitzGibbon]

The cluster connector is different between '85 and '93. The contact pins on the newer cluster are round instead of square. While the two styles of plug will physically fit together, they won't always make good contact.

You are correct on the fuel level sender differences... as I recall, '85 was the last year for the thermistor-type low fuel switch.

Andy

[warmas]

I just this week replaced the flat pin instrument cluster of my 1985 K100 with the round pin cluster of a later model. I had my doubts.
My plugs fit properly and seem to make good contact. All cluster functions operate properly.
Since I replaced the thermistor in the fuel level sender the light illuminates quite prematurely.
My understanding of the fuel level sender is it provides a ground to the light controller.
An NTC thermistor varies resistance with temperature. Apparently mine is now too warm while still immersed in fuel.
Am unsure of the change in thermistor needed.

[Laitch]

Apparently mine is now too warm while still immersed in fuel.
Am unsure of the change in thermistor needed.

Here are the oem resistance values. What does your Mouser indicate?

[warmas]

I have studied the drawings and specifications for this circuit. I speak electricity very poorly.

Here is the part I installed: https://www.mouser.com/ProductDetail/803-501BG1K
This thermistor has, as best I can tell, a resistance of 500 ohms at 25C, according to its datasheet.
I think what is ocurring in my case is as the fuel warms the resistance if the thermistor drops enough to signal the controller to illuminate the light, even though the thermistor is still immersed in fuel.
I don't know whether I need a thermistor of more or less resistance or different characteristics.

[Laitch]

Am unsure of the change in thermistor needed.

Here is a description of the OEM function on your moto.

[warmas]

Thanks,
I have also read the info in your link several times.
I wish I understood it.
But I am getting closer.

[szabgab]

Thanks,
I have also read the info in your link several times.
I wish I understood it.
But I am getting closer.

[rbm]

Warmas,

What is it you don't understand about the explanation in that link?

Ignoring the science behind what is happening with the fuel level sender, if you are replacing the thermistor, the one you select needs to match the characteristics of the one that got destroyed.  There are no thermistors off the shelf that meet BMW specs.  You selected one that is 500 Ohms @25°C.  This is too low of a value and it needs to be a 600 Ohm thermistor.  The closest ones readily available are 560 Ohms or 680 Ohms.  Your best bet is to use a 560 Ohm one and add a series resistor of 40 Ohms.  That way, the fuel level sender circuit still thinks it's driving current into a 600 Ohm load.  If you keep your 500 Ohm thermistor, then add a series 100 Ohm resistor.

[warmas]

All:
Thanks for all the tips.
The 500 ohm thermistor I used was, I thought, the closest available in axial termination in resistance value to the 600 ohm BMW thermistor.
But, I plan to follow the advice here given to see if the light performs tolerably reliably.
Firstly, insert a 100 ohm resistor opposite the ground side of the thermistor and check performance.

[Laitch]

I have studied the drawings and specifications for this circuit. I speak electricity very poorly.

The 500 ohm thermistor I used was, I thought, the closest available in axial termination in resistance value to the 600 ohm BMW thermistor.

Don't sell yourself short, warmas. :giggles