Electronic Fuel Sender Repair - (Early pre-float type)

[DJEwen]

I have identified my fuel indication light issue on my aftermarket speedometer as a further fuel sender thermoresistor failure.

I'm now the owner of 3x fuel senders - 2x Fuel Senders which supply power to the pump well but have faulty thermometers giving no fuel light indication at 4 or 7 L and 1x Sender with whats thought to be a faulty plug that supply's intermittent power to the pump but indicates fuel levels sufficiently.

Now, I could take a gamble and buy another in the hope that both functions work but I could be buying fuel senders for the rest of my days. I have opened up the senders and it looks rather simple to de-solder the thermometers and solder in replacements, I believe these are my options.

• Remove damaged resistors from one unit and operational ones from the other to make one healthy unit (runs the risk of the currently healthy but old resistor failing further down the line)
• Remove damaged resistors from one unit, procure brand new replacements and install to make one healthy unit

Has anyone completed this repair themselves or know of a source where someone has?

Some good work by a seasoned K restorer returned some useful information online and potential replacement products but the ohms value needed is unknown. There is detail on another forum in regards to voltage and ohms resistance at fuel level etc but nowhere specifically that details the exact thermoresistors themselves and what their resistor value is. http://www.k100-forum.com/t1503-fuel-light-unit
"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)"

• Does anyone know the resistor value needed to replace these?
• Do I have to use diode type glass encapsulated thermoresistors (I believe OEM fitment) or will metal film resistors perform the same task?

• Metal Film Resistors: https://www.ebay.co.uk/itm/223053635281?ul_noapp=true
• Glass Encapsulated Resistors: https://www.ebay.com/itm/391205464470?
  ul_ref=https%253A%252F%252Frover.ebay.com%252Frover%252F0%252F0%252F0%253Fmpre%253Dhttps%25253A%25252F%25252Fwww.ebay.com%25252Fulk%25252Fitm%25252F391205464470%2526rvr_id%253D0%2526rvr_ts%253D01737f3416a0ac72add138e7ffe129be&ul_noapp=true

This is well over my head but I'm confident I could do the replacement if someone could assist in what I need to purchace?

Thanks in advance.

Dave.

[mehdib]

Did you try searching?






(Hi Dave!)

[rbm]

That link also gives the specs for the appropriate thermistor.  It needs to be 600 Ohms at 25°C, 0.5 watts and negative temperature coefficient type.  There are many possible aftermarket replacements for this device through places like Digikey, Mouser, Future Electronics, and  RS Components to name a few.  It make resemble an axial through-hole resistor but it is not.  You are most likely to find disc-type thermistors rather than axial, but they should still function.  The trick will be making them fit.

[DJEwen]

Did you try searching?






(Hi Dave!)

Haha.. you Bar Steward!

That link also gives the specs for the appropriate thermistor.  It needs to be 600 Ohms at 25°C, 0.5 watts and negative temperature coefficient type.  There are many possible aftermarket replacements for this device through places like Digikey, Mouser, Future Electronics, and  RS Components to name a few.  It make resemble an axial through-hole resistor but it is not.  You are most likely to find disc-type thermistors rather than axial, but they should still function.  The trick will be making them fit.

Thanks, Robert. I did see the sentence: "when they are immersed in fuel the heat is dissipated and the resistance is approximately 600 ohms"  To me, a non electrical guy, that doesn't tell me I need 600 ohms but merely that the resistance is 600 ohms, so I understand now that's the same thing, thanks - So now all I have to do is find a couple 600 Ohms at 25°C, 0.5 watts and negative temperature coefficient type glass encapsulated thermistor(s)...

[johnny]

greetings...

that depends on how much heat the fuel can absorb... coldl fuel compared to boiling hot fuel...

j o

[DJEwen]

You are most likely to find disc-type thermistors rather than axial, but they should still function.  The trick will be making them fit.

I see what you mean with regards to the disc type. I reckon i could cut slots in the cover for them to protrude through.

[warmas]

I have recently performed, with the most helpful assistance of this Forum, the identical exercise you now attempt.
http://www.motobrick.com/index.php/topic,12078.0.html

[DJEwen]

I have recently performed, with the most helpful assistance of this Forum, the identical exercise you now attempt.
http://www.motobrick.com/index.php/topic,12078.0.html

Hi Warmas, thanks for sharing. I have read your input to duckytran's thread, so whats the status of your modification now, do you have both a 500 & 100 ohm thermistor inline? Maybe you could post some pictures of your modification?

[warmas]

I have only one thermistor, 500 ohm, installed, in the lower of the two fuel levels.
I could not find a 600 ohm thermistor readily.
The 500 ohm functions, i.e. it turns on the light controller, but prematurely. Once the fuel warms up the  low fuel light illuminates, regardless of level.
I have not installed yet a 100 ohm resistor, as am unsure of it's compatibility with fuel.
I don't have any pictures, got the thermistor from Mouser, and rely on my intermittent odometer  for fuel level.
Tank is dry at max 190 miles, or less, according to speeds.

[DJEwen]

Just to update this thread, back in April (2019) and unable to obtain exact replacements for the broken thermistors and unable to find a work around, I removed the working thermistors from the damaged sender and the faulty ones from the working sender then combined the two together. this gave me a working fuel light at 4L.

Yesterday (August 2019) that thermistor failed. My moto had been sitting in the garage for 3 weeks with under 4L of fuel in it, I filled it up yesterday yet the light remained on. This is frustrating as when chatting to a biker recently about the fuel sender issue he warned "they don't like sitting in free air". I had fuel indication for 4 months.

I have been on the phone to Digikey and the closest I can get is 603 Ohms at 25°C, 0.5 watts and positive temperature coefficient type, we need to be the negative type (NTC). Talking to them in more detail they explain BMW would have had the thermistors manufactured specifically for the moto's hence the inability to obtain them off the shelf. Im not giving up yet however, I'll call around everyone.

[mibh]

i very much hope you can find an off the shelf replacement for these.

[rbm]

When you made the replacement, did you use a multimeter to measure the current running through the circuit to make sure that it was still 95mA? There is an adjustment pot on the fuel sender board to adjust that current (R3).  Since you're changing values in a sensitive part of the design, you might have to adjust other components on the fuel sender card to compensate.  I'm thinking R11 and R12 might need to change value.

I doubt that the thermistors died on their own because the fuel was not covering them.  I could believe that they might have been ruined if you have a mal-adjusted R3.

[DJEwen]

When you made the replacement, did you use a multimeter to measure the current running through the circuit to make sure that it was still 95mA? There is an adjustment pot on the fuel sender board to adjust that current (R3).  Since you're changing values in a sensitive part of the design, you might have to adjust other components on the fuel sender card to compensate.  I'm thinking R11 and R12 might need to change value.

I doubt that the thermistors died on their own because the fuel was not covering them.  I could believe that they might have been ruined if you have a mal-adjusted R3.

Hi RBM. I didn’t measure them after replacing, no. I’m clearly well over my head here but I thought that removing an original and working 600 Ohms @25°C, 0.5 watts NTC thermistor from a working BMW K series fuel sender (with faulty plug) and soldering it directly into my non working one is a direct swap? Why would I need to make adjustments?

[rbm]

It could be that the cause of the death of the original thermistors was excessive heating current going through them.  Replacing parts with equivalent used parts and not correcting the excessive current would lead to a second set of failures.  It's a prudent move to check all adjustments if you don't know the root cause for a failure

[DJEwen]

It could be that the cause of the death of the original thermistors was excessive heating current going through them.  Replacing parts with equivalent used parts and not correcting the excessive current would lead to a second set of failures.  It's a prudent move to check all adjustments if you don't know the root cause for a failure

*presuming that there is ‘excessive current’.

How do I go about testing them, RBM, what values would I be looking for and how are they adjusted? Thanks in advance.

[rbm]

Well it's going to be difficult to measure that current now since you have a failed metering circuit but you're not sure what has failed on the circuit.

When you get to a point that you have corrected any cause for the failure, you can follow these instructions.  See my OEM circuits reference page.  You'll need to have access inside the OEM cluster to be able to make the adjustment.  You can see in the photo of the board on that reference page the two potentiometers that will need to be adjusted.  You'll want to insert a multimeter in current mode in series with T1 and J2 so that it sees the current being delivered to TR1.  Set the range on the multimeter to 200mA or 500mA (not sure the capabilities of your meter).  Power up the system and observe the measured current.  It should be 95mA.  If it is not that value, then adjust R3 on the OEM fuel sender circuit card inside the OEM instrument cluster so that the current measures 95mA.  Do the same for T2 and J4 that will adjust the current being delivered to TR2 using R4.

[DJEwen]

Well it's going to be difficult to measure that current now since you have a failed metering circuit but you're not sure what has failed on the circuit.

When you get to a point that you have corrected any cause for the failure, you can follow these instructions.  See my OEM circuits reference page.  You'll need to have access inside the OEM cluster to be able to make the adjustment.  You can see in the photo of the board on that reference page the two potentiometers that will need to be adjusted.  You'll want to insert a multimeter in current mode in series with T1 and J2 so that it sees the current being delivered to TR1.  Set the range on the multimeter to 200mA or 500mA (not sure the capabilities of your meter).  Power up the system and observe the measured current.  It should be 95mA.  If it is not that value, then adjust R3 on the OEM fuel sender circuit card inside the OEM instrument cluster so that the current measures 95mA.  Do the same for T2 and J4 that will adjust the current being delivered to TR2 using R4.

My new multi-meter has arrived.

Ok, so I fall at the first hurdle. Although I still have my original instrument cluster, the moto is no longer running it in favor of an Acewell CA-085. What other options do we have here, RBM?