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Daniel Wilson
Daniel Wilson,
Category: Medium and Heavy Trucks
Satisfied Customers: 5408
Experience:  Owner at DJ Wilson
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I'm interested reading fuel level directly from the fuel

Customer Question

i'm interested reading fuel level directly from the fuel sending unit wire. I have a device which is capable of reading analog values, and latter translate to any format i want.
We have succeeded on some vehicles, but on some other not. On the ones that we have succeeded we can read voltage anywhere close to 1.5v when tank is full and 6.5v when empty. But on some other tanks/fuel senders, the voltage meter shows 0v.
I need to understand how does fuel sender works, and does it work the same way on all vehicle? If not, what do i need to do to achieve my goal ?
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Derek Neustaeter replied 1 year ago.

Hello, My name is ***** ***** Im a qualified Heavy Equipment Mechanic. I will do my best to explain to you the different types of sensors used for fuel sending units.

Older fuel sending units use an electrical sensor called a Rheostat. This is a 2 wire sensor that uses a variable resistor to drop a portion of the voltage supplied by the battery. One wire has a supply, and the other, a ground

Newer equipment with fuel sending units use a 3 wire sensor. One wire is called the "supply" which supplies a source voltage of 5v from a computer. The second one is called the "signal" which measures a signal from the 5v depending on the position of the sensor. The third one is a "return" or ground, which is a negative ground supplied by the computer as well.

A 2 wire sensor is much easier to test, using a standard multimeter or ohm meter. A 3 wire sensor is best to test and observe using a scan tool as you need a 5v supply, a signal, and a ground applied to the sensor.

I hope this answer helps you understand the differences in some fuel sending units, compared to others and why some are easy to test, and others aren't. If you have any other questions, I would be happy to help! If you could leave me a feedback of the service I supplied today, that would be greatly appreciated!


Customer: replied 1 year ago.
Hello Derek, the device that i use is a tracker which has an analog input. Will i be able to read (fuel level) voltage coming from fuel sender with this device for both scenarios you mentioning above ? I was told at a car service that some fuel senders are controlled by negative voltage, this is why I'm not able to read it. I don't understand this, is it possible ?
Expert:  Derek Neustaeter replied 1 year ago.

With an analog sensor, the direction of current flow is not relavent. With a digital 3 wire sensor, some are ground controlled.

The reason you are not able to test the 3 wire sensor is no becase of the ability to control the ground, but because of the nature of the sensor itself. The best way to test these sensors is through the use of a scan tool as the computer supplies the correct voltage to the sensor to be operated.

Does this answer your questions?

Customer: replied 1 year ago.
actually is what we tried today:
The fuel sender of an JCB190 has only two wires. While the unit was running, we connected the black probe of the voltmeter/multi-meter on ground/chassis and the red probe on each wire separately and we got 0 volts on the voltmeter.
While on the vehicles where i'm able to read fuel level, I get voltage readings from 1.5v to 6v depending on fuel level (from one of the wires) using the same measuring method.
Expert:  Derek Neustaeter replied 1 year ago.

Ok, I understand what you are working on now.

If you are working on a 2 wire sensor, one wire will have an input voltage, and one wire is a ground. What you need to do is check your input for voltage. Attach you + probe to the input, and your - probe to the battery ground. If you read voltage doing this test, but have no voltage when connecting your - probe to the sensor ground, and your wire to your ground is good, your fuel sending unit is faulty.

Customer: replied 1 year ago.
Fuel sending unit works fine as it shows fuel status on the dashboard gauge....but here is another hint.
If i connect the + probe on one of the fuel sender wires, and the - probe to the positive pole of the battery, i get steady -12volts
If i connect the + prove to the second wire of the fuel sender, and the - probe to the positive pole of the battery i get fluctuating voltage between 11.75 to 11.9. And the fuel tank is 1/4 full.
Expert:  Derek Neustaeter replied 1 year ago.

Do you have any voltage on the - side of the sensor to ground? Why are you testing it if it works properly, I don't understand your problem?

Customer: replied 1 year ago.
you should read the first post....
The whole fuel measuring system works fine on the vehicle. All i need to do, is to read the fuel level (voltage) using my analog reading tracker, and then transmit this through the modem of my device to my office/server. My device sends a report every minute, so i know the status of fuel level on the vehicle every minute.
I have successfully been able to read fuel level (voltage) from fuel sender on some vehicles, but not on some others. Fuel sending units and how they operate are not my expertise and i'm trying to get help to understand how they work and what do i need to do it order to achieve my goal.
Expert:  Derek Neustaeter replied 1 year ago.

If you ready my my previous post you would see I am trying to help you diagnose the problem. I am not a mind reader, and don't have all the information you have available to me to understand what you are asking. I had no idea what tracker you were refering to, nor what is was for. Please read my posts carefully and answer thoroughly so that I can help you to the best of my ability.

As I asked previously, is there a positive voltage from the ground side of the sensor? (+ probe on ground side of sensor, - probe on negative of battery)

Customer: replied 1 year ago.
Connecting minus probe on vehicle chassis and + probe on either wire of the fuel sender, i get fluctuating voltage on output less than 0.5v; did not pay much attention to the correct values but between 0.2v and 0.6v. (Same output from both wires).
Expert:  Derek Neustaeter replied 1 year ago.

So if you have 12v's on the input, and .2-.6 on the output. The sensor is block 11.4-11.8v depending on the level of the fuel float.

Customer: replied 1 year ago.
I'm not saying that you are wrong , but this very narrow interval between tank empty and tank full is just not convincing.
Is this the expected behavior on old 2 wire sending units ? On newer vehicles, (pickup trucks for example), on two wire sending units we get voltage from 1v to 7v depending on fuel level.
Expert:  Derek Neustaeter replied 1 year ago.

It completely depends on the manufacturer. Different manufacturers supply a different amount of voltage to the sensor. If the sending unit is operating normally, and those are the voltage readings you are getting, I would take them as correct.

Customer: replied 1 year ago.
HI Derek,
i can't take those values as correct.A different of 1V between an empty tank and a full tank would mean that the difference in OHM on the fuel sender resistance between empty and full should be also very small. Are there any Fuel Senders with such narrow span on the resistor values?
Expert:  Derek Neustaeter replied 1 year ago.

This is quite an unuasual and very specific question that I can't help you answer. I will see if any other expert has an answer.

Expert:  Daniel Wilson replied 1 year ago.

Hi my name is***** will try to help further. There are so many different style fuel level sensors as you know each constructed differently and spec'd differently per manufacturer. On this particular sensor run an OHM meter on the ground side and monitor your resistance to ground. Also with power disconnected probe the two terminals while moving the sensor float and record your resistance values. If this style is not compatible with your monitoring system then install a separate sender that is. Click on this link for an option.

Let me know

Expert:  Daniel Wilson replied 1 year ago.

Installing a fuel level sensor that has accurate readings from multiple sensing points is important. Some sending units will not give you this unfortunately. The link I posted above is for a sending unit spec'd out for the application you are using it for.

Customer: replied 1 year ago.
Hello Dan, sorry for the late reply.
We are quite knowledgeable in external fuel sending units, and we are aware of the model you sent me. There are actually better ones which we use today. However, for several reasons we cannot install additional FLS units on some vehicle, so we have to deal with what is available on the vehicle. And as i have stated before, we have success with some vehicles but not with some others. The following two links and solutions describe perfectly what we need. it looks like we need to install/connect a pull up resistor ...
Expert:  Daniel Wilson replied 1 year ago.

Hi, Thanks for the update. I agree with you. That is what you need on this style sending unit. As you know this will likely require playing around with a couple different OHM resisters to get the closest possible reading of accuracy. There is a possibility you could still have to mentally adjust the values between full and empty depending on your final selection.