I need to trick a fluid sensor to return a full signal

I recall using, just for testing 4-20 chanels of an alarms pannel, a portable variable constant current source.

But, just for testing.

4-20 mA is not common on cars. I would expect a variable voltage, a variable duty cycle PWM, or a LIN signal.

atferrari said:

I recall using, just for testing 4-20 chanels of an alarms pannel, a portable variable constant current source.

But, just for testing.

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Definitely worth a try, I see them online for about 10€. Thank you for the suggestion!

Diver300 said:

4-20 mA is not common on cars. I would expect a variable voltage, a variable duty cycle PWM, or a LIN signal.

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Thank you for your comment. Any tips on how I would go about producing a LIN signal?

A LINear signal is just the average DC voltage of any PWM duty cycle with a potential range of 0.5 to 4.5V for 10 % to 90% PWM.

Measure what you have then try something else.

My oops

hhoarder said:

Thank you for your comment. Any tips on how I would go about producing a LIN signal?

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With any signal, you need to find out what needs to be sent first.

One way would be to record a working sensor. It is possible that a sensor that is stuck at 0% would give you a clue as allow you to guess what needs to be sent.

In LIN, the frame ID is always sent by the master, and it's likely that the sensor is the slave, so it might be possible to look at what frames IDs are being sent by the master and to guess the frame content.

A microcontroller / arduino / Raspberry Pi or similar would be needed if it is a LIN system.

Diver300 said:

With any signal, you need to find out what needs to be sent first.

One way would be to record a working sensor. It is possible that a sensor that is stuck at 0% would give you a clue as allow you to guess what needs to be sent.

In LIN, the frame ID is always sent by the master, and it's likely that the sensor is the slave, so it might be possible to look at what frames IDs are being sent by the master and to guess the frame content.

A microcontroller / arduino / Raspberry Pi or similar would be needed if it is a LIN system.

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I'm afraid I don't have any other sensor to try. However, I do have a few Raspberries and one older Arduino. I've already found a library that can sniff LIN messages, so if the sensor is indeed using LIN, I should hopefully be able to figure out the protocol and then send my own messages.

Thank you very much for pointing me in this direction!

I've raised the car again and measured some stuff, but I've gotten unexpected results.

Measuring between + and - gives 12V as expected. However, it's showing 7.2V between + and signal wire. Am I correct in assuming that the car is sending a 7.2V to the sensor? Would that mean it's trying to initiate some sort of start-up protocol?

Edit: the values above were measured inside the plug that's going from the car to the sensor. I've disconnected the plug and then measured the pins inside, the sensor was never involved.

hhoarder said:

I've raised the car again and measured some stuff, but I've gotten unexpected results.

Measuring between + and - gives 12V as expected. However, it's showing 7.2V between + and signal wire. Am I correct in assuming that the car is sending a 7.2V to the sensor? Would that mean it's trying to initiate some sort of start-up protocol?

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I suspect that the signal from the sensor is just a voltage, so more voltage represents more fluid.

Can you measure the voltage from the signal wire to ground. It should be 12 - 7.2 = 4.8 V but it's worth checking.

Can you also measure the voltages with the sensor disconnected?

What does the car say that the fluid level is when the sensor is disconnected?

Diver300 said:

I suspect that the signal from the sensor is just a voltage, so more voltage represents more fluid.

Can you measure the voltage from the signal wire to ground. It should be 12 - 7.2 = 4.8 V but it's worth checking.

Can you also measure the voltages with the sensor disconnected?

What does the car say that the fluid level is when the sensor is disconnected?

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I'm sorry, I should have been more specific - I have disconnected the plug from the car to the sensor and I've measured the voltages inside the plug itself. The sensor was completely disconnected during this time. So the plug going into the sensor had 7.2V between + and SIG.

The raw fluid value is still showing as 0, just as it was when the sensor was actually connected.

hhoarder said:

I'm sorry, I should have been more specific - I have disconnected the plug from the car to the sensor and I've measured the voltages inside the plug itself. The sensor was completely disconnected during this time. So the plug going into the sensor had 7.2V between + and SIG.

The raw fluid value is still showing as 0, just as it was when the sensor was actually connected.

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It could be a pull-up resistor, the sensor could pull it low, and could still be any kind of data, a variable voltage, or a variable current - you really need a working sensor to determine it.

Nigel Goodwin said:

It could be a pull-up resistor, the sensor could pull it low, and could still be any kind of data, a variable voltage, or a variable current - you really need a working sensor to determine it.

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Your pull-up resistor gave me an idea - I went back and extended the wires between the plug and the sensor in such a way that I could measure voltages while it was all connected.

I'm still getting 12V between + and -, but the + and SIG was fluctuating between (roughly) 6 and 8 volts. I'm sure it was more nuanced as that, but I only have a multimeter and no scope. So you're right, it was a pull-up resistor and something started to happen once I connected the sensor. I'm guessing there was some sort of communication between the ECU and the sensor.

Since the voltages were always higher than 5V, I probably can't use Arduino to try and see what's going on. I've found some USB devices that can supposedly connect to the CAN hub and read this kind of data stream, but who knows if that would do me any good.