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Problem with reverse voltage on old truck dashboard

Lambish

Sep 5, 2022
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Sep 5, 2022
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I've a numeric dashboard project for my old truck that has a faulty dashboard at the moment (fuel level and water temp has wrong value, the sensors are good). In this project, I've 4 sensors that are basically a resistor that changes with the value of the content (NTC400 water temp sensor, fuel capacitive sensor and 2 pressure sensors for air tank). But today I've a problem with some reverse voltage in measuring sensor ohms value. This project is based on Arduino, It use a voltage divider to get the sensor resistance value.

For more context, I've tested air tank pressure sensor to get the value in ohms @ X bars. The sensor is orginally connected like that on the old dashboard: dashboard sensor output => sensor => ground (I assume, it's an ohms mesurement with a voltage divider as the old dashboard gauge is only made from resistors and other small stuf, not ADC or anything else from what I can see inside). All tests was done with a calibrated multimeter and not with the Arduino (the value in Arduino was not good until I set an offset, but like explained below, the reverse voltage value is not always the same).

With the master relay OFF I've correct values from 10 to 140 ohms (0 to 7.5 bars) and when I engage the master relay, the value became totally broken (50 ohms to 80 ohms @ 7.5bar)... When I check the voltage between ground and the dashboard input of the sensor I get 65 mV (Ground => Voltmeter => sensor input => ground) like shown in attachement.

This value changes as I engage some equipments (not all of them) from 50 to 95 mV on this input (with NTC400 water temp sensor the value isn't the same, about 20 mV when he other one is at 65 mV).

Just to test,I've check between truck ground and earth (my field) I've 330/350mV. This value only changes with the wind and not with equipments activation (electrostatic charge I think) but the value of the reverse voltage doesn't change (I've test this in case of the earth connection discharge an eventual electrostatic charge)

So, how can I "suppress" this reverse voltage (I don't know if it's the good name for this) to get the real good sensor value (ohms) like when master relay is not energized? Or another way to get my sensor value maybe ? I can set an offset in the arduino (it work as the equipement activated not change), but I think it will change with humidity or other parameters (Equipements activated). However, the old Analogic dashboard work perfectly for the air pressure values (the water and fuel gauges are broken and gave wrong value..., that's why I've this numeric dashboard project) but I can't use it anymore and a new one is few thousand euros...

EDIT : This values doesn't change when engine is running ! Here is a schematic of how I connect the multimeter.The ground I used is the ground on the battery => 702A is the dashboard plug (disconnected from it)
 

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Bluejets

Oct 5, 2014
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Draw a diagram of what you have.
Include any power supply you are using.
At the outset when using millivolts as input to Arduino you might need to use instrument amplifier and mapping for the Arduino to successfully recognise any changes.
Code you have written will also affect the outcome.
Perhaps for the latter, the Arduino forum would be a better choice given that is all they do.
As always, a circuit/photo is worth pages full of written text.
 

Lambish

Sep 5, 2022
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Sep 5, 2022
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I've already joined the diagram of how I test with the multimeter on the actual diagram but i'll do another one if you prefer to have power supply on. Tell me if it's more clear to understand how I test the sensor and how I've this values.
Like i've sayed, all the measurements was made with the multimeter and not the Arduino.
The Arduino part is not so many important because I think this problem also killed my previous gauges and has caused some EDC (engine calculator) problem like shut off because Turbo pressure is too low for him (it's also a resistive sensor), that's why I need to find how I can suppress this reverse voltage for the all truck.
 

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Bluejets

Oct 5, 2014
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Can't follow what you are trying to do.
Are you measureing resistance with voltage (master relay) on...???
Or are you measuring millivolts with the master relay both ON and OFF.
You have shown resistance and millivolts..... ??????

Is the square at the lower part some kind of sensor and is top round circle your guage...??
Standard schematic using standard electronic symbols would be much more helpful.
Some here may have seen the ones you use but not this bloke.
 

Lambish

Sep 5, 2022
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Sep 5, 2022
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Ok, maybe it need a standard electronic diagram for more understanding, mine with manufacturer diagram is a little bit confusing as i understand.

What i'm trying to do is gettting the sensor Resistance, this resistance represente the air tank pressure (for this example but any other sensors I need to read have the same problem).

The dashboard is disconnected (like explained on drawing).

Like explained on drawing and text I've take measurement with master ON && OFF and get the result I've sayed.
Yes the square with P AIR is the sensor and the round the gauge, but it's not connected.
Here is the new diagram, i'll edit the orginal post with this one ;) sorry for this confusion... I hope it will be more understandable now (I don't know if the air pressure sensor symbol is good)
 

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Bluejets

Oct 5, 2014
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Nope, makes no difference, still about as clear as mud.

Firstly you didn't answer the questions I put to you in the first paragraph.

I don't get the connection between this master on off and your sensor/multimeter.
Also just what is the multimeter measuring and how.
Draw a complete diagram of your system otherwise it makes no sense. (at least to me)
Are you injecting the 24v somehow into the sensor (perhaps a bad idea) ??
What does the mV /ohm on two levels represent??
 

Lambish

Sep 5, 2022
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Ok, I can't really do it better. Let me explain another time.
I'm no measuring with voltage applyed on the sensor. During my test, the only connection between my sensor and other equipement (energized or not) is via the ground.

I've no direct connection between master relay and other equipement except the ground busbar. The complet diagram of what I test is what i show you, but if you need a 1000+ relays and other equipements on the truck it make no sense to me :/

The multimeter measure the resistance and the voltage (of course, I change the position... but not the wiring) between dashboard (disconnected) plug and ground busbar.

I don't inject any power in the sensor. I only energize other equipements connected on the same ground busbar.
It represent the values I get when I test (one time with master OFF and after with master ON). One time in ohms mesurement and after in DC voltage measurement position. Of course I can't make both at same time, I switch multimeter position

I hope this time is good for you, I can't really explained better than I've made...
 

Bluejets

Oct 5, 2014
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Ok, I can't really do it better. Let me explain another time.
I'm no measuring with voltage applyed on the sensor. During my test, the only connection between my sensor and other equipement (energized or not) is via the ground.

I've no direct connection between master relay and other equipement except the ground busbar. The complet diagram of what I test is what i show you, but if you need a 1000+ relays and other equipements on the truck it make no sense to me :/

The multimeter measure the resistance and the voltage (of course, I change the position... but not the wiring) between dashboard (disconnected) plug and ground busbar.

I don't inject any power in the sensor. I only energize other equipements connected on the same ground busbar.
It represent the values I get when I test (one time with master OFF and after with master ON). One time in ohms mesurement and after in DC voltage measurement position. Of course I can't make both at same time, I switch multimeter position

I hope this time is good for you, I can't really explained better than I've made...

Can only be feedback somewhere in the complete circuit (which is not shown).

If one has a sensor connected only to ground..............
(has really to have something else connected there as well, circuits need two ends to work)
...................and you measure voltage across that sensor, then it has to be feedback from some other device.

Usually in autos , this means a bad ground connection to another device and it is somehow using your sensor as a return path.
These can be difficult (even for trained auto leckys) to diagnose.

Example......old FJ Holden. Grandfather would arrive home, drive into the garage, put foot on the brake, turn off the ignition switch..........but engine kept running.
Take foot off the brake....engine stopped......
Diagnosed as bad earth on the brake lights.

Many times it can be when vehicles return after some maintenance and the "mechanic" has left an earth strap disconnected somewhere on the motor, gearbox or similar.
As the motor is on rubber mounts, stray voltages can appear in all sorts of places.

You mention "numeric dashboard" which would be especially sensitive to any of the above.
 
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Lambish

Sep 5, 2022
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Sep 5, 2022
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Thank you for the informations and your return about the car problem. I'll check the bus bar and see if a circuit have a bad grounding. The bus bar was clean and I only have 0.01ohms between busbar / battery.

I'm sure to have a feedback from other equipements, but I've test many of them and a lot of equipements make this feedback increase (or deacrease when I deactivate). It will be difficult to find the real problem source.

I found a solution with Arduino, but i wan't to find the source of this problem too. Thank for your help ;)
 
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