VDO water level meter problems.

[Big John]

Not sure if any one can help with some advice, a friend of mine has asked me look at a problem on his boat with the water level gauge/sender.

He has the VDO 02240401 capacitive sensor,(this is the 2 rod electrode capacitive type) having a 4 to 20mA output fitted to his tank and a 4 - 20mA gauge on the dash board. For what ever reason the gauge always shows full even though the tank is not. Even if you take the sensor unit out of the tank the gauge does not move. The electrics are very simple, two wires to the sensor one being the negative feed to the sensor the return wire to the gauge. The gauge has 12v+ feed wire and the return wire from the sensor.

Is there a simple way of testing either unit?

 

[harvey38]

Put a multi meter across the sender and see if the reading changes as you move the sender up and down, sounds like the sender is dud. I recently had a similar issue with a fuel sender, the gauge would ping across to the full side due to an open circuit.

 

[Big John]

I used my capacitor measurement meter across the two poles on the sender. Capacitance changes as the electrodes go into and out of the water.

 

[Chris_d]

I used my capacitor measurement meter across the two poles on the sender. Capacitance changes as the electrodes go into and out of the water.

Not sure you can measure the change in capacitance at the terminals there will be circuitry inside converting it to a 4-20mA signal, try removing one wire only and connecting the multimeter as an ammeter on the mA setting.

 

[Big John]

Thanks Chris_d, Thats one thing I did not try. Hope to get back to his boat next week. I will let you know.

 

[Big John]

My friend today dropped off the gauge from his boat and I wired it up on the bench as per the diagram.
As before with the sensor in fresh air the gauge went to full. Inserted my multimeter on mA. On full deflection the gauge is receiving 20mA.
I add a variable resistor in the line from the sensor to the gauge and turn the out put to 4mA the gauge drops to empty and goes to full if I increase the mA's to 20mA. Conclusive that the sensor is faulty which I suspected from the beginning but have now confirmed.
These sensors are about £180 so I will very carfully attempt to get inside and see if there is any obvious problem.

 

[Chris_d]

My friend today dropped off the gauge from his boat and I wired it up on the bench as per the diagram.
As before with the sensor in fresh air the gauge went to full. Inserted my multimeter on mA. On full deflection the gauge is receiving 20mA.
I add a variable resistor in the line from the sensor to the gauge and turn the out put to 4mA the gauge drops to empty and goes to full if I increase the mA's to 20mA. Conclusive that the sensor is faulty which I suspected from the beginning but have now confirmed.
These sensors are about £180 so I will very carfully attempt to get inside and see if there is any obvious problem.

Sounds conclusive, proper fault finding that

 

[Big John]

Having removed the two rods/electrodes (easy bit) and carefullu eased of the top section which holds the the two electrodes I then discovered the whole PCB and wiring inside is potted with hard rubber. Obviously the rubber is stop moisture getting in and me. Back to the drawing board as to how if posible to removed the inside. I think it is going to be imposible. Will look for alternative 2-4mA water level electronics to be able to use the same tank fixing.

 

[Bran]

Try disconnecting one or both of the electrodes from the pcb if possible, then power up the gauge. If the reading is still full scale then the issue is in the pcb.
If the gauge drops to zero the issue is some sort of contamination causing a semi short between the two electrodes, may be possible to clean or reduce the contamination but can be very difficult to do this.
It is possible to work on potted electronics but it takes time and patience!

 

[Big John]

UPDATE - The rubber in theVDO sensor housing is too hard to be able to remove without damage to the PCB. Other types of 4-20mA water level sensors not suitable for such a small tank. Decision taken to re-think and now we are going to use a resistive water level gauge from AliExpress. It’s a no brainer when you look at the price compared to the VDO unit replacement cost. Also the mechanical fixing has the same tank hole dimensions and screw hole positions as the VDO unit. The wiring needs a slight modification but other wise it should be a simple exchange.

https://www.aliexpress.com/item/100...031d16c4793b2e472305f791acf&afSmartRedirect=y

 

[Parabordi]

UPDATE - The rubber in theVDO sensor housing is too hard to be able to remove without damage to the PCB. Other types of 4-20mA water level sensors not suitable for such a small tank. Decision taken to re-think and now we are going to use a resistive water level gauge from AliExpress. It’s a no brainer when you look at the price compared to the VDO unit replacement cost. Also the mechanical fixing has the same taThis is nk hole dimensions and screw hole positions as the VDO unit. The wiring needs a slight modification but other wise it should be a simple exchange.

https://www.aliexpress.com/item/100...031d16c4793b2e472305f791acf&afSmartRedirect=y

This is what i did, i had three such capacitive gauges and the same issue. Was cheaper to buy new gauges with senders.

 

[oldgit]

Have a couple of plastic tanks which use those stick on the outside surface sensor thingies for fresh and black water levels.
All hopelessly temperamental and wildly inaccurate.
Replaced with an £15.00 chinese old fashioned float + gauge in the Poo tank and just brim the freshwater and guess the level.

 

[Big John]

UPDATE: The Chinese water level sensor and gauge arrived to day and I was very pleased to see that the mounting ring from the old VDO unit fitted exactly the new resistive sensor from AliExpress. I had checked the data file from the supplier with angles and screw positions but it was good to see it matched perfectly. Those of you who are familiar with the way the VDO sensors are fitted to the tank will appreciate this. No drilling required. A straight swap over.

 

[Big John]

UPDATE: Fitted the new level sensor and gauge in my friends boat. The existing twin wire connection to the sensor were used. One wire was 12v- and the other wire being the signal wire back to the gauge.
The guage on his boat was not located in the main instrument panel but was in a remote location.
The old wiring to the gauge was two wire, a switched 12v+ and the other the signal wire from the sensor.
The new gauge required 12v+, the signal wire and an additional lead connected to 12v-. A bit of time was required where to get a 12v- but as the fuse panel was near by that was easily corrected.
The new sensor and gauge are now working perfect.
Had the gauge been fitted in the main instrument panel connection would have been easy.