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Old 07-07-2011, 10:58   #1
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VDO Water Gauge - Help

I have a VDO gauge on my water tank">fresh water tank that is not working properly. I'm looking for some help from anyone that has done some troubleshooting on these gauges.

The water gauge reads about 1/4 full when the tank is Empty and about 5/8 when the tank is full. I'm trying to determine if I have a bad gauge or a bad sending unit. Both are original to the boat and are approaching 10 years old. This has been going on for some time, but as I'm trying to get the boat ready for extended cruising I'd like get it fixed. In any case, last weekend the tank was nearly empty and the ice maker was out of it's hole allowing me access to the tank so I decided to do some trouble shooting. The sending unit does not have a brand label on it but has a label saying 240/33 ohms on it which means it's a US standard unit. It has an S(sender) and a G(ground) post. While the tank was empty I disconnected the S line from the back of the gauge and measured the resistance to ground and found it to be 328 ohms. My immediate reaction was aha... bad sending unit or wiring, so I measured the resistance between the posts on the sending unit. It was also 328 ohms. So the conclusion was bad sending unit..... maybe maybe not. I reconnected the gauge and the gauge read 1/4 full. Since 240 ohms is supposed to be the empty reading I would have expected the gauge to read empty or actually be pegged at empty. I disconnected the sending unit again (infinite ohms) and the gauge pegged at empty which is what I would have expected. In the meantime my wife filled the water tank and sure enough when it was full it read 5/8 again. Just for grins I decide to see what the ohms were at the sending unit. 328 again!!!!! How can this be??? The gauge definitely detected a change in resistance as the gauge moved from 1/4 to 5/8 but my ohm meter detected no change. Bad ohm meter? So I find a 5% 680 ohm resistor and it reads 656. Certainly within the 5%. So the Ohm meter seems to be working. Since this is a 12 volt guage I decide to measure the voltage coming out of the gauge at the sending post and it's 7.68v with the sending unit disconnected. The problem is I don't know what the voltage coming out is supposed to be. On such a simple device I would have expected it to be battery voltage which at the time was at float at 13.1V, but this may not be what is expected. If the gauge is calibrated based on a nominal 12V output I can see why with only 7.68 coming out of it it would not read the ohms correctly. The only thing I've been able to find on line are installation instructions which provide no guidance on expected voltages.

I'm still at a loss to explain why the gauge sees a resistance change and the ohm meter doesn't. If anyone has had experience troubleshooting these gauges any suggestions will be greatly appreciated. If anyone knows what voltage I should be expecting out of the S post on the gauge or can point me to a reference that would be really helpful.

Thanks in advance for any help.

Bill
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Old 07-07-2011, 14:49   #2
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Re: VDO water Gauge help

I have a similar problem but Eurpopean gauge standard. Have not measured voltage but did replace the sender...
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Old 07-07-2011, 15:13   #3
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Re: VDO water Gauge help

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Originally Posted by SV Demeter View Post
I have a similar problem but Eurpopean gauge standard. Have not measured voltage but did replace the sender...
I take it replacing the sensor was not a successful fix since you are head banging. I did happen to notice that 7.68V is very close to 5/8 of 12 volts. At least as close as I can read on that analog guage. That might explain the full reading, but I don't understand why it won't go below 1/4. I'm guessing that the output from the gauge to the sender is messed up but I'd like to make sure. Replacing the sender looks like a nightmare as there's less than a fott clearance on top of a 2 foot deep tank.

It looks like VDO is discontinuing their line of gauges that works with 240/33 ohm sending units so I better figure this out soon.
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Old 07-07-2011, 15:15   #4
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On the back of gauge , short ground to sender wire and if it goes to full .your gauge is fine
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Old 07-07-2011, 15:19   #5
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Re: VDO water Gauge help

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Originally Posted by Benebobby View Post
On the back of gauge , short ground to sender wire and if it goes to full .your gauge is fine
Thanks, I'll give that a shot.
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Old 07-07-2011, 15:29   #6
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Re: VDO water Gauge help

As far as I know VDO uses two different systems in the 'old' days. One is based on resistance of the sender unit and one is base on current flow. The latter is actually a bit better as it compensates for length of wire between sender and gauge. Also the variation in supply voltage is no longer a problem.
So what could be is that the sender unit is actually not meant for the type of gauge. Also; check the supply voltage on the gauge. Make sure there is not a bad connection it the supply feed.
Good Luck,

Arno
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Old 07-07-2011, 15:56   #7
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Re: VDO water Gauge help

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Originally Posted by aluijten View Post
As far as I know VDO uses two different systems in the 'old' days. One is based on resistance of the sender unit and one is base on current flow. The latter is actually a bit better as it compensates for length of wire between sender and gauge. Also the variation in supply voltage is no longer a problem.
So what could be is that the sender unit is actually not meant for the type of gauge. Also; check the supply voltage on the gauge. Make sure there is not a bad connection it the supply feed.
Good Luck,

Arno
They were installed when the boat was new and worked for about 5 years before intermittent problems started. About a year later it never showed correctly again. If they had never worked I might suspect they were incompatible. I measured the supply voltage at the gauge and it was the same as the battery voltage, 13.1v. While there is no information on the gauge itself I'm assuming it is the resistance type because of the label on the sending unit.
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Old 07-07-2011, 17:32   #8
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I'm at my boat now and when I connect the two leads on the sender the gauge pegs to zero. When disconnected from the sender the gauge pegs full. My gauge is connected to a three position switch that monitors two tanks
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Old 08-07-2011, 10:18   #9
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Re: VDO water Gauge help

See Basic Engine Gauge Theory and Testing
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Old 08-07-2011, 11:34   #10
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Re: VDO water Gauge help

Thanks Gord, I should have known to do a search of your posts.

I'm assuming that the water level meter being a resistance type is really an ammeter. With infinite resistance 0 amps and at 33 ohms 364 milliamps at full deflection and 21 milliamps at Empty with 12Volt input. With zero ohms (shorting the sender post to ground) you will get infinite amps (dead short) minus the effect of the internal resistance of the gauge whether you have 12 volts coming out of the sender post or 7.68 as I have measured. If the gauge is really an ammeter then it is going to show full deflection at either voltage. According to the Gauge test this would indicate a good gauge even if it was substantially out of calibration and therefore marginally usable. That's why I am looking for what voltage to expect on the sender post. I would have expected battery voltage. I seriously doubt these cheap gauges have correction circuitry to account for variations in battery voltages. I will try the full deflection test, but my guess is that it will tell me my gauge is good. It just seems too much of a coincidence that the tank level is showing 5/8 full and the output voltage of the sender post is also 5/8 of nominal. 5/8 of the voltage applied to a fixed resisitance results in 5/8 of the expected amps if I remember my basic Ohm's law and can still do ratio and proportion math. I'm taking my good ohm meter to the boat next time and I'll recheck the ohms. I've also located a 33 ohm and 240 ohm resistor in my spare electronics parts box and will test the meter at it's theoretical calibration points.

Bill
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Old 11-07-2011, 19:47   #11
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Re: VDO Water Gauge - Help

I hope you are having better luck than I am... Heres what I have found. First off my boat is a Wauquiez Amphitrite built in 1984 in France so I am reasonably sure the guages for the water tanks are on some sort of european standard. I have two gauges which monitor 4 tanks. I am pretty well certain the problem is that I have the wrong sending unit because the gauge in question reads correctly for the other tank it monitors so does the other gauge on both of its tanks. There is a three position toggle switch to select which of the two tanks I want to check. I replaced the sending unit with a vdo european scale model but this did not help.

Tonight at the boat I took some measurements on the sender that is not reading right and also on one which is. First i measured voltage. When I remove the wires from the sender I get 8.2 volts on the good gauge and 8.4 on the bad one. Im willing to say this is close enough to equal given the differnent lenght circuits and voltage drop. I then removed the good sender and measured roughly 1 volt when in the empty position and 5.4 when full. I also measured ohms but am not sure I trust my old fluke 75 100%. I got 22-25 ohms on empty and 140-150 on full. Again this was on the good sender which I removed from the tank disconnected the wires from and manually actuated the float from empty to full positions. I think I need to find this type sender. On top of this VDO sender are the following numbers

40/102

828/1/1 10.00

I did not remove the bad sender because Im pretty sure its just the wrong type of sender and I ran out of beer. With the wires disconected from this sender im seeing 66-77 ohms when I know the tank is full. Im also seeing 3.2volts with the wires connected. Again Im not sure I trust the accuracy of my meter and admit Im not so great at using it when measuring resistance. Also of note when ever I disconnect the wires from the sender my gauge on both the good and bad tanks pegs full. If I connect the two wires together the gauge pegs empty.

Did I just get a bad sending unit or is there some funky european sender with variable voltage like Arno mentioned a few posts back? If so where do I find one of those? I suppose there is the possibility that the float on the sender I replaced is getting stuck on something inside the tank and in fact is okay. The gauge when it is connected stays right about 1/2 full which for a european gauge could be about 66-77 ohms. I guess I better get more beer and pull the sender. But I was pretty carefull when I set it up and installed.Boy will I feel stupid if it turns out the sender is jsut catching somthing inside the tank...
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Old 13-07-2011, 20:19   #12
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Re: VDO Water Gauge - Help

Ok so it looks like the problem is in fact the sender. I opened up the tank access and the sender i installed just 2 months ago new was covered in rust...When I move the float up and down the gauge works correctly. Where can i find a sender suitable for use in a water tank that is on the european ohm scale? I did find this:

Sailboat Supplies, Engine Parts and Boat Parts

but the price is rediculous and if it looks anything like the picture it will rust just as fast as the $30 one thats in there now.

These look great but they seem to be on the US ohm scale.

Fuel Tank Senders, Gas Tank Senders, Water Tank Senders, Holding Tank Senders by WEMA USA, Inc.

maybe they can make a euro scale model...
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Old 13-07-2011, 22:03   #13
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Re: VDO Water Gauge - Help

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Originally Posted by SV Demeter View Post
Ok so it looks like the problem is in fact the sender. I opened up the tank access and the sender i installed just 2 months ago new was covered in rust...When I move the float up and down the gauge works correctly. Where can i find a sender suitable for use in a water tank that is on the european ohm scale? I did find this:

Sailboat Supplies, Engine Parts and Boat Parts

but the price is rediculous and if it looks anything like the picture it will rust just as fast as the $30 one thats in there now.

These look great but they seem to be on the US ohm scale.

Fuel Tank Senders, Gas Tank Senders, Water Tank Senders, Holding Tank Senders by WEMA USA, Inc.

maybe they can make a euro scale model...
I've bought all new sending units ( Ultrasonic Tank Sender )
Perhaps they'll work for you.

Regards,
Extemp.
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Old 14-07-2011, 07:37   #14
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Re: VDO Water Gauge - Help

Thanks Extemp but at nearly $300 I think Im gonna pass although they do look nice. WEMA emailed me back this morning advising they sent my request for 10-180 ohm scale to their engineering group. Im hopeful they can supply one. Otherwise I will probably get the VDO sender. Apparently there is a SS version for about $150.
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