Residual voltage in water pump circuit

[sgtangel]

Hi all,
I have a 1993 Swift Corvette Diamond, with a standard Truma water inlet (old style, not cartridge) and 12v whale water pump. The van is powered by an Amperor products 3 stage leisure charger, which replaced the standard built in swift charger which was faulty when we bought the van 4 yrs ago. It was professionally installed by the dealer, and appears to be working absolutely fine.

My query relates to the 12v circuit for the water pump.
Whilst preparing the van for our first trip away next week, I was preparing to sterilise the water system, but the pump would not operate. I did a few checks, and found that the 12v on the battery pole had worked loose, so repaired that. I re-checked, still no pump operation. (this was just fortunate I found this)

I then checked the voltage at the pump socket, and this is where I have my query.

With the amperor turned OFF, therefore only on battery, the pump circuit is at zero volts. When I turn a tap on, 13 volts....... this is exactly what I would expect. (disclaimer - I am NOT an electrician, only a keen diy`er)

With the amperor turned ON, the pump circuit is fluctuating between 6 and 7 volts, and when I turn a tap on, it goes to a solid 13 volts.

I have never, to my knowledge, had occasion to check the residual circuit of the socket with the amperor on, so could not honestly say if that residual 6/7 volts is normal, HOWEVER (and here`s the surprise) my fault finding diagnosed a jammed pump, and after freeing it off, the pump operates fine....... and when plugged in, everything works exactly as it should....

So, had the pump not jammed, I would have been unaware of the residual voltage, and this could be absolutely normal.... but now I know about it, I need to make sure it is safe....

Its like a countdown conundrum.......

Any words of wisdom from people far cleverer than me ??

 

[VicMallows]

I've re-read your post several times, and hope I have correctly understood your symptoms!

Firstly, I don't think the jammed pump is relevant, and was just a coincidence.

I assume you are probably using a DVM (digital volt meter). These (especially cheaper ones) are notorious for indicating spurious readings, especially of any tiny AC signal finding its way onto the (supposedly open circuit) wire you are measuring. You are probably seeing a spurious voltage (effectively 'interference') created by the battery charger. ( If I wanted to verify this, I would reach for my traditional AVO8 mechanical meter, which does not suffer in the same way because of the way it 'loads' the circuit). However, I do not believe this 'spurious voltage' to be of any concern.

 

[sgtangel]

Thanks for the reply VicM, I totally agree, I think the jammed pump was simply that - a jammed pump !
It was, however, the reason I tested the circuit, and found this anomoly....

I am indeed using a digital meter, and in the grand scheme of things, it is probably classed as a cheap one, so hopefully you are correct.

I have had the van sitting for the last 4 hours or so, with everything on as you would when on site, and periodically checking the pump (whilst running the sterilising solution through at the same time) and it all seems normal, so I have to think that this residual voltage has been there all along, and it is not a safety issue.

I may borrow another meter just to see what results I get

 

[ProfJohnL]

I too agree with Vic's suspicions.

To be more certain its nothing to worry about, if you can find a 12V car bulb, and connect a couple of wires to the terminals of the bulb, then use the wires like the probes of the multimeter. First check the bulb by touching the wires to the battery. If it lights the bulb is ok.

Make sure the pump switch is turned OFF Then touch the wires to the pumps socket. I suspect there will be nothing in which case there is nothing to worry about.

But if the bulb even glows then you do have a problem, and you should refer the matter to who ever installed the charger.

 

[bertieboy1]

Prof,
Can't agree with your test with a light bulb,you can have a voltage present and the light bulb won't even glow.With the taps closed the voltage on the pump motor should be zero,your test only says volts may or may not be present and doesn't give a positive result.
Nothing wrong with using a digital voltmeter.

 

[Hutch]

I agree with Bertie boy. Even a £10 multie meter , is very accurate for our usae. But even a bulb and two wire , if there is a spark then you have a voltage leak there, this shows a problem , corroded contact on a tap micro switch,

 

[ProfJohnL]

Prof,
Can't agree with your test with a light bulb,you can have a voltage present and the light bulb won't even glow.With the taps closed the voltage on the pump motor should be zero,your test only says volts may or may not be present and doesn't give a positive result.
Nothing wrong with using a digital voltmeter.

Hello Bertie,
We know from the St Angel' report that when the Amperor charger is turned off and the pump switch is turned off, and the pump is disconnected, there is no voltage detected on the water inlet socket when using a DVM. But if the Amperor unit is then turned on and nothing else is changed, the DVM will start to show a "fluctuating" voltage between 6 and 7 Volts.

What this tells me is that with the pump switch "off" it fully disconnects the pumps positive power line from the 12V supply from the battery. So the positive line is electrically floating, and the meters input resistance of 10MOhms is sufficient to allow all induced potentials from air born sourced interference to be grounded, preventing any potential build up on the conductor.

However when the Amperor unit is turned on it is probably producing electro-magnetic emissions which are strong enough to overcome the grounding effect of the meter;s 10M R input. This could be due to the pumps positive wiring inside the caravan passing close enough to the Amperor unit to induce an EM signal on the wire. This is supported by the information that the signal fluctuates, and it is a lower voltage than the normal DC output of the unit.

It is therefore unlikely the imposed signal would be strong enough power the pump, but to prove that point I suggested using a car bulb. The bulb has a much lower resistance than DVM, and if the bulb glowed it would definitely indicate the was a more complex issue to be resolved. It goes a little deeper, becasue the bulb would be a more sensitive indicator than the pump, in so far that at low voltages the bulb would glow where as the same low voltage would not be enough to cause the pump to turn, BUT the current would still be flowing through the motor its brushes and coils and could cause premature failure of the pump.

The other side to this is, if the bulb does not glow it proves the signal has no significant current capability, and this will not pose a problem to teh life of the pump when its connected.

Hello Hutch
I use some very sensitive DVM's in electronics hobbies I have , and I am acutely aware of how the type of measurement instrument can affect the circuit you are investigating. Whilst DVMs are really fantastic devices and can be a highly appropriate instrument for much work, sometimes a lower tech approach can be a more realistic indicator. As for accuracy, yes they can be more accurate than analogue devises, but don't be misled by the notion that digital is always more accurate. DVM's have plenty of ways they can be inaccurate !

very much aware of having to understand the

 

[Hutch]

Prof are you admitting that a spark means there is a voltage supply from some where, . Some times use long wires and a bulb on $28 million heli. . A spark means power.

 

[ProfJohnL]

Prof are you admitting that a spark means there is a voltage supply from some where, . Some times use long wires and a bulb on $28 million heli. . A spark means power.

I have not even mentioned a spark? This is nothing about sparks, its about the detection of a substantive current if its there!

However, for accuracy you can get sparks without voltage, :huh: but sparks always means power, consider sparks from a friction collisions or those released from a fire or other chemical reactions. :evil:

 

[Dustydog]

So in simple dog language what is the problem
The pump?
Or does the new charger have a built in residual power supply of some sort.
Looking at its specification sheet it does have a Standby Current of 1.5mA . Is this relevant

 

[ProfJohnL]

So in simple dog language what is the problem
The pump?
Or does the new charger have a built in residual power supply of some sort.
Looking at its specification sheet it does have a Standby Current of 1.5mA . Is this relevant

There probably is not a problem, but without being able to investigate in person I couldn't be specific. The bulb suggestion would hopefully enable the OP to find out.

 

[sgtangel]

Hi all, thankyou for the healthy discussions - it is always appreciated to get advice from these forums.
We have the van all packed up for our first trip away, and I intend to check into this a bit more whilst I am away (weather permitting)
Everything seemed to be working absolutely normally as I was flushing the system, and I suspect that, had the pump not jammed, I would have been blissfully unaware of this occurrance, however I am intrigued nonetheless, and keen to be 100% certain there is nothing dangerous going on.

Stay safe everyone... the world is a sadder place this week

 

[Gafferbill]

Prompted by your posting, I have just checked my caravan since it is a 1995 Swift Challenger fitted with an Amperor 3 stage charger.
I fitted the charger some 6 years ago to replace the original and it has worked well with no problems.

I did the same tests as you described using a volt meter .........there is no residual voltage present at the water pump terminals on my caravan and I would not expect there to be.
I would not be happy with your situation but cannot help over the internet.
I would do what you intend, investigate further get everything checked out.