# Using a Multimeter – Help please!



## jiwawa (Jun 22, 2007)

We have an intermittent fault in our 12v electricity. Lights, heat, fridge light are not affected but power comes and goes to the water pump (all taps), toilet flush (which is connected to the fresh water tank and therefore the same pump) and the SOG fan.

There are a number of fine wires in around the SOG fan/toilet area which I suspect might have been stretched/broken. I’m thinking if the wire for the toilet flush is broken – would that stop all the connections to the pump?

I want to test them with an Altai digital multimeter but I need help! I know to put the black lead into the COM port, and red into the V/Ohm port.

The range selector – do I point this to the symbol that looks like an arrow running into a +, or do I point to the Ohms range (choice of 2000K, 200K, 20K, 2000, 200)?

When the meter is turned on and prongs not touching, the display reads 1 (it also says BAT – does that mean I need to change it?

What should it read if (a) there’s no break in the wire? And (b) if there is? 

I should presumably test them when it’s all NOT working, is that correct? I’m snookered at the moment, cos it IS all working!


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## listerdiesel (Aug 3, 2012)

With leads connected as described, for DC you want to select the symbol that is a straight line with a dotted line underneath, for AC you want the selection with the ~ on it.

Start at a high range reading if that is an option, but for auto-ranging just have a prod.

It is best to find the -VE terminal first, and try and fix that in place while you prod around with the Red lead.

Make sure that the mains supply is unplugged.

Peter

Edit: Here's a picture of one of our workshop Fluke meters:










From OFF it goes to V~ which is AC, then V with the symbol I can't type, which is DC, then mV DC then Ohms and so on.


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## pippin (Nov 15, 2007)

You are correct in the placing of the test meter leads.

You should *not* use the Ohms/resistance settings to measure voltage otherwise you may well burn the meter out.

I see that someone else has posted as I am am typing this so will pause!

OK now to carry on!

You are looking for 12V DC so select the range with the next highest value above that, probably 20V.

As mentioned, clip the -ve (black lead) to a known chassis or earth connection.

Then prod around to find a 12V connection.

With no pumps/SOG running you should read something more than 12V.

With the pumps/SOG running the voltage will drop a little, perhaps half a volt.

With the fault present then the volts will drop a significant amount.

CAUTION!

Be careful not to short circuit any connections with the meter probes.

Do not go anywhere near any of the mains sockets.

Confusingly, some MHs are wired with the 12V DC: 
Blue = +12V 
Brown = Zero or chassis.

Quite the opposite to what one would expect - it certainly fooled me for a while on our Hymer!


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## jiwawa (Jun 22, 2007)

Am I right in thinking I need to be testing this when the fault is there? Then I test for the 12v current coming through and if it isn't there, that's my problem? I take it the -ve terminal is the ground? And how can you tell which it is?

Or can I turn off the 12V supply and test for continuity - would that help me find the fault?

We're not on EHU so no danger of frying myself - I hope!


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## pippin (Nov 15, 2007)

OK give me a phone call - you have a PM!


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## listerdiesel (Aug 3, 2012)

Most meters are auto-ranging these days, but check before you apply prods.

I'd have a look at the fuse/switch panel first, and see if you have a damaged fuse or bad connection there.

Pippin's details are good, read through before venturing into the electrics.

Peter


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## Ian_n_Suzy (Feb 15, 2009)

Hi,

Another option is to find a good earth (negative terminal of your leisure battery would be one option) and connect the black (earth) lead to this, then look for 12v on the positive side with the probe at various points were you suspect the problem.

If the meter has a continuity test (usually beeps), this can be used to check for a break in the wiring.

Good luck


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## pippin (Nov 15, 2007)

You must not try the beep continuity test if there is a voltage present!


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## Spiritofherald (May 18, 2012)

If you're looking for broken wires and are still baffled by the meter then go to somewhere like Halfords and buy a cheap 12V test light. This device is as simple as it gets and will tell if you're getting 12V to a connection (lights up) or not (fails to light up).

This is the sort of thing I mean - http://www.ebay.co.uk/sch/i.html?_sacat=0&_from=R40&_nkw=12v test lamp&_sop=15


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## pippin (Nov 15, 2007)

I have just had a long (Skype) phone call with the good lady (in Spain)and explained everything to her.

We can hold off the replies on here for the time being.

Thanks guys.


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## jiwawa (Jun 22, 2007)

Thanks folks - and especially to Pippin who has just given me an excellent tutorial over the phone with him having his Elektroblok diagram, and me having the real thing in front of me!

I guess this is gonna take some time, but I've no other pressing appointments!

Will let you know how it all goes.


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## Ian_n_Suzy (Feb 15, 2009)

pippin said:


> You must not try the beep continuity test if there is a voltage present!


Yes, as Pippin correctly states, continuity is not used to detect voltage, but, err, continuity (apologies, I wrongly took this for granted).


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## jiwawa (Jun 22, 2007)

Update: when talking with Pippin on the phone, I removed the fuse governing the pump, and it felt a bit loose. It's the end fuse on the block so maybe it had taken a knock. Anyway, I decided to see if that was where the problem had been before getting into trying to trace the wires which sounds like trying to find a needle in a haystack! 

So far so good. I had wrongly assumed that if the loss of power was intermittent, then it couldn't be the fuse. But I hadn't thought of an ill-fitting fuse. 

So, if it goes again then I have the directions to follow the wires. Thanks to all, and particularly Pippin.


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## jiwawa (Jun 22, 2007)

We did very well for about 5 weeks, then started to have the problem again. It would be on again after a short drive, then fail after a while. 

Couldn't be the batteries as the rest of 12v was working. Again, it was pointing to loose connection. So I got down to tracing the wires. Thanks to previous info, I was able to find the connection was good as far as a huge. bank of blade-type terminals. Then the wires disappeared into a cable... to where? 

It was a while before I realised it had to go to the pump BEFORE it split kitchen/toilet. So had a look at the tank. No voltage coming through. Then I noticed there was rather more wire visible on the earth than on the live. Sure enough, that was where the problem was (well, I hope so!). The wire was sitting in the mouth of the terminal, but not secured. So sometimes it would make contact, sometimes not. 

It's now secured, so hopefully my problems are over. 

Many thanks to all for yr help on my learning curve! 

BTW, is it always true that if the earth is disconnected, there won't be any power through the live?


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## Pudsey_Bear (Sep 25, 2008)

JWW said:


> BTW, is it always true that if the earth is disconnected, there won't be any power through the live?


Yes that is usually true, unless there is a fault in the system or it can pick up an earth elsewhere. IE if you have a metal box and it is screwed to the chassis it will be earthed by contact, and also earthed by a earth wire, it is possible to disconnect the earth wire, and the box with still be earthed.


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