Using a multimeter on a vehicle electrical system

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brian47

brian47

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Bridgend, Mid Glam. Somebody's got to live here.
There some very good, useful and highly informative threads here on the forum about vehicle electrics, how and why, etc.

@cowasaki's Car electricals tutorials
Part 1: https://www.landyzone.co.uk/land-rover/tutorial-car-electrics.245168/
Part 2: https://www.landyzone.co.uk/land-rover/cowasakis-electrical-tutorial-part-2.285354/
And @The Mad Hat Man's Basic Relay info: https://www.landyzone.co.uk/land-rover/basic-relay-info.48117/

But as with any of the less familiar tools at our disposal a certain amount of instruction might be a reasonable idea, and the electrical multimeter no exception.
Like any of the other threads, this is not intended to tell anybody who might already be well versed in the use of a multimeter, but there are some members who might have access to such a device but have little knowledge regarding how to get the best out of their meter.
I've tried to set out the very basic methods of using a multimeter, particularly the low cost digital types of multimeter, in the hope of de-mystifying the measurement of voltage, resistance and current on a 12volt vehicle electrical system. As with all things electrical, "A little knowledge can be a dangerous thing". If you still don't feel confident to tackle an electrical job, then don't. Always seek assistance.

As a bit of a safety note, it's unlikely that you'll suffer an electric shock from a 12 volt car battery, although I have known one bloke who could feel "a tingle", but a car battery does contain enough electrical power to cause a fire if things go wrong or an incorrect connection is made. When carrying out tests on electrical circuits, especially active circuits sparks could well occur. Most people have a natural aversion to sparks and they can, even will make you jump a bit.

The attachment below is in the .pdf format so that any of the pictures don't go missing some time in the future.
 

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I know all this as I've been using these for years, but I love things like this that re-enforce that I am in fact doing it correctly, as I bought one of these and by guessing and trial and error and the odd bit of internet help have sort of figured it out.
I really wish I had had something like this when I first bought a multimeter it would have been really really useful. :)
Now although I can service a series 11a it would be amazing if someone did a service manual like this, Then I could give it my wife and she could service her own landrover :) and I could sit and relax at the weekend :)
 
Thanks for taking the initiative and the time to do this Brian.
I'm one of those this is directed at. I know what a multimeter is. I've seen many.
But I haven't the first clue how to use one correctly. I need to learn, because I'm just
about to start a light Landy refurb. Will read all of your post with rapt attention.
Thanks again.
Mike :)
 
A power probe/scope is also very handy, a not so handy tool is a battery tester, I've used a few with mixed results, mostly condemning good batteries and bad ones tested as good.
 
Excellent tutorial, I loved my AVO 8 before digital meters came on the scene. Ah the good old days.

Is it worth mentioning very high voltage circuits like spark plugs, coils and HID bulbs.
 
One of the questions that I am often asked is "How can I separate my Auxiliary battery from my main battery so that I can use a winch or powered 12volt accessories but still get the battery charged when the engine is running?". The assumption here is that you have got or want a secondary battery fitted.
In reality the answer is simple, connect the two batteries together using a cable that will cope with at least 4 0 amps (6mm Thinwall flexible cable red & Black) but fit a split charge voltage sensing (VSR) relay in the circuit.
In both cases, Ebay is your friend. Search for 6mm2 thinwall cable, Voltage sensing relay and in line strip fuses and holders.
No need to worry about connecting plug in relays and sensing the alternator charging.
The system works by the VSR detecting the voltage rise as the alternator starts to charge the main start battery and then, making a connection effectively putting the two batteries in parallel. When the engine stops and the alternator stops charging then when the voltage drops the VSR drops out.
The result is that using the winch when you have connected it to the auxiliary battery means that the car battery will not go flat as a result and you will never have to call your mate out for a jump start.
Couple of things to note.
The leads between the batteries don't have to be bigger than the charge (amps) your alternator will produce.
The leads to the load need to be able to carry the output load (amps) required by whatever is on the end (winch?)
charging LR002.jpg
 
I'm no expert where batteries are concerned but my limited knowledge says to me that if your auxiliary battery is flat and your main battery is full when the relay connects the 2 batteries the auxiliary battery will take whatever the main battery will give.
Since the main battery can give way more than 40Amps surely this requires some form of current limiting before the fuse blows or the cable melts?
 
Here's a quickie that I used to use when I was teaching electrics to new recruits who had little idea about the subject and Ohms law was something that the old bill knew about if they wanted to give them a hard time.
Most people understand water so ...
Pressure in a system is the head of water (Volts). The higher the water supply vessel is the higher the pressure
The flow of water out of a tap is the current (Amps)
The amount of water that will be allowed to flow will be determined by the resistance to the flow, tap and pipe size for example. (Ohms)
The amount of water that actually flows in a system depends on the pressure of the water and the size of the pipework or the amount the tap is opened.
The water that ends up on the floor is the result of the pressure and the amount the tap is open (Watts)
 
backinblack said:
I'm no expert where batteries are concerned but my limited knowledge says to me that if your auxiliary battery is flat and your main battery is full when the relay connects the 2 batteries the auxiliary battery will take whatever the main battery will give.
Since the main battery can give way more than 40Amps surely this requires some form of current limiting before the fuse blows or the cable melts?
Click to expand...
The aux battery when flat has a higher internal resistance and this will prevent the rush of current.
 
Given this analagy a fuse in a circuit wouldn't blow, it would just limit the current flow through it in the same way a tap would restrict the flow of water.

I don't know how much current a flat battery will take if it had an unlimited supply but if the battery was flat and a winch was being used (with the engine runniing to energise the relay) I suspect that could exceed the 40A in this example.
 
backinblack said:
Given this analagy a fuse in a circuit wouldn't blow, it would just limit the current flow through it in the same way a tap would restrict the flow of water.
Click to expand...

A fuse is designed to go when the current reaches the fuse rating A fuse has designed internal resistance, as has the connecting cable of course but the fuse should be the weakest point in the circuit for obvious reasons. Current flowing in a circuit will, because of the fuses resistance, heat it up. When it has reached the set temperature it will melt. Bingo.
You will notice this heating effect whenever current flows in a circuit. Feel the jump leads when they have been cranking over a big'un. Feel the coiled up wandering lead when you are using a 3kw electric fire. All down to Ohms law.
You are right of course as a resistor in a circuit will limit the current. Depending on the wattage capability, it will get hot as it resists the flow of electricity.
PS. the water analogy doesn't work all the time by the way but for new students it was a good start point.
 
Having blown the 40amp fuse in a split charge relay unit I'd agree with a flat battery taking more than 40A given the opportunity.. .
One of my DMMs has has 6.5 digit resolution and is accurate to +-28ppm is that enough? :)
 
3D Printer said:
A fuse is designed to go when the current reaches the fuse rating A fuse has designed internal resistance, as has the connecting cable of course but the fuse should be the weakest point in the circuit for obvious reasons. Current flowing in a circuit will, because of the fuses resistance, heat it up. When it has reached the set temperature it will melt. Bingo.
Click to expand...

My point exactly.

3D Printer said:
You will notice this heating effect whenever current flows in a circuit. Feel the jump leads when they have been cranking over a big'un. Feel the coiled up wandering lead when you are using a 3kw electric fire. All down to Ohms law.
Click to expand...

The fact that the cable gets hot is down to it being under rated for the load and not down to Ohms law. Ohms law is merely a way of calculating Currrent, volts or resistance. In your example of a 3kw fire this would need a 13A cable, if you use a 5A cable with a 5A fuse it will blow, in much the same way as the 40A fuse in the dual battery setup. As per @TheQ point above.

The only reason I mention this is to make people think about what they are doing carefully and not just follow your example and waste time and money or worse fitting something under rated for the job.
A quick google search says an L322 alternator is over 90A (far higher than the 40A you mention)
 
As an example of under rated cable, many years ago, in a barrack block at RAF coltishall, the 16 man rooms had been subdivided into 3 man rooms each with a single socket in the wall so it was common to have a four block extension cable or two. This was of course removed and hidden during inspections.
The three next door to me turned on an illegal kettle, then a illegal fan heater was turned on, after a while there was a smell of burning plastic. After some panic it was found the cable nearest the socket under a mat had melted...
It was a 10amp rated orange garden cable, I suspect the fuse had been replaced with a thirteen Amp fuse
But the guys spent some hours scraping off melted orange plastic off the wooden floor, then had to repolish it before the next inspection..
 
"The fact that the cable gets hot is down to it being under rated for the load and not down to Ohms law"
Not exactly correct. The cable is "under rated" because it has too much resistance for the current that someone is trying to draw through it. Ohms law is definitely involved. A correctly sized cable would measure lower resistance. Power (watts) = I (current) squared x R (resistance). More resistance equals more power lost in the cable. As you can see, squaring the current gives you a big number so to keep the power loss down you need a very small resistance.

On my boat I have a 200 amp fuse in the cable that runs my electric windlass which is much like a winch.

One note on multimeters: when you turn yours on does it display all of its symbols and does that include a Low Battery symbol? If not I would suggest you buy a different meter or test/change batteries regularly because low batteries will give you false readings and cause much frustration.
 
Ohms law wouldn't try and stuff 30 amps down a 13 amp cable. I agree it will tell you why you shouldn't do it but it is just a mathematical equation.
The fact that the cable gets hot is down to it being under rated for the load.

All my test meters tell me when the battery is low, if I am in any doubt I just use one of the others to test it :)
 
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