The V8 Owners Forum -

DaveEFI wrote:Simple surely to calculate the draw? You're only really going to need to know any added high current devices like cooling fans - and they tend to say how much they take.

So what is the current draw on yours ?

Both when accessories are switched off/on, and when the battery is in various states of discharge ?

You may think you know, or may think you can calculate...but you'll struggle to get it right, because there are more variables than you think.

Just normal daytime driving, no fans, no heater, no lights etc mine is sitting at around 45A draw just to run the engine. That's with a well charged battery.

Obviously if the battery was lower, it would be drawing more current to re-charge it on top of any electrical loads. Then add lights, heater blower, indicators, heated screen, cooling fans, wipers.

I originally had a 105A alternator, which I always thought was more than enough. But it really didnt take much for me to hit a 70A load, I stopped testing at that point.. And rated at 105A or not, I dont like my alternator running at 70% capacity which can happen for extended periods if driving on a cold wet night for example

I now have a 170A alternator.

it's surprising what's going on in the background you dont know about until you actually test it.

(Sorry to have missed a few recent posts.)

The reason for my original question is that something is wrong with my electrics and I’m not sure what it is – OK, I built my own loom from scratch. I have all plug-in sub-looms into one junction box containing relays, fuses, etc. as per this.



Basic wiring is Alternator > 100amp fuse > Starter > Battery then Battery > main 70amp midi fuse (end of red cable going to my ‘junction box’). I now split my 12v feed to a permanent distribution panel being one half of the bolt terminals in picture and 12v ignition on via a master 40 amp relay and to the other half of the bolt terminals. From there each circuit goes via fuses and relays off to all sub looms.

Initially I made my own main fuse holder up from wood and found that the two midi fuse nuts were loose due to getting hot and shrinking the wood behind. I then made up a new plastic fuse holder and now find the plastic has melted! At the moment I can only think that as I had drilled out the midi fuse mounting holes to fit, I have reduced by too much the fuse metal and it gets hot. Having bought a proper fuse holder and new fuse I now need to check the total amps being used. Downstream of the distribution all is fine IMHO.

I’ve had a puzzle for some time in that battery voltage is 12.6 but at distribution panel it is around 11.8. With engine running battery voltage is 13.2 and still around 11.8 at distribution panel. I’ve justified all loom circuits by removing fuses in turn to find volt drop – I’ve an MS ECU, an ABS ECU and Traction Control ECU – so happy that my main minimum current draws. I cannot detect a short / defect.

I would have expected my alternator to pump out 14.4 v – it’s a standard LR 100amp unit (44,000m only) so I have a puzzle.

I have calculated total / typical current usage as per first post so guess I could be near to 70 amps draw but as the car is not on the road, so far I cannot have reached that sort of current.

That’s why I want an ammeter.

Any help suggestions appreciated.

With the engine running, alternator charging and measured locally to the alternator, I would be hoping for at least high 13's

13.2 doesnt sound that great really. And those volt drops do sound pretty crazy.
Is all earthing good ?


IMO get a clamp meter like I posted. You will easily and quickly be able to hook up to multiple points to measure current loads so you can adjust wiring or fuses to suit.

if you need something more compact than the big meter clamp. These are an option. Although it's a BNC plug intended for some oscilloscope. They basically just output a voltage based on current. So could be read with a digital multimeter if you didnt have anything else. Only mention it because of the smaller head which is easier to get on a single wire in a bunch of wires.
Although it may not be big enough to go around some of the larger wires like starter etc

http://www.ebay.co.uk/itm/Hantek-Oscill ... 20d027a33e


But dont forget. When the engine is running, ALL the power to run everything comes from the alternator. It will power accessories and any excess will re-charge the battery. So the output from the alternator is the main one you need to measure. And this wiring needs to be substantial to carry this permanent high load.
100A fuse is maybe sensible, but I could see the fuse getting warm if it was charging at high currents for long periods. But probably wise having it there anyway. As long as the fuse and connections are rated for continuous current ?

The alternator works out how much charge it needs to give by (effectively) measuring battery volts. So it needs a very good low resistance connection to the battery. It's not usual to have a fuse in the alternator output - as a fuse can't blow quick enough to protect the alternator in event of a short. And any extra connections - to say a fuse - will introduce resistance.

With a good battery etc the alternator should settle at 13.8v, measured across the battery. If more than about 0.1v either side of that, it needs investigating.

You can look for problems without an ammeter - simply check for voltage drop across each part of a circuit. Set your DVM to 2 volts and measure along that circuit rather than attempt to try and measure the actual voltage at either end and calculate the drop - this is far more accurate. And also check the voltage drop on the chassis side.

Volt drop will indicate a wiring problem.

But it still wont tell you current loadings throughout the system

The 100A fuse....could be argued for and against.

I'd agree that it isnt typical on older systems. But high amperage fuses are quite common on modern cars these days. Although not 100% sure about their role. But modern cars do have lots of high current fuses and circuits, or fusible links.
So wouldnt be that surprised if one is on the alternator line

Fuses are not to protect devices like alternators, or any smaller loads. People mistake them for that. Fuses are there there to protect wiring from overload and fire risk.
As a secondary they might also help to protect devices, but that is not their primary role.

Although slightly ironic that on some VAG cars their main high current fusible link box has a tendency to melt and be a fire risk lol

If you had a long and "at risk" cable run from the alternator to battery, then a suitable fuse may be a good idea. But it would need to have substantial terminals and of suitable design to handle high currents for extended periods

Fuses of this kind are never fast blow as such. But in the event of an impact damaging wiring, they will still blow and may help to prevent wiring meltdown and fire.

There a big difference between a replaceable fuse and a fusible link.

Of course a fuse is generally there to protect the wiring - but a fusible link more to prevent the risk of fire, etc. I was simply guessing why Richard had included one on the alternator output.

Measuring voltage drop accurately won't tell how much current something is drawing, but will indicate where the problem is quickly. You can then measure the actual current draw if needed. But since Richard made his own loom I assume he knows what each individual item takes.

Will a clamp ammeter work on dc?
I thought they only worked on Ac
Ian

Yes the one's I linked to work with DC.

Ive been using them for several years, and they are very very handy.

Even more so when it's the one hooked up to the scope.

And I have always said suitable fuse, not simply a fuse.. I dont know what type has been used here though

The 100amp fuse is a standard TVR feature (a 1997 engine) and no doubt standard on the LR Disco source engine as I assume TVR would not add it.

I’ve spent very many hours (actually days) trying to find the fault and eventually found the loose main (70 amp fuse) terminals mentioned above and that cured several strange readings. As part of the process I checked just about every circuit I could so am confident of the downstream distribution circuits and earths. I also checked every volt drop of each sub circuit and that seemed to justify the 11.8v without engine running - it’s the ECUs I have that draw the current for the 12.6 > 11.8v. All other items were negligible.

As I may have an alternator fault, I need to check current drain especially as the plastic at my main 70 amp fuse terminals has (partially) melted. As I said, I had drilled out the fuse’s mounting holes from 5mm to 6mm and this may account for it getting too hot. Surprised but a replacement will tell me.

That’s why I want to measure amps from very small up to 100 in my junction box feed and indeed in other wiring.

My alternator is a Magnet Marelli A127M that came with the TVR engine. The battery is next to the junction box in the picture so close to engine / starter / alternator and not a long cable run at all.

ECU's in general shouldnt really draw a lot of current, as they dont really do that much.
If the engine ecu has a coil driver or several built in, that can draw some current.

But even injectors etc, really dont use that much.

The bulk of the load on my car is the two pumps. They alone draw about 22-23A during normal use.
I know my coils can pull up to around 23A too, although that's peak current draw, and only for milliseconds at a time.
Dont think I specifically tried my ecu to see what it is drawing.

I just clamped around the main feed from alternator to battery, and it displayed 43.5A with engine sitting idling in the garage.

I then tried it on a drive to see how things stacked up, and was very surprised.


I would say that even if you got up to 40-50A fairly continuous, it wouldnt take long for the fuse terminals to heat up, and the fuse itself, unless it is one designed to handle those sort of continuous loads.

Sitting idling in traffic at night with the heater on in the rain is probably worst case scenario lol.

I stopped turning things on when I got to about 70A ! I'm sure I could have got another 20A.
But as I was already having alternator problems, didnt want to push the one I had any further knowing it would nearly be maxed out.

Under normal conditions, an alternator won't be harmed by being 'maxed out' - they are inherently self limiting.

Have you considered going to PWM control of your pumps? They won't actually be shifting 20 odd amps worth of petrol needed to run the engine at idle, as it were.

If I remember correctly, Richard's MS has the four wasted spark coil drivers fitted inside the MS. However, these ground the coils so shouldn't be causing a high current flow on the MS +12v feed - only on the ground connection, as there would normally be a separate +12v feed to the coils. Same with the injectors.

If a component is rated for 100A and you run it at or near it's limit for long periods. Chances are things will heat up and suffer damage

much in the same way the 70A fuse has already. Much the same the way my 30A fuse for my pumps did

Yes I'd like to do PWM for my pumps....seems that isnt as easy as I'd like though. At least not on a budget.
But it isnt a major issue anyway.

stevieturbo wrote:If a component is rated for 100A and you run it at or near it's limit for long periods. Chances are things will heat up and suffer damage

[snip]

Several older cars could exceed the maximum alternator output with vehicle load - and by quite some margin. One such was my P6 3500S. But it didn't cause alternator failure - just a poorly charged or even flat battery.
It's not the same thing as running an engine at full power - an alternator is (or should be) perfectly happy delivering its maximum output.

The windings, rectifier and circuitry will be designed to handle a certain load.

Same with any electrical device, it wont just simply go higher and higher with no worries.
Otherwise there wouldnt be 70A alternators, 100A alternators, 140A etc etc

And conductors and terminals carrying that current for long periods...if they arent suitable rated, they will heat up. And heat isnt always a good thing