Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Fog Light Wiring

A little confused by your logic in reducing fuse 12 rating to only 5 amps now Dave. As you are aware this fuse covers the offside side lights and dash illumination and it's rating of 7.5 amps was chosen by the factory, with same rating fuse 11 for the independent nearside sidelight circuit, to cover the loads from OE filament bulbs. The previously redundant dedicated wiring in the loom you have employed for your fog lights takes its power from the same o/s circuit/fuse, so you have therefore added to the ex factory load albeit by only a small amount now by using LED bulbs. It seems odd to me therefore you now choose a smaller rating fuse unless it course you have changed all sidelight bulbs to LED as well

Not sure what your voltage measurements prove other than showing they are as expected. With regard to the fragile PCB board, as explained already the front fog light wiring you have employed is independent of this PCB. There is a sidelight power feed to this PCB but only to provide a conditional trigger for rear fog light when selected.

Post 2014 MMC had rear fogs on when selected conditional of headlamp selection, main or dip. This conforms with our BMW Mini. Previously they required only sidelights on which struck me as wrong.

Because one of these.....



Is way cheaper and easier to fit than going through all this again.



After including the two Marchal LEDs my calculations definitely show a 5a fuse is the most appropriate value, and my testing supports these calculations, all this suggests to me Morgan’s 7.5a fuse choice may not have been ideal.

We shouldn't blindly assume Morgan got everything right, and however small the risk to my way of thinking a blown 5a fuse is infinitely preferable to all the other failure scenarios.

With it being the foglights there's no harm in testing it thoroughly in daylight .... can't beat a real-world field test ... and if it blows then back to 7.5.

K

Sorry, hopefully not too nerdish, but we are all here to learn and consequently, I cannot resist tackling some of the technical aspects for accuracy, through these threads.

Let’s step through the electrical theory, it a stage at a time:
We have Ohms Law illustrated by volts divided by amps equals resistance. Also if we multiply volts by amps we get power in watts.
So all these quantities are inter related and proportional to each other BUT the point that I wish to make is that Resistance is the ONLY constant, (only affected marginally by temperature, but lets keep it simple.)

Referring to the earlier thread No. 791827, when we look at an electrical load in Watts, that is the power consumed at the NOMINAL RATED voltage of the equipment (12 volts in automobile cases)

So if 12 v is applied to this load of 30 watts as quoted, it will consume 30 / 12 = 2.5 A
By virtue of Ohms Law, this means that the actual Resistance of the load, which is fixed, is 12 / 2.5 = 4.8 ohms.

Now, If under normal running, the system voltage is actually 14v (due to alternator etc) , the current that will flow, again by virtue of Ohms Law will be (Volts / Resistance)14 / 4.8 = 2.91 amps. ; and similarly we are now actually getting almost 40 watts in total from the same bulbs. (e.g. with headlights switched on, rev the engine from tick over, and you see the lights brighten)
As we can see this is almost 0.8 amps above the calculation in the earlier posts.

I stated in an earlier thread that the selection of fuses for auto use should be subject to a load of about 70% or so maximum for reliability. So here is just one reason why that margin is necessary, as the current flowing will increase with alternator charging.
Also, if a fuse is run constantly at its full rating, although in theory, it should be able to do that forever, in practice, due to variations in ambient temperature, current spikes, etc. and the general heating of the fuse element working at full tilt, one could find that the fuse “gets tired” and melts at some time in the future yet can be replaced with a fresh one that will in turn, carry on for a long time before the process repeats.

Coming back to Monte’s case, if the 30 w total load is correct, then we can see that his 5A fuse is loaded to about just under 60% so happily should be ok in this case.

Just as a final note, bear in mind that as I stated earlier, fuses are not that great at protecting for overload conditions. Their prime function is to protect against short circuits and safely disconnect the circuit before the whole thing burns out. A car battery under a dead short can easily produce several THOUSAND amps. So the wiring causing the short circuit would virtually vaporise in seconds, catch fire and burn our beloved car to a cinder. So although it is important to select a fuse with a rating in excess of the total load, it is essential to never choose a fuse rating that is above the rating of the wire that it is protecting.

Here endeth!

Fair point Dave if you can get away with the lower value although not sure you included all loads on that fuse in your calculations. I understand your concern following PCB repair but note this fuse feeds only the rear fog relay coil on PCB which you say didn't fail during the burnt land episode. Note also the PCB has other power feeds including a 20 amp fused permanently live one!

Apologies if I appeared a little harsh with my criticism but it did seem an incongruous move to reduce fuse value after your previous overheated fuse issue in this location despite switch to LED's.

Regarding MMC making the wrong fuse choice, I imagine they erred on reliability especially given same fuse values for the sidelight circuits are used on later cars.

Something like this would be useful in asserting the actual load current a fuse has to handle. Or you could probably manage with probes of a multimeter as long as the DC current range was adequate.

^ Not seen one of those before Richard, looks a handy bit of kit