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

Glad you got there

Spot on sir

Congratulation that you sortet everything!

By way of an update on this one I did actually stick with the 7.5a o/s side light fuse in the end and with no blowing of fuse 12 at all, I figured it would work fine as two 5w side lights and two 45w fogs totals 100w and if you divide 100w by 13.6v you get 7.35a, admittedly a bit on the edge but the 7.5a fuse definitely refused to blow.

However, more recently and after one very enthusiastic acceleration event the plastic insulation around fuse 12 got hot and started to melt, this should not be confused with overloading a fuse. In my experience if a fuse melts but fails to blow it's an indication there's a bad connection somewhere, so I went hunting and with the prime suspect being the Marshals it didn't take long before I found the culprit.



As we can see in my above video the bulb is a very loose fit in the bulb holder, the BA21S bulbs used by Marchal are of the single contact type so the bulb body is the ground, clearly the poor fit and subsequent bulb chattering within the bulb holder was causing a make and break condition so I’m confident this is why the plastic body of fuse 12 melted last weekend, it also makes sense why the violent acceleration event triggered the issue.

The solution to stop the chattering was simply to put a bit more preload on the sprung contact, this pulls the bulb tightly into the holder against its bayonet pins, now, although I'm confident I've solved the melting fuse issue and proved the standard 7.5a fuse is fine I’ve still decided to follow what I did with my Lucas PL700 headlights and fit LED bulbs in my fog lights too.

I was pretty much convinced this would involve converting the bulb holders to H11 but to my amazement Classic Bulbs offer the rare Marchal BA21S bulbs in LED, they're not cheap but they will last indefinitely and according to Classic Bulbs will also draw over 75% less current than the original incandescent bulbs.



https://www.classicbulbs.co.uk/prod...m-6-12-24v-led-head-spot-lamp-ba21s-base

Just like my headlight LEDs they'll live under these yellow glass globes.





When the fogs are off this will give the correct golden glow the original Marchal globe style yellow bulbs produced back in the day.

Well done for persevering on that.

A fuse that melts like this but refuses to blow is definitely the result of high resistance, resistance creates heat and is typically either a function of cable of insufficient gauge, or simply bad connection, as it was in my case.



My Marchal fog lights had given months of reliable service, I'm sure someone will be along to tell me how illegal this is but I tend to use them as daytime running lights, Monty is Sport Green so tends to disappear to other drivers in the day unless I light up, yellow light seems to work well as it gets you noticed by other road users without blinding or dazzling them.

However, more recently something changed with the reliability, I suffered two melting fuse incidents and on both occasions the fuse point blank refused to blow. In both cases the melting started after the car took a jolt, the first time was a pothole but the second was just some hard acceleration, it all made sense when I found the loose fitting bulb holders in the Marchal lamps.

All sorted now, and simply solved by pre-tensioning the sprung contact, it was probably only marginally tentioned from new but had further relaxed over the last 12 months of use causing the bad connection, however, despite finding the issue and proving it was not the result of excessive current I've still decided to go LED.



These BA21S Marchal LEDs are 3000k so emiit a yellow light straight out of the box, but it'll be interesting to see how they perform because for an LED to emit a yellow light it must do so by adding a bit of blue and green to its natural white light output, as I understand it this can slightly lower the LUX output, perhaps I should have gone for 'warm white' as I'll be using the yellow glass globes anyway?

I'll report back on the results once fitted but Classic Bulbs do say these LEDs put out 1,800 lumens which is the same as I'd expect from the original 45w Marchal incandescent bulbs, so as long as they perform just as well I'll be happy as they worked great by day and night, of course the real advantage of going LED is they draw significantly less current which for many reasons is always a good thing in my book

Not quite correct there Dave. Unless you have a constant power load as distinct from purely resistive as your bulbs are, any unintended extra resistance in a circuit will actually reduce current as per Ohms law I=E/R. What it can do though is produce a volt drop at point of localised resistance as with a bad connection, translating as power dissipated as heat. This explains why images of melted connectors appear regularly on this forum.

Your fuse issue was because it was working for a substantial period of time at close to its maximum current though, as well as likely having slow blow characteristics. Consequently, and this may seem ironic, the poor contact you found on bulb holder may have reduced current just enough to stop the fuse blowing.

The answer would have been uprating the fuse value as originally suggested which would have stopped the fuse overheating whilst providing a tad more power to your bulbs. However your decision to go with LED equivalents sounds a good call.

FWIW my car is a 100% LED illumination fit now. I even reduced fuse values on the side and headlamp circuits to provide as much protection as possible given reduced load current.

Richard was bang on there.

Fuses, though a simple form of protection can in fact be quite complex in operation. It all depends on the fuse element heating up enough to melt and disconnect the circuit, before the wiring or protected component burns out! Ambient temperature will have an effect so fuses in a warm environment may not be able to carry the same current for as long as a fuse in a cooler environment, because they already have a head-start temperature wise.

The rating of a fuse is generally what it should be able to carry continuously, without melting. However, in the case of automobiles, it is advised in some quarters, that you should not exceed a maximum current loading of about 70% of the fuse rating for reliability.
Fuses are excellent under fault conditions, (short circuit/ very low resistance faults) creating disconnection in a fraction of a second.
However, under overload conditions, they are not quite so clever, or quick, as they take longer to heat up and melt. A typical auto fuse loaded up to say a third over its rating may take up to 10 minutes to finally blow and that can mean quite a cumulative temperature rise. ( just like Dave/ Montague has seen with his spot light circuit)

I'm sure you're right chaps and of course I bow to your superior knowledge on the subject, I’m just working on the principle that if you choose a fuse value that's correct, or even on the low side of ideal, it’ll blow long before the insulation around it melts.

After all, is this not the very working principle of a fuse?

If the result of fitting a fuse of a lower than ideal value is for the fuse not to blow, but to get so hot it actually melts its own insulation causing an undeniable fire hazard, you’d likely be safer running no fuses at all. As a young mechanic I was taught that if I find evidence of this initially counterintuitive condition, I should go looking for a point of high resistance. Indeed I subsequently went on to see this melting fuse issue on a number of occasions over the years, typically it was traced to corrosion somewhere in the circuit, but in one case it was down to a loose terminal.

Anyway, back to the subject of LEDs, I’ve been a fan for years, I put my TVR on LED H4s back in 2014 with good results. Of course things have moved on considerably in recent years with the LEDs I fitted to Monty last year proving to be exceptional, however, the Marchal LEDs remain untested so I’ll report back as soon as I've thoroughly tested them.

Merry Christmas to you all

I wouldn't worry too much about LED colour Dave just choose your preference. You can always buy a variant with higher lumen output, if not now then in the future. From a practical point of view daylight white seem to be best for nightime illumination. Certainly the LED strip lights in my garage with three selectable colour temperatures between warm and daylight white are all set to the latter.

With regard to using your fogs for daylight awareness, you may be interested in the switchback LED's I have fitted in front indicator position. Description of conversion starts here. Definitely the best electrical mod I've done to Morton and all using existing loom cables. Car passes MOT without mention despite not being able to turn them off once ignition on, as ex factory.