Fusible links

[1972nail]

I replied to a post on another forum where the member asked what rating the fusible links were. His loom had been chopped about and the links removed and he reaslised that an important safety feature had been lost. His car is a late 4 headlight model with 2 links of equal size. Early Firenzas and 2 headlight models have one larger link.

Here's my reply :-

The headlight one is quite simple to calculate using the modified Ohm's Law which adds Watts into the calculation where amps = watts/volts.

So the total load on the headlights of a Snoot with all 4 lights on is 4x55w - 220 watts. The voltage is nominally 12V so the calculation goes like this...

Amps = 220/12 = 18.3333 Amps.

If you allow a safety factor for things like a low battery of say 10V then the current goes up to 22A. The headlight thermal cut out on HC's is rated at 28A

For a 4 headlight flat front the calculation is slightly different in that during flashing of the headlights all 6 filaments are illuminated... 4x60 + 2x55 = 350W (assuming that the highest legal wattage is being used)

Again Amps = 350/12 =29.166. Then the safety factor for a low battery the current goes up to 35A.....hmmmm the thermal cut out is rated at 28A!

Ah but, normal operation of the flat front four light system only puts a load of 20A to 24A on the cut out (60W bulbs) and flashing the lights only operates the 6 filaments for a very short time, short enough that the cut out doesn't have time to heat up enough to react.

So back to the fusible links. The headlight rating would probably be OK at 35 to 40A. The other one on a four headlight car with 2 fusiblke links would probably be OK at the same value. On 2 headlight cars and those 4 headlight ones with 1 link (early Firenzas and modified 2 headlight cars) 50 to 60A might be a better bet..

Bear in mind that the headlight circuit has no other fuses, only the thermal cut out and that the rest of the electrics on the other link are protected by four 35A fuses on early cars and three 35's and one 45 or 50A fuse on later ones. Also that the ignition and starter circuits have no fusing arrangements at all, leaving the ignition resistance wire circuit totally unprotected, hence my bypass modification.

[fistfullofV5's]

I'll be needing to employ this knowledge on my 2000sl as the fusible link disintegrated when I was stripping the engine bay for painting. Have been thinking about making a separate engine bay fusebox and tucking it out of the way as I'm not overly confident the original setup won't burst into flames at some point.
Regards,Dave.

[1972nail]

Was it the actual link wire that disintegrated or just the double male brass connector? The link wire is the fusible bit.

[fistfullofV5's]

Firstly, I must apologise for a very late reply to your response. It was the plastic housing with the spade terminals that is fixed to the battery tray area. Very pleased that this might be salvageable.
Regards, Dave