Still con-fused about fuses

[Member 10617]

Randall. You wrote that if I am going to use modern fuses, they should be rated 20 amps (LH fuse box) and 25 amps (RH fuse box)respectively.

At present, I have a modern 35 amp fuse in the LH fuse box (new from Moss) and a 50 amp fuse in the RH fuse box (new from Moss). The reason for the 50 amp fuse in the RH box is that when I had a 35 amp fuse in it, my new electric fan did not work. The 35 amp fuse did not blow, the fan just did not work (a mystery to me). When I put in a 50 amp fuse in the RH box, the fan commenced to working again.

What are your thoughts about this? Should I drop down in LH to modern 20 amps fuse(or perhaps 25 amps) and in RH to modern 25 amps fuse? With 25 amps, I don't know if my electric fan will work, but I would soon find out.

What is "standard" practice these days?

As it stands right now, the 35 in LH and 50 in RH seem to be working.

 

[Andrew Mace]

Ed, any fuse that can handle normal current flow is "fine" so long as there's no problem with that circuit. It's only when that circuit gets overloaded due to a short or other fault that the fuse rating can make the difference between a sacrificial fuse and a plume of acrid smoke billowing up from under the dash. :frown:

How are the Moss fuses marked? Are they Lucas, and/or do they say -- for example -- 35 amp; 17 amp continuous? If so, you're good to go. If not, I'd call Moss and confirm exactly what they are.

 

[Twosheds]

The reason for the 50 amp fuse in the RH box is that when I had a 35 amp fuse in it, my new electric fan did not work. The 35 amp fuse did not blow, the fan just did not work (a mystery to me). When I put in a 50 amp fuse in the RH box, the fan commenced to working again.

Maybe the 35 amp fuse was not passing current even though it looked to be not blown. I have experimented with this.

Or, when you replaced the fuse, you inadvertantly cleaned corrosion from the fusebox contacts.

 

[Member 10617]

The Moss fuses are simply marked on a piece of paper in the tube as "35 A" and "50 A". They don't say "continuous."

I have two fuses that came with the car that say "Lucas 50A 25 Hold."

As for the possibility that the 35 amp fuse was actually not passing current, everytihg else governed by that fuse worked... except the fan.

Update; I just called Moss and they say that the 35 amp fuses are continuous at 17.5 amps and blow at 35 amps; the 50 amp fuses are continuous at 25 amps and blow at 50. Does that clear things up?

 

[TR3driver]

Maybe the 35 amp fuse was not passing current even though it looked to be not blown. I have experimented with this.

That would be my guess. In my case, the "root cause" was eventually traced to light (not visible) corrosion or contamination inside the fuse clip. This was causing the fuse/clip connection to get hot under sustained load, which in turn was melting the solder inside the fuse.

I have no experience with buying fuses from Moss, but it seems likely to me that they are selling reproduction Lucas fuses (meaning the ratings you have are reasonable). One check you could make is to measure the overall length of the fuse, as American fuses are slightly longer (1.25" vs 1.20"). It also wouldn't hurt to call Moss and ask.

Frankly, if it were my car, I would have the fan on a separate fuse. In fact, I do have my fan on a separate fuse

A simple in-line fuse holder (available at any auto part store) would do fine; or what I did was to buy a new covered Lucas fuse holder of the type used in TR6 (4 fuses) as I knew I wanted more fuses for other things (currently brake lights and headlights).

 

[Andrew Mace]

Update; I just called Moss and they say that the 35 amp fuses are continuous at 17.5 amps and blow at 35 amps; the 50 amp fuses are continuous at 25 amps and blow at 50. Does that clear things up?

Interesting that they are not marked with the "continuous" or "hold" rating, as Lucas fuses have been so marked since -- I think -- sometime in the early 1970s!

 

[TR3driver]

Interesting that they are not marked with the "continuous" or "hold" rating, as Lucas fuses have been so marked since -- I think -- sometime in the early 1970s!

OTOH, I have a "Clear Hooters" brand fuse in my parts bin that is marked simply "25 A".

 

[TR3driver]

As for the possibility that the 35 amp fuse was actually not passing current, everytihg else governed by that fuse worked... except the fan.

Than that would indicate the fuse was purely coincidence and the problem lay elsewhere. Fuses either conduct or they don't; they can't control which device gets current!

That comment also leads me to wonder : When you say you have 50 in the right hand fuse, do you mean the side that has 3 terminals? (meaning the fuse that provides power to the wiper motor, brake lights, fuel gauge, etc.) If so, I would doubly urge you to put a proper fuse back in. This was a poor design in the first place, which you have made worse by installing an oversize fuse.

Again, although it works fine now, the danger is when you have a short. If the fuse does not blow, the wires will overheat, and the original PVC insulation <span style="font-weight: bold">IS</span> flammable if it gets hot enough. 25 amps is way too much current for the relatively small wires used for all those devices.

I was just picking apart the harness from my old TR3A yesterday, with the graphic remains of a short. The insulation on the UW wire is simply gone (burned away) and the other wires are both partially melted and black from smoke. The outer covering is charred and partially burnt away. Just a few minutes more, and it would have been a blaze. Maybe it will never happen to you, but why take a chance?

 

[Geo Hahn]

The ones I just got from TRF are simply marked 35amp (stamped on the cap) though I'll presume they are correct (given the source). They have the blade inside, not the single wire and do not have the external solder like the old Lucas -- which is too bad as those (external solder) are much easier to open up and insert a repro Lucas paper.

 

[Geo Hahn]

...The insulation on the UW wire is simply gone (burned away) and the other wires are both partially melted and black from smoke...

Perhaps worth mentioning the possibility of adding a fusible link to our cars. I'll credit a long ago post by Randall that prompted me to do this. They're readily available at your FLAPS and can be installed w/o any permanent change to the harness.

I believe Peter Egan once said that a TR3 wiring loom burning is a smell you will never forget.

 

[Member 10617]

Randall,

Once again, many thanks to you (and others). I took out the modern (Moss) 50 amp fuse from the right hand fuse box (wiper motor, fuel guage, etc.) and installed a modern (Moss) 35 amp fuse. This time the electric fan worked -- no problem (still a mystery) -- but I'm good with that.

So now I have RH and LH fuse box: modern (Moss) 35 amp. fuses; no 50 amp fuses. The 35 amp, as the Moss technician told me this morning, is 17.5 continuous ("hold") and blows at 35. Does that sound OK to you, or should I install 25 A fuses?

 

[TR3driver]

So now I have RH and LH fuse box: modern (Moss) 35 amp. fuses; no 50 amp fuses. The 35 amp, as the Moss technician told me this morning, is 17.5 continuous ("hold") and blows at 35. Does that sound OK to you, or should I install 25 A fuses?

That sounds fine, Ed. The only issue <span style="font-weight: bold">might</span> be if the load exceeds 17.5 amps with everything running. But most likely, you are good to go.

 

[Member 10617]

Randall,

I have very little on that circuit. Only the things that are normally on the RH fuse box circuit. The only thing that has been added is a new electric fan, and there was one there before I had the new radiator installed.

When I got the car, there was a 20 amp fuse in this fuse box. I changed it to 35 amps when I read the Triumph handbook calling for 35 amp fuses. A local TR guy said I should use 35 amp fuses. Nevertheless, I am now tryng out a 20 amp fuse in the box and there seems to be no problem with it. Do you think I would be wise to go with a 20 amp fuse in the RH fuse box and a 35 amp and a 25 amp fuse in the spare fuse holders, just in case?

 

[Geo Hahn]

...Nevertheless, I am now tryng out a 20 amp fuse in the box and there seems to be no problem with it...

You may be mixing the amp system of US and British systems since you mention a 20 amp fuse.

Simplest thing I think is to use the British 35 amp fuse in both spots. The 20 amp (US) is higher than you want.

An alternative if you must buy fuses at a local auto parts place may be the 14 amp fuse as this gets you within the 'continuous' rating of a British 35 amp with the added bonus that it is close to the physical size of the (shorter) 35 amp fuse Lucas used to supply.

 

[Member 10617]

George,

You have shined a bright light on this murky subject. Many thanks. Yes, I have been equating the US (Buss, etc.)and British (Lucas, Remax, Moss) amp systems (didn't know they were different). So... in conclusion... I will put an original Lucas 35 amp fuse in each of the fuse boxes and a Moss (Lucas-type) 35 amp fuse in each of the spare fuse boxes, and will leave it at that.

 

[TR3driver]

Only the things that are normally on the RH fuse box circuit.

Problem is, that can be quite a bit under some circumstances.

Roughly speaking:
Brake lights : 4.2 amps (assuming you are using 1157 tail/brake bulbs)
Wiper motor : 5 amps (I'm guessing here, ISTR measuring mine at 8 amps with the wipers on dry glass)
Heater motor : 5 amps (again a guess)
Turn signals : 2 amps (1156 bulbs = 4.2 amps when lit, average depends on your flasher)

As you can see, that comes quite close to the original 17.5 (continuous) fuse, but only under certain unusual circumstances (eg, waiting to turn at a stop light with the heater and wipers on).

<div class="ubbcode-block"><div class="ubbcode-header">Quote:] The only thing that has been added is a new electric fan,[/QUOTE]
I don't know how much your fan draws. But mine draws a pretty healthy surge when it first comes on, enough to peg my 10 amp DMM; and about 8 amps even after the fan is up to speed.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]When I got the car, there was a 20 amp fuse in this fuse box.[/QUOTE]Was that 20 amps Lucas method? Or 20 amps American method? (Or even 20 amps ISO method?) It does make a difference!
<div class="ubbcode-block"><div class="ubbcode-header">Quote:] Do you think I would be wise to go with a 20 amp fuse in the RH fuse box and a 35 amp and a 25 amp fuse in the spare fuse holders, just in case? [/QUOTE]20 amps American method (ie SFE-20 or AGC-20) would be OK. But 20 amps Lucas method would be totally inadequate! Sorry to belabor the point, but the fuse marking is somewhat meaningless without knowing what method produced it.

My preference would be to keep the 35 amp (Lucas method) fuses on both sides; and add another fuse for the fan.

However, since it seems to be working for you, you could leave the fan on the 35 amp fuse with everything else, and just change the fuse when/if it blows. Don't forget though, that your brake lights won't work, nor will the fan; until you notice that the fuse is blown and pull over to replace it.

 

[Member 10617]

Randall,

The 20 amp fuse that was in the car when I bought it, in the RH fuse box, was a BUSS SFE-20 amp.

Should I keep a (Lucas method) 50 amp fuse in the car just in case the fan blows the 35amp (Lucas method) fuse, or should I just replace a blown 35 amp (Lucas method) fuse with another 35 amp fuse until I get this sorted out? You earlier warned against using a (Lucas method) oversize 50 amp fuse.

The next time I have the car in the shop, I'll ask about having a separate fuse installed for the electric fan. At present, it runs off the RH fuse.

Thanks for the warning about the brake lights.

Question: Do the headlamps also work off the RH fuse, or do they operated directly off the generator control box? If they work off the control box, does that mean that the headlights are not fused? I'm thinking of having some high intensity headlights installed.

 

[Geo Hahn]

Headlamps are not fused and there is no relay (full current thru the switch). Some headlamp upgrades will not be bothered by this, depends on what you go with.

I connected my fan to the horn side of the fuseblock... at least there is nothing other than horns on there and I don't generally lay on the horn for long periods while also running the fan.

That change (moving to the other side of the block) might someting you can easily do.

 

[mrv8q]

So, just to muddle the waters a bit, as Geo and Randall know, a 35A and 50A British-standard fuse is called for. P36 of the Instruction book: "The fuse carrier is located forward of the control box, and houses 2 operating and 2 spare fuses. The top fuse (50amp.) protects the horn, while the other fuse (35 amp.) protects those items which can only operate when the ignition is switched on.....".

As long as you're using "British standard" fuses, which I assume MOSS is selling, you should be all right....

 

[TR3driver]

So, just to muddle the waters a bit, as Geo and Randall know, a 35A and 50A British-standard fuse is called for. P36 of the Instruction book:

Yes, but that is a fairly early edition. Later editions show both fuses as 35 amps. I can't lay my hands on my 4th ed at the moment, but both 5th & 6th show 2x35.