In a message dated 98-10-01 15:26:14 EDT, ulmerK@UNCWIL.EDU writes:
> Can anyone tell me a good resource for replacement fuses for a 1974
> Triumph TR6
Kevin,
Yes, I can. Just go to any auto parts store; however, DON'T buy the 35 amp
sizes, as listed in the Triumph manual for Lucas fuses. Below is a copy of an
article I am trying to write for the VTR web site on this subject. The
material is all there, but it needs a good bit of editing yet before being
posted.
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According to the owner's manual, and stamped on a slip of paper inside the
fuse itself, the correct fuse rating for a triumph TR6 is 35 amps. This means
if a fuse blows, and I don't have a spare, I can just go to the local auto
parts store and pick up a 35 amp fuse from the rack and pop it in the holder,
right? WRONG! Never, ever, replace a Lucas 35 amp fuse with a 35-amp fuse of
American origin.
Why not? Although there is really not that much difference between the
structure of a Lucas and an American fuse, there is a big difference in the
way they are rated. Every fuse has a time/current characteristic curve,
depending on the type of fuse. Below a certain current, the typical fuse will
last indefinitely without blowing. Above a little higher current, the fuse
will blow in a few minutes. As the current increases, the time required before
the fuse blows is decreased. Above a maximum current for a particular fuse,
the time to blow is reduced to near instantaneous. There are other types of
fuses, some (fast-blow) designed to blow very quickly if the current exceeds
the rating by only a bit, and others (slow-blow) designed to blow over a
longer time, even if the current is higher than rated. For an American fuse, a
20 amp fast blow will blow fairly quickly above 20 amps, while a 20 amp slow-
blow will take a while at higher amps (not a great while - just long enough to
ignore short current spikes). Normally, though, these other types of fuses are
available only on special order from electronic supply houses, and not from
the local auto parts store.
This would be a good place to explain the purpose of fuses in an automobile.
There is a lot of misunderstanding about this. The purpose of fuses is to
protect the WIRING, and ONLY to protect the wiring. A fuse is NOT meant to
protect a radio, heater motor, lights, etc. It is not meant for that purpose
because it can't perform that function. It is entirely possible for a radio,
for example, to be damaged due to the heat of an internal short while it is
pulling less current than the fuse rating, and even while pulling less current
than it uses when played at high volume. A fuse can't protect a lamp, because
the only way to get more current through a lamp is for it to have failed
already. A fuse serves its purpose by offering itself up as a sacrifice when a
short occurs to save the wire from burning. It is much better to replace a
fuse than to replace burnt wiring, or, even worse, a burnt car! (As an
interesting aside, the National Electric Code, which governs most electrical
installations in this country, is published by the National Fire Protection
Association, rather than by an electrical engineering organization as one
might think)
This gets us then to the rating of the Lucas fuses. According to the Haynes
manual for the MGB, the 35 amp Lucas "...fuses are 17 amp current rated, 35
amp blow rated." What does that mean? Well, not being a fuse expert, I'm going
to have to take a guess, but I assume that over a long time period, current
"around" 17 amps will blow the fuse, and 35 amps will blow it instantaneously.
By comparison, a 35 amp American fuse will handle current very near 35 amps
indefinitely. I don't know how much it would take to blow it instantly, but it
would be a lot more than 35 amps, and certainly a lot more thean the wiring in
a Triumph is rated for.
How then should replacement fuse sizes be determined? Ideally, they should be
sized according to the current carrying capacity of the wires they are feeding
(and the wire, in turn, should be sized for the load it will carry). If the
wire is rated for 30 amps, then it is safe to use a thirty-amp fuse as a
MAXIMUM!. A 15-amp fuse should be used for a wire that is rated at 15 amp. Of
course, a wire that is rated at 15 amp will not burst into flames if 15.5 amps
are ran through it. Running 20 amps through it, though, for a long period of
time will probably heat it up enough to melt the insulation, and maybe even
catch it on fire. For a very short period of time, you might even be able to
shoot 30 or more amps through it without any problem, depending on how the
wire is routed. If it is in open air in a cool atmosphere, it will handle it
better than it would if it were in a wiring harness running close to the
exhaust manifold. I can state from experience that a 16 gauge wire, rated at
about 10 amps, will carry the total load of a TR6 (including headlights),
without burning, if you route the wire through the cockpit, away from the
engine. In an emergency once, I ran my car that way for about an hour. I held
the wire in my hand just to monitor the temperature, and it got quite warm. In
a wiring harness, it probably would have burned (the insulating effect of the
other wires and the harness wrapping would have held the heat in).
Most wires in a Triumph, with the exception of the battery and main alternator
cables, are rated at 8 amp continuous, with a few rated at 5.75, and a very
few rated at 17.5. When you are choosing the fuse based on the wire size, you
have to keep in mind that a 17.5 amp wire may feed into an 8-amp wire, which
in turn may feed a 5.75 amp wire. The fuse should be sized for the 5.75 amp
wire, and not the 17.5 amp. This means that the fuses in a TR6 (or any
Triumph, or any Triumph for that matter) probably should be all sized at 6
amps or less, unless you have traced the wires and know for sure the sizes
involved. If you do this, however, I doubt that you could use the car for
blowing fuses. Fortunately, there is a lot of conservatism built into the wire
ratings.
For practicality sake, I would size the fuses to be just over the maximum
current draw for all loads fed by a given fuse. For a stock TR6, and typical
for other Triumphs, this would be:
"RED" fuse - This fuse feeds red wires and supplies all the tail, parking,
marker, and dash lights, and pulls less than 6 amps.
"PURPLE" fuse. - This fuse feeds purple wires and supplies the glovebox lamp,
courtesy lamps, trunk light, the horns, the hi-beam flasher, and the hazard
flasher circuit. With the exception of the horns and the hi-beam flasher, the
load is less than 5 amps on this fuse. For a stock headlight, flashing the hi-
beams pulls about 9 amps, and the horns draw around 5 amps. If you have
quartz-halogen lights, the current will be more, but you really should have
them on a separate, un-fused, power source anyway (yes, I said "un-fused" -
that is not a typo!), and they should be relay operated. It is very unlikely
that you will have the doors, trunk, and the glovebox open while blowing the
horn and flashing the lights. Even if you should do this and blow the fuse,
the headlights, both hi and low beams, will still work, as they are fed from
another circuit. The hi-beam flasher merely bypasses the headlight switch.
"GREEN" fuse - This fuse feeds the green wires, supplying power to almost all
of the loads that are switched on with the key, the most notable exception
being the ignition circuit, which is fed directly from the igniton switch with
no fuse. The load on this one is a little harder to determine, as you will
seldom have all loads on at the same time, but the maximum load, with
everything on, is about 20 amps.
My recommendation? For a TR6, I recommend 10 amp for the RED fuse, 15 for the
PURPLE, and 20 for the GREEN. These are the sizes I'm using in my own TR6. Of
course, if you are completely rewiring your car from scratch, making your own
harness as I did, then you can size the wires, distribute the loads, and size
the fuses as they should have been done in the first place. When I'm asked why
I used nine fuses, my reply is "because Triumph used 6 too few from the
factory." It should go without saying that you should always carry spares. To
simplify spares, as the fuse box only has room for two, you can use a 15 amp
fuse as a spare for both the RED circuit and the PURPLE circuit as well.
For cars other than the TR6, you will need to use a schematic to determine the
wattage of all devices fed by each fuse. Add the wattages and divide by 12 to
get the approximate load in amps. For each circuit then, use the smallest size
available that will handle about 130% of the rated current.
Wattages or current ratings are not available for all devices, so the follow
typical examples can be used:
Windshield wiper motor: 14 watts/1.2 amps
Horns (pair): 5 - 6 amps
Heater motor: 3 - 5 amps
Gauges: 1/4 amp
Starter solenoid: up to 30 amp!
Ignition coil: 4 amp maximum
Relays: 1/4 amp
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/index.html
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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