> I have cleaned the contacts on the purple & green fuses and reconnected
the
> power. No change (radio, interior courtesy light, cigar plug - all on the
> purple circuit are dead), but this time when I check out the fuse, I see
> it's blown. I'm pretty sure it wasn't before so I take it out, get a new
> one, put it in, and the very second I connect the brown wire back, the
fuse
> blows. So I'm leaving it for the moment, because obviously something
> somewhere is causing it to go.
So you do have a short after all. This may not have been blowing the fuse
originally if the fuse was making a bad connection - the bad connection
acting like a self-resetting circuit breaker as the short came and went.
However, the connection of the radio to the heater switch wouldn't cause the
purple fuse to blow - unless the D (in this case) PO has buggered-up other
wiring as well. It seems unlikely that the purple circuits (horm interior
lights) come and go at the same time as the green circuits do as one is
powered all the time and the other from the ignition. If there were a
connection between the two you would not be able to switch off the engine
and your battery would flatten while parked.
> My Haynes manual (short book, for MGB 1969 to 1981) only has wiring
> diagrams up to 1977 (export). Mine is a 1978, but I'm using the 1976
> diagram as I assume it hasn't changed much since then. Or should I use
the
> '77 export one? At any rate, how can I work backward to figure it out - I
> can't use a multimeter because the thing blows the second I give it
> juice. The only thing I can think of is to one-by-one disconnect each
> device (or disconnect all and reconnect one-by-one) until I find the one
> that blows it. Any other thoughts?
Connect a high-power 12v bulb e.g. an old headlamp bulb in place of the
fuse. If the bulb glows brightly the short is present, if not it isn't.
> Also, can someone explain to me how the fuse block works? I was confused
> because mine has upper and lower connectors for source - for instance two
> browns run to the one fuse, but I'm guessing the bottom brown is the
"brown
> out" - that is, that the top one has the juice and the bottom one
continues
> it down the circuit. Is this how it goes?
Yes, the brown and white (or white.brown) side of the fusebox is the 'power
in' side for all fuses. Only one white or brown wire is the 12v feed in,
any other wires of the same colour are taking 12v from it.
> The other thing that is weird is that I have the four fuses... but only
the
> bottom two seem to be used? I'm not sure how this happens. Purple and
> green are the "out" on the bottom part of the fuse block, fed by brown and
> some other color I forget right this second - may be white or pale blue.
At
> any rate, they're fuses #3 and #4 when counting from the top. This is
> fine, but what confuses me is the other two (the first two fuses going
down).
>
> The first two fuses, one seems to have an "out" but nothing connected to
> the "in" side. The other one is the reverse - seems to have an "in" but
> nothing connected to the "out" side. The wires are more red with black, I
> think. Is this some kind of way of doing double-fusing where they're
> joined internally to run through two fuses? If not, how does that part
> work? Haynes doesn't really say much about it. I'm not fiddling with
them
> anyway as my problem is on purple, but I was curious.
The top two fuses are for the parking lights - one fuse per side. A single
red/green comes in at the front but the fusebox has an internal link between
the fuses on that side. On the other side you should have four reds, two
per fuse. These are the 'outs' to the parking lights - one wire per corner.
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