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Re: Weirdo electrical problem

To: mgs@autox.team.net
Subject: Re: Weirdo electrical problem
From: Tab Julius <tab@penworks.com>
Date: Tue, 20 Aug 2002 18:20:28 -0400
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.

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?

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?

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.

TIA for any suggestions...

- Tab

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