Carl Stern writes:
>
> The Prince of Darkness strikes during daylight...
-5 right off the bat. Lucas has nothing to do with Smiths gauges or MGB
wiring. Mostly likely cause is a P.O. that left off the fues box lid.
> On my morning commute, I noticed that my tach, temperature and fuel
>gauges went out. (Oil pressure gauge still on). I stopped at an appt. for
>about 20 minutes and went I came back out and fired up, everything was working
>the rest of the way in to work.
> Is this my voltage stabilizer getting ready to die? What would cause it
>to become temporarily unstable? Or is there something else going on?
>This is a 74 MGB.
>
> Carl
and Scott Fisher replies:
>To solve this, I took the fuses out, polished the metal on the connectors,
>then pinched them closer together to restore the spring tension on them.
>I also replaced all the fuses on general principles. I never had another
>fusebox related problem on that car. (Just don't ask me about the
>brake lights... :-)
Bingo! Exactly! Correct answer!! (almost :) I must add that if the fuse
box cover is missing, get one and put it on. Under the hood is a very
hostile enviroment for something as fragile as a spring tension connector
carrying 15 amps or so.
Diagnosing Carl's problem, we find that the three gauges that died are all
electrical. The speedo and oil-pressure are mechanical. Also, the tach is
not driven by the voltage stabilizer, so it's safe, for now. The most likely
cause is a bad connection somewhere in the fused ignition power supply. The
gauges draw very little power, so aren't likely to overload a bad connection
by themselves. So, we trace back down the green wires to where some more
substantial loads are handled. These loads are the turn signals, backup lights,
and brake lights. The point at which all these junction is the fuse panel.
So the most likely cause is a bad connection at the fuse, or the fuse itself
is bad, cracked filament.
If any one ever has this happen, all electric gauges dieing at once, check
the turn signals. If they also do not work, stop and fix the problem like
right now! Why? Not only are you without signals and gaugeless, you don't
have any brake lights.
>Speaking of fuses, don't forget that a fuse can occasionally fail due to
>a manufacturing defect. I've seen one or two of these thirty-five-cent
>fuses fail when one end of the metal strip became detached from the bullets
>at the end of the fuse, but it wasn't obvious to a cursory inspection.
>If a blown fuse could solve the problem -- that is, if the problem affects
>all the electrical devices on a single ciruit -- *always* swap in a known
>good fuse before doing any more troubleshooting. If it solves the problem,
>then peer down the inside of the fuse you took out and see if it's bad.
>You might be surprised -- and you'll definitely be relieved that you
>don't have to pull all the wires to look for open circuits elsewhere in
>the guts of the system.
Another way to say this is that to test a fuse, a VOM or test light must be
used. Visual inspection isn't worth even the time it took you. Most fuses
fail due to old age, usually a hairline crack in the filament. How many
"off again, on again" electrical stories have you heard from old car owners?
As a final note, bad connections at the fuse/fuse box are nothing new, or
unique to British cars. Matter of fact, compared to the old German style
fuses, the British are down right reliable. Of course, the "high" rating
is helped by the fact that the British didn't fuse many of the more important
circuits; headlights, ignition, fuel pump... :>
Randy
randy@taylor.UUCP
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