> Recently the fuel gauge, tachometer, and temperature gauge on my 79
> MGB went out. I tracked this down to a blown fuse (2nd from bottom).
> Unfortunately it keeps blowing fuses - I'm now on my 3rd replacement
> fuse after trying 20A, 25A, and 30A fuses. Any suggestions on what the
> likely culprit might be?
Is the fuse blowing coincidental with anything, like putting the car lights,
etc? If so then this will be your clue. Or, if the fuse blows the moment you
install it, then you probably have a direct short.
If a constant short exists, then you are lucky because this is the easiest
to fix. Either use Denise's method if you have a meter, or Mike's idea if
you only have extra fuses. Or better yet, Paul has suggested that a 12V lamp
can be used to test. Just place the lamp across the open fuse holder, and it
should light up which would be seen as the same current that would blow the
fuse. Now, just pull the red wires, one at a time, off of the fuse holder.
When you the light goes off, you have the offending circuit. What could be
simpler?
Note that you should have the Haynes or Bentley wiring diagram for proper
wire tracing.
I am working on a method for intermittent wiring shorts, having just had to
replace the wires to my reverse backup switch. The method involves
soldering, or clipping a PTC fuse (polymeric Positive Temperature
Coefficient solid state fuse) across the fuse holder, in parallel with the
fuse. So that when the fuse blows , so will the PTC. But once the short
clears, and since the problem is intermittent, the short will clear, the PTC
can carry a portion of the load current. Not the whole 35 amps, but say
20-25 amps.
This will allow a person to troubleshoot the system and have semi-normal
operation for the next few hours (say to limp home from a trip). Or days, if
the problem is very illusive. My wiring short only happened about once per
week and seemed to disappear very quickly. The PTC helped me run for a
couple weeks until I had it diagnosed and fixed. After fixing the problem,
the regular fuse can then be reinstalled and hopefully you will get a long
time before the next Lucas moment!!
If anyone is interested, I will do a write up of my method, including part
numbers.
Kind regards,
Tim Economu
1969 MGBGT
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