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MG MGB Technical - smoking rev counter

The rev counter on a friends MGB gave a puff of smoke and then stopped working, this is the second time this has happened. Any ideas would be much appreciated. Many thanks in advance.
D Bates

What year is the car?

Positive or negative earth?
Dave O'Neill2

What did he have to do to get it working again before? Smoke sounds like a short somewhere. Don't see how polarity would make a difference, although whether it was a 72 or earlier current-pulse tach, or a 73 or later voltage pulse type might.
PaulH Solihull

The year of the car is 1971. Apparently the smoke appeared behind the glass. He didn't fix the problem he replaced the revcounter. I didn't know he had a problem until the second time it happened.
D Bates

<<Don't see how polarity would make a difference>>

Just trying to get some background information.

It could make a difference if it had been converted from positive to negative earth and the tacho hadn't!
Dave O'Neill2

Are their any other electrical problems like over charging the battery?
Check the output of the voltage regulator. Sounds like the output may well be high and cooking the electronic parts inside the rev counter.
MGmike

M McAndrew

Again, not enough info. *If* the car had just been converted, switched on for the first time, and the tach filled with smoke, and *if* it was immediately replaced and exactly the same thing happened, then polarity may well be an issue. But if the smoke suddenly occured during a period of normal use, and another period of normal use occurred before the next failure, then it isn't. It does beg the question of how two can fill with smoke though, I'd expect massively high voltage before alternator over-voltage would do that.

Is he sure it is smoke? Mine mist-up internally from time to time in spring and autumn.
PaulH Solihull

Paul, valid point, info is key and the fuller the better but take a chill pill and help guide the man to a conclusion. It's enough to just ask for more info or ask questions to guide rather than confront those who don't appear to have the troubleshooting knowledge/process and background we seem to have.
BTW who mentioned polarity change "converted"? If you know your B's (as I think you do) a 71 car would be -ve earth and yes, a high voltage would be required to smoke it out... but better to check the basics first.
There are only two "normal" sources of failure that would cause smoke inside the tacho (assuming its not mist as it stopped working!). 1, High input voltage to the electronics causing one of the capacitors or tranies to fail 2, high current in one of the circuits causing a wire to burn up. What circuits? The HV or ignition circuit which could short to earth inside the the head unit and cause smoke (I agree this would be accompanied with a significant misfire).. or the LV electronic circuit which the regulator pays a big part in.. hence the question about other things related to voltage increase..

In the absence of full info, check and eliminate the basics and, no matter how unlikely, if the possible has been eliminated the impossible is the most likely!!!!

MGmike
M McAndrew

This thread was discussed between 17/03/2012 and 20/03/2012

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