The Importance of Voltmeters

Having some experience with MGB electrical issues, I decided that a nice Christmas present might be a voltmeter.  This would allow me to keep an eye on the electrical system and take appropriate measures.  Indeed it did, but that's a story for another paragraph!  First, I had to create a spot for it since the '67 did not have a voltmeter as a standard instrument.  There was some space to the left (to the left, to the left!) but it had been occupied by a rocker switch.  Since the meter would fit perfectly there, I began to consider how to create the proper opening.

The first thought was a hole saw.  But I realized I had a very small tolerance.  The step around the meter is only around a 1/16".  So, if the saw wandered at all, I might see daylight behind the gauge.

So, I decided on the long but controlled method.  I traced out a circle around the gauge body and drilled all around it.  I broke three drill bits in the process of drilling about 200 holes.  

 Then I set in with a file.  I'm fortunate that the dashboard is a mild steel stamping.  Speaking of which, it's a pretty fancy stamping with lots of depth and lots of wrap.  We would be proud of that today and it was made in mass production 52 years ago!

 After filing it looks remarkably like a circle!  To the extent that the meter actually fits!

To be fair, there were more than a few trials before that actually went in.  In any case, it made it and it works.  To its credit, it went to zero on my way from work one day.  This should have already been obvious since Lucas provides a red light on the tachometer which comes on if the dynamo is not charging the battery.  Sadly, this light had fallen out of the tachometer so I would have had no clue without the voltmeter.

This led to another investigation:

This is the inside of my new dynamo, which I installed last Spring when a horrible screeching noise overwhelmed the normal, noisy exhaust.  This turned out to be a failed bearing in the dynamo.  I bought the cheap imitation of the original and soon learned the error of my ways.  If you look closely, you can see a broken wire in the foreground.  I guess that was rubbing against the housing and finally wore through. Several friends and relations looked at that photo and said "Is that your finger in there?" and "How did you push a sausage down into the dynamo?".  Those are, however, the field coils wrapped in weirdly flesh colored tape.

Fortunately, I had permanently borrowed some solder from my Dad's workshop.

So, I was able to reconnect the break.  In the process, I also learned how this works.  The dynamo is much like a basic brushed DC motor but running backwards.  However, in a typical motor, there are permanent magnets to provide the magnetic field.  Instead, here they use another set of coils to provide the field.  Hence, they are called Field Coils!  Why bother with this instead of throwing in some magnets?  Because it allows electromechanical voltage regulation!  There is a relay that cuts off the field coils if the voltage gets too high.  That is, mechanical points open when the voltage is too high.  When you look at the complexity of today's cars, it's hard to imagine that BB gets me back and forth to work every day it doesn't rain!  But she does.  Happy Motoring!