On 01/05/2018 10:52, Dave Plowman (News) wrote:
In article ,
Graeme wrote:

The Morris again. Just to recap, 1968, now -ve earth but still dynamo,
control box etc.

Suddenly started to flatten the battery whilst parked, but charges when
running. Restarting a couple of hours after a good run, no problem. 24
hours after a run, battery sluggish but will just about turn over
engine. 48 hours after a good run, starting by handle only.

I have convinced myself the problem is the Lucas control box, which is
almost certainly original. This is a Minor, so there is nothing to be
left on when the car is not running, other than lights, which I have
checked, ditto interior light. About the only other electrical
accessories are indicators, wipers and simple on/off heater blower, but
they are wired through the ignition. No radio, heated rear screen or
anything like that.

Given that control boxes are around twenty quid, I'm tempted to just
replace the old one and see if that fixes the problem, but have I missed
anything obvious?


IIRC, there are contacts inside the control box (a relay) which isolate
the battery from the dynamo when it isn't charging. With no fan belt on
the car, and charge in the battery, if you make those contacts by hand,
the dynamo will motor. And I have seen them welded closed. That would draw
considerable current and flatten the battery very quickly. BTW, closing
those contacts by hand was a quick and dirty way of polarizing a new
dynamo, or re-polarizing one after changing the ground polarity.

But I'd not claim to be an expert on those control boxes - not had one for
a very long time indeed. Perhaps 40 years, so likely forgotten what I once
knew - if I indeed did. ;-)

Correct, the boxes are normally openable IIRC (they might be riveted
closed) and the contacts are fairly obvious. So easy enough to check
before buying a new one. I'm sure there will be drawings on the web.