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MG MGB Technical - Charging Problem

I would appreciate help in analyzing a charging problem on my TD with a 3-main MGB engine. The charging light is showing an issue. The symptom is that the light does not extinguish unless I gun the engine to about 2000 or 3000 RPMs. After that the charging seems to revert to normal (e.g. the light extinguishes at about 1200 RPMs). This usually occurs when I first start the engine. However, the problem only occurs intermittently. Most times the problem does not exist.

My first thought was that the brushes in the generator were worn or were hanging up. I disassembled the generator and found that the brushes were nearly new. I did reduced the width of the brushes in case they were hanging up. When I first started the engine the same symptoms occurred. The control box is about 2000 miles old and came from Moss. I purchased a pre-adjusted control box to avoid possible adjustment problems. What is the most likely cause and solution for this problem?

thanks, in advance, for your help.

Frank Grimaldi
Frank Grimaldi

Are the dynamo and control box matched? Or is one for a TD and one for a B?

You need to measure the cut-in and drop-out voltages, and compare them with the specs. Connect the voltmeter between the D (output) terminal of the dynamo and earth, and gradually increase engine speed until the cut-out relay (the one with the normally open contact) just operates, which should be between 12.7 and 13.3 volts. Then slowly reduce engine speed until the cut-out relay just releases, which should be between 9.5 and 11 volts. What revs this happens at will also depend on the relative diameters of dynamo and crankshaft.
PaulH Solihull

This thread was discussed on 12/12/2011

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