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MG MGB Technical - Alternator/Ignition Light

I have the proverbial electrical problem. I have a 78 roadster, mostly original but with pertronix electronic ignition. Ignition light started glowing at idle, fading as the rpm's increased. I changed the alternator with a rebuilt lucas. Easy enough swap, except that, now, the ignition light never comes on. Pulled the bulb - isn't blown. Battery is still putting out 12 volts, so I know alternator is charging. Checked all the ground wires that I can think of, traced the ignition-lamp-alternator loop and can't find anything out of sort. Nothing. I can pull the plug off the alternator, and still nothing. What am I missing?
Jeremy Lawrence

Just because your battery is putting out 12v doesn't mean you're charging. In fact--the voltage reading should be higher. Recheck and make sure you've got it correctly hooked up--reference the schematic as you're doing this. Wiring could have been "re-engineered" by a PO. Use an accurate meter to test.
R. L Carleen

I had the same thing happen and after months of the light not working found the third wire on the alternator plug never got pluged in properly. The plastic part was not in good shape so when I thought I plugged it in it just bent over and never connected. I wished I checked it first because changing bulb was a real pain in the butt. Also most auto parts stores will do a free test of the battery/charging system if the think it might sell some parts.
Bob Ekstrand

Assuming that you have power from IGN to the IND lamp, and wiring is in order, The Alternator is faulty. Voltage delivered to the Batt at 3000rpm should be 13.8 to 14.4 approx. with a fully charged battery and the lights on. Voltage from Alt+ to Batt+ should be no more than 0.2, same from Alt- (case) to B-. The light will NOT come on with the plug out. See the other thread "alternator problems"
FRM
http://www.usachoice.net/gofanu
FR Millmore

This thread was discussed between 10/09/2004 and 11/09/2004

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.