MG Car Repair: 1978 MGB Ignition light, wiring loom, isolator switch

Question
QUESTION: When I turn on the ignition to the 'on' position #without starting the car# and then back to the 'off' position the brake light remains on.  Also, the battery is not charging.

Thank you for any advise.

John

ANSWER: Hi John.  These symptoms point to a faulty diode in the wiring loom.  There may be one or possibly two diodes depending on the exact year and model of your car.  The diode(s) can be found under the dashboard near the middle of the car.  Access from inside the car looking up underneath the glovebox.  They are available from the following company link:

http://www.mgbsource.com/products/MG/1976/MGB/DIODE--EMERGENCY-BRAKE-LIGHT--IF-F  

This link is useful because there is a tiny picture of a diode to show you what to look for.   They are effectively a one-way valve.  You cannot fit them the wrong way round, because one end is male and the other end is female.

Test the old diode(s) with an ohm meter.  They should give a low reading one way, but an infinite reading the other way.  If both diodes are OK, get back to me.

---------- FOLLOW-UP ----------

QUESTION: On my 1978 MGB, when I turn the key to the on position the ignition light does not light up.  I replaced the lamp, pulled the plug off and re-attached to the alternator and cleaned the connections at the starter.  I have an on/off switch on the (-) battery terminal.  In the past, I could start the car and could turn the on/off battery switch  off and it would run.  Now when I start the car and turn the switch to off the car will die.  The gauges work.  Need HELP.

Thanks

Thanks.

Answer
Hi John.  I didn't know that you had a master battery switch.  I think your isolator switch has caused all this trouble.  You must NEVER operate the switch to disconnect the battery while the engine is running.  If you do, then

1) There is a good chance that you will blow the alternator, and

2) While the battery is disconnected, the alternator cannot recharge it.  If you start the car a few times and never recharge the battery, then the battery will go flat.  

Sorry to tell you this, but I think you need a replacement alternator.