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MG MGB Technical - flickering ignition light

Help needed from auto-electricians: just changed to a new set of 6V batteries in a 74GT and now the red ignition light flickers at idle speed. Light goes out when engine revs are increased and, strangely, the flickering also stops when the headlights, defogger, or heater fan are turned on (with the engine at idle speed). Use of smaller electrical items like turn signals will not stop the flickering. Any help is appreciated.
willie

When additional load is applied does the light glow constantly or go out altogether? A 74 should have an alternator and in my experience an alternator, unlike a dynamo and control box, has considerable hysterisis between switching on and switching off by which I mean that when it is on one has to raise the revs to about 900rpm before it goes out, and once off the revs have to be lowered to about 500 rpm before it comes back on again.

A flickering light usually indicates a bad connection, if it is erratic and associated with road bumps. Possibly a bad alternator or regulator if more regular than that and while stationary. I trust you didn't connect the batteries backwards initially, which wouldn't do the alternator any good at all. OTOH it may simply be coincidence that the alternator has chosen now to go flaky, I can't see how correct connection of new batteries in itself would cause a problem ... unless possibly one of the new batteries is faulty. Tell us what the voltage on the brown and the revs are when the flickering occurs, when it goes out when the revs are raised, and when the additional load is applied at idle.
Paul Hunt

anyone have the email address for Steve S, Virginia
USA? He's been helping me with my MGB however, my
computer crashed and I lost all my emails..
Thanks in advance! D.
dave myers

Paul, thanks for your suggestions. When an electrical load is applied, the flickering goes away completely. The erractic light is not associated with road bumps - it will happen at idle speed with the car stationary. Also, when I was installing the batteries, I did for a momemt put the + cable on the negative post on one battery, but the ignition was off - would this ruin the alternator?
willie

Here are the voltmeter readings taken at brown wire fusebox clip:
Ignition off: 13.2V
Idle with no electric load: 14.3V
Idle with headlights anf heater fan on: 12.3V
Did I answer my own question - it's time for a new alternator?
willie

Willie. My readings for the ignition off are about 12.0 to 12.5 volts, somewhat lower than yours. The readings for the idle are about 14.5 volts, similar to yours. But, my readings do not drop below 13.5 volts with the system loaded which yours do. Thus, my reasoning is similar to yours-- the alternator is bad. This is all most of us can check. I had a problem similar to yours and took the car to a friend who was a professional mechanic owning better equipment than I can afford. He hooked up his meter which showed the charging rate of the alternator in amps. My alternator, which should have been capable of charging at 34 amps under load was only capable of producing 17 amps under load. The battery was picking up the excess load, hence, the flickering charging indicator light.

Hopefully, Paul Hunt will come on to verify my experience and add to it. If not, send him an e-mail through his website, "The Pages of Bee and Vee" and ask him to respond. Tell him both of us are eagerly awaiting his thoughts. Les
Les Bengtson

Willie, if you connected the battery terminals wrong, then there is a good chance you did damage your alternator. I have a 73 MGB and I was having the same problem with my red light coming on at idle. It turned out that it just got worse over time and I ended up replacing my alterntor and the problem was solved.
Robert Browning

"the flickering goes away completely"

Hi Willie - still doesn't answer my question, does the light go out or glow continuously? However given the other info the question may now be irrelevant.

Connecting just *one* cable to the wrong terminal will do no harm, connecting *both* to the wrong terminals *will* damage the alternator, if plugged in to the loom, irrespective of whether the ignition is on or not.

Whilst the normal standing voltage of a battery should be about the 12.5v Les mentions if the engine has recently been running and the alternator charging then the battery takes several minutes with the engine stopped, and everything switched off, to get back down to that level. The 'no load' voltage of 14.3v is fine, but the 12.3v with headlights and heater fan on is very wrong. Measure the voltage on the brown at the alternator plug and if you get the same conditions then yes the alternator is bad. Could be worn brushes or dirty slip-rings, or a bad regulator, or bad diodes. Frankly better to exchange it for a guaranteed unit than fiddle with it.

If you measure a 'normal' voltage of about 14 at the alt (check right on the alt spades if you can) but still 12 at the fusebox then you have bad connections between the two, the usual place is at the starter solenoid, but check the alternator plug and spade terminals as well.
Paul Hunt

Paul - forgot to answer the initial question: with electrical load on at idle, the ignition light goes out completely. Also when I was connecting the batteries, I did connect each battery individually to the wrong terminal for a momemt, never was both batteries connected to the wrong terminals for any length of time. Can this ruin the diodes? Thanks for your help.
willie

Willie - connecting both terminals (or all terminals in the case of twin 6v batteries) *at all* has a very good chance of damaging diodes and other electronics pretty-well instantaneously, unfortunately.

Your warning light going *out* with an increase in load tends to confirm electronics problems rather than bad connections. Bad connections tend to cause an increase in glow as the load increases.
Paul Hunt

This thread was discussed between 18/02/2005 and 22/02/2005

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