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MG MGB Technical - More Electrical Fun Than Anyone Should Have

Back in July I spent about three weeks chasing an electrical problem that started with a loose/semi-broken wire and ended with bad connectors on the alternator. Of course I decided to replace the alternator before I found that it was the connectors.

Well, the problem has sort of returned. I think I have it narrowed down now to a bad alternator. Just need someone to verify my findings. After recharging the battery I checked the voltage at the battery (12+ volts). I then checked the voltage at the business end of the big connector (that has big brown wire in it) that goes into the alternator (12+ volts). Finally, with the car started, I checked the voltage that was coming out of the alternator which resulted in about 1 volt. The output was essentially the same at idle and around 2.5k RPM.

Another oddity of this problem is that the red/orange light on the dash is intermittent. Sometimes it is on and sometimes off depending on how bumpy the road is.

Cheers!
-Jason
1970 mgb roadster
Jason Coolbaugh

Depending on your set up, a good connection to the indicator light is essential.
George B.

Are you saying that without a good connection at the indicator light the voltage output of the alternator is going to be low?
Jason Coolbaugh

Yes.
George B.

George has it right. That can be a very elusive problem to solve. I had a constant hassle with the lack of battery power for a long time without realizing that the little red light never lit up - bulb was burned out. I replaced the bulb and the battery has held its charge ever since. Has something to do with turning on diodes or some such thing.

ALan
Alan

For an explaination of this, see:

http://www.vtr.org/maintain/alternator-overview.html
Dan Masters

"Are you saying that without a good connection at the indicator light the voltage output of the alternator is going to be low?"

No. Without the warning light connected it could be zero, or 14v. The warning light is required to start the alt charging, once it has started it is no longer needed. But in my experience a used alt *without* the light connected will start charging when revved to about 3k, then charge normally right down to about 500rpm i.e. as normal.

You should measure the alternator output voltage on the same thick brown wire spade as you measured the battery voltage. 12 point something is correct with the engine stationary or alt not charging (warning light connected and on) but it should show 14.4v with the engine running at a fast idle and minimal electrical load. You should see around 14v on any brown, purple, white or green wire with the engine running like this. If you measured 12v without the engine running, but only 1v when it was, then either you have some bad connections *and* a bad alt, or you hadn't got your meter connected properly, a a flickering warning light over bumpy roads is definitely wrong and also indicating bad connections.

One should always be alive to the warning light *not* coming on when you first turn on the ignition. It also comes on, flickers and dies when you turn off the ignition, if the engine has not already stalled, in which case it should be on anyway.
Paul Hunt

Jason,
The large spade connector at the starter is known to loosen and sometimes even fall off it's lug. I had this happen to me and had some of the symptoms you describe -- when I went over a bump the ign light was come on momentarily. It would be worthwhile to check the connector for crud and for tightness....
Rob Edwards

By 1973, how much earlier I don't know, the brown wires on the solenoid had lugs bolted to the stud with the main battery cable so much more secure. Still can be loose, or not making a proper connection even if tight.
Paul Hunt

Paul,
I didn't realise that they had changed the connection -- sounds like a change for the better! My car is a '69 and uses the spade connector, so if Jason's is a ring lug, has the first year of that design....

HTH!
Rob Edwards

Rob - although I have the bolted type it chose the middle of the annual test to go high resistance. Absolutely no warnings beforehand, and it hadn't been touched for at least eight years. Testers mechanic couldn't work out where the problem was, I went down and sorted it out, fortunately while it was still on their ramp.
Paul Hunt

This thread was discussed between 22/11/2002 and 24/11/2002

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