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MG MGB Technical - Coil or Alternator

Hi guys

After a head de-coke, engine flush and coolant/pipe replacement the old engine just refuses to fire up on its own battery. Its a 77 (RB) MGB GT with Lumenition electronic ignition.
The car will start when connected to another via jumpleads, but get succesively lower on battery output each attempt. It was like this pre de-coke.
Started on jump leads with donor on medium revs. Starts immediately afterwards but gets slower and slower on subsequent attempts.
The battery is new. The fuel is good/new and getting there. The fanbelt is/has been tried tight/slack/correct.Plugs getting spark.
Alternator....Coil....earth/s....other ?
Any comments would be appreciated.






P Liles

Have you measured the battery voltage with the engine running?
Dave O'Neill 2

P-
Whereas the alternator only starts charging when the engine speed is raised to approximately 900 RPM, once it has started charging, it will continue to do so down to about 600 RPM or so under all “normal” electrical loads. Once started and blipped over 900 RPM it should charge at its idle speed of 750 RPM or whatever without the alternator warning light glowing. The alternator warning light should switch on and off at those speeds, and not flicker or glow dimly at any speed. If it does either, then you have a problem. A dim alternator warning light when running a load (e.g., lights, brakes, etc.) is a sure sign of partial diode failure, which frequently causes a low charging voltage. If the problem occurs when not running under a load, then you will need to measure the voltage on each side of the alternator warning light while it is glowing dimly. You will need to perform a Load Test by using a voltmeter in order to check the voltage. Since the battery produces direct current (DC) power, set the Voltmeter to DC rather than to AC. Connect its red lead (wire) to the positive (+) terminal and the black lead (wire) to the negative (-) terminal. With no accessories on, start the car and increase the engine speed to around 1,000 RPM. The voltage should register at around 14 Volts. Anything less than 12 Volts may indicate a problem. Next, turn on the headlights, radio, and anything else that draws electrical power. Increase the engine speed and recheck the Voltmeter. Again, the voltage should register at around 14 Volts. If you have a failing alternator, the voltage will be well below 14 Volts. If so, then it is time to rebuild or replace your alternator.
Stephen Strange

If it's cranking over but not firing up then it could well be the coil or fuel. But if it's not even turning over then the coil, fuel etc. is irrelevant, the problem is the battery, starter, or their connections.

The sequence of events isn't clear. If you mean you can start it with jump leads and run the engine, then switch off, then *without* jump leads it will crank and start, but over a few attempts cranking gets slower, then the battery is flat. The fact that jump-starting and running it then allows the car to start with its own battery for a few times seems to indicate it *is* charging, but as I say is flat and needs a much longer charge than it is getting. Alternatively it could simply be the donor battery giving the cars battery a bit of charge, and the real problem is that the alternator isn't charging at all.

As Stephen says it should be charging at around 14v, certainly more than 12.8v. If it isn't charging, or it's charging low, check the connections before splashing out on a new alternator. You did plug it back in after the decoke didn't you? And cars haven't needed decokes for 40 years! If it is plugged in unplug it and check for 12v on the brown wire in the harness plug. Turn on the ignition and check for 12v on the brown/yellow. If no 12v on the brown the no alternator will charge, and if there but not on the brown/yellow then the engine will probably have to be revved to about 3000 rpm before it will start charging.

I'll stop there till we get some more information.
Paul Hunt 2010

This thread was discussed between 21/05/2010 and 25/05/2010

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