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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. ?
Any comments would be appreciated.

P Liles

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

voltmeterreas 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
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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