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MG MGB Technical - Starting issues/alternator

I have read some interesting threads in the archives which are of assistance, but would like to throw this one out as well.

I have a voltmeter in the cig lighter which I find of great assistance. I know that I have an on going issue with my alternator, but I think I have some other underlying problems. The voltage would read in the high 13V range for days on end and then it would start fluctuating to below 13V, dropping as low as 12.4 and then jump back up. For the past week at startup the voltage reads somewhere between 13.7 and 14.1. As soon as the car is warm (the temp needle starts to rise, then the voltage drops to about 12.45, so obviously the alternator is not putting out or something else.

For the past couple of weeks, the car has had a real trouble starting. This is where I think I may have a bad battery or something. I have the battery on trickle charge overnight, so its fully charged in the morning. When I put the key in the ignition and turn (not crank) the voltage drops from about 12.75-12.80 to about 12.45. As soon as I crank it the battery sounds strong at first, but rapidly drops below 12V. From there on the battery sounds weak and the voltage fluctuates from as low as 7V to about 10 and eventually the car fires up. While driving now the voltage reads about 12.45. Thats fine as long as I don't have to put my lights on! Starting after that is not an issue at all unless it has been left standing for a few hours.

The battery is about 20 months old and I have check my connections at the battery etc and things look fine.

Thanks



GG Ginty

GG - Sounds like you have a battery that is going south or you have a sneak current when the ignition is shut off. You don't say what year your car is, but the alternator can sometimes have a failure that will cause a drain on the battery when the ignition is turned off. If the car is one with a light in the trunk, that light can be staying on and draining the battery. Also, connecting the battery to a trickle charger can result in an over charge situation over time. Remove the caps from the battery (yes, even maintenance free batteries have filler caps - the two rectangular portions on the top can be carefully pried up, using a putty knife to reveal filler holes in the top of the battery) and check the level of the electrolyte in the battery. There should sufficient electrolyte in each cell to cover the plates - if not add some distiled water to bring it up to the proper level.

A MGB with a properly operating alternator should maintain a battery at the proper level of charge without having to resort to a trickle charge on it at night. If you are worried about the charge level after getting the problem sorted, invest in a Battery Maintainer (sold under various names) and attach it to the battery when the car is parked for long periods. This unit will charge the battery to optimum level, then shut off until the battery voltage drops to a predetermined level, at which point it again charges the battery, then shut off and so on. Cheers - Dave
David DuBois

The voltage readings in the first paragraph indicate charging problems. If you get the same kind of readings with a mater on the alternator output spade then it is the alternator, otherwise is likely to be bad connections between there and the solenoid.

If the battery really is fully charged overnight and you still get those readings when cranking then I'd say the problem is as likely to be bad connections in the cranking circuit as a bad battery. Use your meter right on the battery posts, and if you get the same readings then it is the battery. If you get more than 10v when cranking, but cranking is still slow, then it is connections, one of which is at the solenoid with the alternator cables...
PaulH Solihull

Not sure what year MGB you have or which Lucas alternator you are using in it (or is it an aftermarket alternator conversion?). With some of the very early Lucas alternators (16AC and 16ACR) the voltage regulation circuit was set to a relatively high rpms (about 1000) before it would begin charging. At idle, this often meant that not much charging was being done and you would see the sort of voltage drop you describe - and all would actually be "normal". The later Lucas 18ACR units and beyond supposedly took care of this problem.

It could be that you do have a charging problem and perhaps need a new alternator or a fresh set of brushes in the one you have. However, another factor often overlooked is the health of your main battery cable. If your cable appears to be original, it could be suspect if you discover corrosion has traveled underneath the insulation and rotted away the strands of your cable at the battery end. Folks are often fooled by the look of the moulded on cable terminals, thinking that they keep acid and corrosion out, when on occasion, they actually keep it in and hidden. As a cable corrodes, fewer and fewer strands of that cable will be left to carry the load demanded and you will see more dramatic voltage drops as when cranking the engine. It is worth a few moments to check this out, and if all is well, nothing's lost, and if not, you may have found your culprit.
Robert Muenchausen

On my cars the alternator starts charging at about 900rpm, which isn't that different to 1000rpm. However once started they charge down to about 600rpm, and while charging the system voltage should certainly be above 12.8v, although if the revs are low and lots of electrical loads are switched on it can approach it. If the warning light comes on with the ignition, and goes off when the engine is started and revved to about 900/1000 rpm, and remains off, it is charging. If it stops charging the light will come on again. Voltage should certainly not jump about between the voltages you describe if the warning light remains off, neither should it do so (or the light come on) if the revs are in excess of 1000rpm, if that happens then there is a fault, either inside the alternator or with it's connections to the battery cable at the solenoid. Even if it is the working light circuit that is breaking intermittently, the alternator should charge normally once it has started charging, the warning light circuit is only used as a pump-primer to start charging at about 900rpm. Once started it doesn't need the light and will charge down to the aforementioned 600rpm.
PaulH Solihull

Thanks.. you folks are a wealth of information. I was thinking about replacing the battery cable this winter. My B is a 74 and was thinking of ordering one from Moss.

GG Ginty

This thread was discussed between 26/11/2010 and 07/12/2010

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