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MG MGA - Charging Problem - Urgent

Hello,
I'm off to Le Mans this weekend, probably the furthers My MGA has been in decades.

I've fitted a new wiring harness and Alternator (Tested OK). I also have another Alternator which apparently works.

I have an amps gauge that goes to the -ve side when the lights come on, but never goes +ve when the engine runs. I have reversed the wires on the Amps gauge, just see if it would read +ve and this works OK.

The Ignition light comes on (Bright) when the key is turned, but seems to be on (Dim) when the engine is running.

I have bypassed the control box as per all the instructions I can find. e.g.
http://www.mgaguru.com/mgtech/electric/ac101.htm

The car is now negative earth with a single 12v battery.

The battery keeps going flat and I have to charge it externally, this is not due to an overnight leak, as I cut the main brown wire that goes to the solenoid (New) and fitted a spade terminal which I disconnect over night.

Any suggestions on how to rectify this by Friday when I leave for Le Mans?

Many Thanks
<MARK>
Mark Hester

I dont know how other people do it but wiring an alternator to a car is extremely simple. There should be 3 wires at the back of the alternator. The 2 larger connections need connecting to the positive of the battery, the 3 connection generally the small one is connected to the ignition light, the other side of which would be connected to the positive side of the ignition switch. If that is the case and the alternator does not charge then it is faulty.
If you have therefore connected it up using some really tricky method check to see if you have live connections at the rear of the alternator.
Bob (robert)

You need to measure what voltage the Alternator is putting out, then you can see whether it is functioning or not.

What is the state of the battery?
dominic clancy

Dominic, Rob.

The battery has been completely flat and fully charges, but no reading on the amps gauge.

How to I check the Alternator output.
Can I connect a wire from the alternator through the amps gauge and then onto the starter solenoid.
Would that work ?

Cheers <MARK>
Mark Hester

How long does the battery hold a charge when left disconnected. (ie does it appear to charge, but in reality is knackered)

What happens when you disconnect the amp gauge and run the engine - you should show a charging voltage of c. 14V across the battery. If the internal regulator is not correctly set then the battery will not charge.

dominic clancy

The battery is new and the previous one only 1 year old, but I replaced it anyway.
I will try and disconnect the amps gauge and measure the battery.

Should the battery measure 12v with engine off and 14v with the engine started ?

Thanks
<MARK>
Mark Hester

The way to check if the alternator is working is to put a voltmeter across the battery. With the engine stopped the battery will register about 12 volts. With the engine running you should see a rise in this voltage to around 13.5 or a little more. I do not like amp meters, dangerous and tell you noting because if the battery is fully charged then the ameter will register zero.
Bob (robert)

I seem to have cheep volts gauges, as when I put them across the battery they can't handle the amps the battery is capable of delivering and blow up.

I may have found the source of the problem. Then back of the amps gauge that was once working is now melted and not working, thus breaking the circuit back to the battery.

As your say Bob, these seem to be dangerous devices. I shall try and hunt down a Smiths volts gauge instead.

Thanks for everyone's help.
Cheers <MARK>
Mark Hester

Mark,

Volt meters are very high resistance meters, that pass negligible Amps, even if directly across the battery, which where the are intended to be used.
Amp meters on the other hand have very low resistance and will allow almost unlimited current flow, and can only be inserted in a circuit (in series).
It sounds as if the 12 volts was across the Amp meter, hence the melted instrument.
Both meters have their place in the scheme of things, but they cannot be interchanged.

Mick
Mick Anderson

Good point about the danger of using an ammeter wired into the charging cicuit. Certainly if mis-connected across the battery I'd expect the resultant current draw to fry the ammeter. I have just replaced the generator and regulator on my MGA 1500 and the system works as it should now except that with headlights on there is a positive current flow through the ammeter of about 4 or 5 amps. It drops back to zero when the battery is charged and the headlamps are off but jumps back positive and stays there when the lights are on. I thought it should gradually return to zero even with the lights on. The ammeter also jumps to positive in time with the turn signal indicator when it flashes. I suspect the ammeter is reading curent draw from the generator rather than charging current to the battery. Any suggestion how to correct this while I look for a reasonably-priced Smiths voltage meter with the right appearance? Thanks
Tom Heath

Tom,

Intercept the thick brown wire going to the Starter Solenoid with the amps gauge wires.
This wire also runs up to the control box, but be carefull as their are two thick brown wires at this end.
This will give you what is being taken or given by the battery.
I also convert this terminal on the Starter Solenoid to a spade type connector, so I can disconnect it when the car is not being used for long periods. Much easier than removing the battery cover and disconnecting the battery.

Cheers <MARK>
Almost Ready for Le Mans...
Mark Hester

It just sounds like your car's not charging, Mark. The charge light is telling you something. Don't make it rocket science. Use a voltmeter across the battery when the engine's running. As said earlier, if it's not at least 13, 14 volts, your battery's gonna go flat. Take your car to a guy that works with automotive electrics and save your money on voltmeters.

I agree about the ammeters in the dash, they just show when you have more load on the system. After you first start the car, they'll show that the battery is being charged back up by reading higher. And mine occasionally pegs itself to the negative until I give it a whack.

Enjoy the alternator, Mark. It's much better at keeping up with the demands of the car than the old generators are. I'm such an old geezer that I enjoy the quirks of the old girls, tho.
Tom

The amps gauge was defiantly at fault.

I have one of those Pro User Portable Power Station PS900's
http://www.wantmore.co.uk/e.catalogue/700-10301.html

Which has a built in Volts Gauge.

I connected it to the car and it read just over 12 volts. Then started the car and revved it a bit it then read 14.2 Volts.

So all is well again.

Many Thanks To all for their comments...

Cheers <MARK>
Mark Hester

Mark

Glad you are all charged up. Bon voyage au Le Mans. Will you be joining up with the main MGA convoy at some stage during the week?

I am in a BRG Roadster PSJ 903. Perhaps see you there.

Steve
Steve Gyles

Steve,

Hope to catch up with others. I have a camping pitch on the infield (all that was left at the time).
I'm leaving on Friday though, so not joining the main convoy. I hope to catch everyone on Saturday for the start of the classics race. Not sure the exact time though.
Cheers <MARK>
mga twin cam WAS152 !!!
http://freespace.virgin.net/marka.hester/MGA_Twin_Cam_Home.htm
Mark Hester

This thread was discussed between 06/06/2005 and 10/06/2005

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