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MG MGB GT V8 Factory Originals Technical - Battery discharges

Here is one for the electrical gurus. Maybe Dan Masters can help me. It seems my V8 conversion is discharging its battery after being shut off. My guess is that I might have a leak through the ground of the indicator light circuit in the alternator. Not sure, just a guess. Any ideas? How would I test? Battery goes completely flat in about 4 to 5 days if not run every day.
James Johanski

Easiest and cheapest way is to disconnect negative cable from battery and put a test light( cheap one, with a real bulb inside- not an L.E.D. type) inline with the negative battery cable and negative battery terminal. Any current drain will cause the test light to light up(perhaps very dim). Next, while the test light is still connected, start disconnecting fuses one by one until the light goes off. If it does not go off, start disconnecting stuff- alternator, lights, radio, etc, until it does. When ever the light goes off it means that that circuit is at fault. You can then just focus on that circuit. You can work in a systematic way to save time, but you'll find the cause any way you do it. Good luck.

A very small drain won't light a test-lamp (maybe wouldn't flatten a battery overnight either) but will cause a *voltmeter* connected in place of the battery ground strap to register a full 12v. If you only see a few volts this is the leakage current through the alternator diodes and is quite normal. If either test is positive then pull the plug from the alt first to eliminate or prove that, then the brown - purple fuse and the hazard flasher fuse if you have one. After that it is a matter of disconnecting brown wires from the solenoid and sundry other places.
Paul Hunt

Thank you. I will try the voltmeter test method and report the results.
James Johanski

I had a similar problem with my 78 MGB V8 conversion. I found the problem to be the switch for the trunk light.
Michael S. Domanowski

I haven't had a chance to go out to the garage (too cold) and try the voltmeter test. I know it is not the trunk lite, as I don't have one. Dan Masters are you out there--got any suggestions?
James Johanski

Put a coat and hat on.
Paul Hunt

Thanks to all for your suggestions. I put a coat and hat on yesterday and headed for the garage. Using a voltmeter on the negative side of the battery, I read the apropriate 12.6 volts with the alternator connected--when I disconnected it the voltage went to zero. As I thought, it was the alternator leaking voltage. I am going to replace it with a new chrome one wire set up.
James Johanski

If you use a one-wire you will lose the diagnostics of the ignition warning light, and even the suppliers of the one-wire as a replacement for a two-wire recommend the installation of an ammeter or a voltmeter along with it. Seems like a lot of extra work (if you don't already have them fitted) for no gain to me.
Paul Hunt

I have a GM one wire in my car and until she goes over about 1K rpm, the ignition light is on. the whole "one wire" is somewhat of a false description, there are actually 3 wires going to the Alt. one big Batt lead and the 2 smaller leads, one of which is for the ignition light. The nice thing about going to the GM was that the base model is 2x the lucas output, they are CHEAP, and if I ever need I can get a bigger output and even chrome! heh
Larry Embrey


Let me clear up a common misconception. The alternator you have is not a one-wire, even though about 90% of the countermen at auto stores think it is, and will tell customers that it is. Unfortunately, many of them also tell customers that it only requires one wire to make it work. One of the most common inquiries I get concerning alternators is "why doesn't my one-wire work?" when they have connected it as a one wire.

There are true one-wire alternators available, and they do indeed require only wone wire to work. They are very expensive compared to the three wire, and you do lose the indicator function.

For the street rod boys, where they try to hide EVERYTHING, having only one wire is a plus, and having only one wire to hook up is great when you are wiring a car from scratch and know very little about electrtical work, but for us, it is of no value at all. The alternators in our cars already have the three wires needed, and it takes just as much knowledge to know what to do with the extra two wires if you don't use them as it does to know how to hook them up.

Street rodders don't like to have anything on their cars that isn't absolutely mandatory, incuding indicator lights, but we already have the alt light, so we might as well use it.

The really big disadvantage of a true one-wire is availability - or lack thereof. You can find a three wire in stock in just about any town of any size in the country, but you'll have to order and wait for a one-wire if you are anywhere but in a large city.

Ready availablity is a definite plus. Last year I caravaned to Florida with a friend in a Chevy 350 powered TR6, using a MODIFIED alternator from some small Japanese car. Key point being "modified." it failed on the way down - try finding a modified alternator on the road. We spent the better part of a day at a small alternator shop at Sebring while the shop owner had the parts shipped in and then modified them so we could use it. A whole day compared to maybe an hour or two. My friend now has a GM one wire (bought before he talked to me about it!), but I wired it so that the plug is there if he ever has to buy one on the road. He can just buy a three wire and go.
Dan Masters

Go easy on him, Dan. Larry's a Ford man. :)

Summit make a conversion kit to make a standard 3 wire into a 1 wire alt. It replaces the voltage regulator with a self energizing voltage regulator.

Here's a website that shows how to do it:

Personally, 3 wires is just fine with me.

I have the same problem with my B (not a v8) at the moment. After a week in the garage without being used there is not quite enough juice left in the battery to start it on a cold morning. Leaving the battery (twin 6v) disconnected for the week and then reconnecting at the weekend is getting to be a pain.

Using the voltmeter test I found a full 12.6v being drawn with the ignition off and everything connected. After disconnecting the stereo this dropped to 2.6v, which I assume can be considered insignificant - does this sound reasonable?

When I dont need the car for a weekend Ill leave the batteries connected and take the fuse out of the stereo power line to see if there is anything else drawing from it.


> For the street rod boys, where they try to hide
> EVERYTHING, having only one wire is a plus, and
> having only one wire to hook up is great when you
> are wiring a car from scratch and know very little
> about electrtical work, but for us, it is of no
> value at all. The alternators in our cars already
> have the three wires needed, and it takes just as
> much knowledge to know what to do with the extra two
> wires if you don't use them as it does to know how
> to hook them up.


I always look up to your tremendous depth of knowledge and your huge generosity in offering detailed info. But now I'm confused. My alt. is just the stock V8 type, hanging in front of the RHS cyl. head, which I think was made by Motorola. It only has one (brown) wire. Nothing else. The ignition/charging/whatever-you-call-it red light on the dash works the same as in any other MGB. Just to understand: why do you say we need three-wire units?


If you only have one wire connected to your alternator and the warning light still operates as it should, I have to confess - I'm stumped!

I know how the alternator might work without the warning light being connected, but I'm clueless as to how the light might work without being connected to the alternator.

See for a detailed description of how an alternator works, and you'll see why I'm confused.

There is only one place in the entire electrical system that a voltage independant of the system voltage exists, and that is at the warning light output diodes inside the alternator. Anywhere else you connect the light to, it will see exactly the same voltage on both sides, and will not light up.

Can anyone clear this up? Help!
Dan Masters

I can imagine how a voltage-sensing gizmo could control the warning light, but the question is 'why?'. And I have found that 2-wire alts will work with only the thick brown connected, but the engine has to be revved to about 3k or so before they start charging. After that they work as normal i.e. down to about 600 rpm or so. And in what way is a one-wire Motorola alt 'stock' for a V8?

Richard - just 2.6v with everything connected but the stereo looks fine. Is there a power amp that isn't getting turned off perhaps?
Paul Hunt

Thanks Paul, No power amp, just a CD changer and probably a poor installation mixed with ageing batteries. Its all coming out next week anyway (respray), so Ill be neater when I re-install it again.

Dan, whoops, sorry for the unfair brain teaser. I popped the hood this AM and there is indeed a second wire, very thin, running from a spade lug on the back of the alt to somewhere in the bowels of the electrical system. I've had it out several times and I just don't recall ever seeing it before ... too many brain cells lost back in HS! This must be the thing you're talking about.

This thread was discussed between 23/10/2002 and 29/10/2002

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