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MG MGB Technical - How Many Amps?

Well I've got 4 different ammeters to choose from for installation in the B. With the engine off, and headlights on, two of the Smiths read 20 amps, one Smiths reads 10 amps, and the Stewart Warner reads 15 amps. Which one should I install?

Can anyone give me advice on which number is closest to actual?

Also, my '67B is positive ground. When the battery is charging should the ammeter needle be pointing to the plus or negative side? I'm assuming it would point to the opposite direction on a negative ground car. Is that correct?
Terrence Goodell

Terrence -

Depends on headligh bulb wattage - anything between 50 and 100W each.

Track down total Wattage of all lamps that are lit, divide by 12 to get a nominal current value.

Option is to get/borrow/steal a more accurate ammeter to measure the current, and then use one of the ones that closest match.


Cheers

Ian f
Ian Fraser

Terrence - Ian is correct, use a diagnostic ammeter to determine what the car is drawing and then chose the instrument that gives the closest reading. Apparently the different instruments are using different shunts. The ammeter should always read to the plus side for charging and the negative side for discharge. Just swap the wires around on the terminals to make it read correctly. That said, my own opinion is that a voltmeter is much more usefull than an ammeter. The voltmeter shows you the condition of the battery rather than just whether or not there is charging current going to it (which may or may not be charging the battery). I put a voltmeter in our MGB and several months later saw imediately when the alternator started going bad, by the reduced voltage that was being read. I was able to take corrective action before there was enough of a problem to strand me.
Good luck - Dave
David DuBois

<soapbox>Don't bother. An ammeter introduces several more potential (!) points of failure and needs very heavy wiring if excessive volt-drops are to be avoided, all of which is unfused. That is, unless you have a remote-shunt type which I doubt. By comparison a voltmeter is very simple to install, only needs very thin wires, and can be connected to any of the green (fused ignition) wires behind the dash. But neither will tell you much more than a working ignition warning light, and none of them will tell you if your car is going to start in the morning.</soapbox>
Paul Hunt

The ampmeter is more trouble than it is worth. Ever connection in the circiut is a potiential trouble spot. Remember that every bit of current that goes to your battery must go through the ampmeter first. Bad connection anywhere along the line will reduce what your battery gets. It may even cause a fire at the connection. USA got away from ampmeters 25 years ago for a good reason.
gerry masterman

Thanks to all for the advice. I'd always heard that installing an ammeter with an alternator was a bad idea, but that it was OK to hook one up with a dynamo.
Terrence Goodell

Bad on both, worse on an alt simply because of the bigger loads/higher currents.
Paul Hunt

Skip it ! i just removed the ammeter from my '67 BGT , it was not overly usefull anyway as it was a 50 - 50 amp job and never indicated much , a voltimiter is much better as mentioned earlier . I got lucky and found an original Smith's matching face voltimiter in a junk box at the local autojumble .
it _is_ better than the " idiot light " as your charging system can die a slow death and never light that damned light , leaving you stranded in the rain with the missus outside a pub around 03:00 , don't ask how I know .

Your charge rate should be over 13 volts , off idle in any case .

Check all the connections between the starter abd voltage regulator as that is where I found all my troubles - DPO "bubba"
wiring .
-Nate
Nate


Humm, damn, I was thinking of adding a ammeter to my 63 B (dyno), along with a voltmeter, oil temp guage and vacum guage.

I figured it was worth knowing if I were draining my battery or not. I intend to do some historic rallying and though this may give me a warning of potential problems.

So I guess I'll drop the idea of an ammeter...

Jamie
jamieo

Jamie,
From experience with my MGA: I have both a Smiths voltmeter and ammeter installed. They work well together. The ammeter tells me the charging requirements of the moment, the volt meter the effect of the rate of charge / discharge over a period of time on the battery. With the extra electrical load from rally lamps I'd say both instruments are important especially with the marginal output of the generator. Careful attention to the installation of a new, quality ammeter, should avoid any electrical problems. -M.S.
Martin Straka

I put a NAPA brand voltmeter and vacuum gauge on mine. Last spring one of the 6 volt batteries had an internal short where sometimes it would put out only 3 volts and other times it put out 6 volts. With the voltmeter, I could turn the key on, see whether it showed 9 or 12 volts, and then prepare for the appropriate starting measures. It did start on 9 volts several times before I finally replaced the bad battery. It had a crack on the top corner of the case. The ammeter is just to tell me if I killed the engine by letting off the clutch too hard, or if the brushes are getting worn in the alternator. On my commute, sometimes I'll get in behind a truck, and watch that vacuum gauge go up 5 points while the speed stays the same, because of the drafting effect!
Ken T
Ken Thompson

Remember , if you're running a generator and want good lights & etc. , set the charging rate to 15 VDC at the armature terminal , charging system isolated from the car (no load) , car fully warmed up per BMC .

-Nate
Nate

Comparing ammeters, voltmeters, and warning lights is like comparing oranges, apples, and bananas. Ammeters, voltmeters, and warning lights all tell you something different, but something of value if you know how they work and what they can tell you.

An ammeter tells you simply how much current is going into or coming out of the battery, and nothing else. An ammeter does NOT tell you how much current your car's electrical system is using. (see: http://members.aol.com/danmas2/ammeters.htm for more info)

A voltmeter will tell you the voltage of your electrical system, but will not tell you if the battery is being charged or not. In fact, the battery can fall out of a rusty battery bin and disinegrate on the highway behind you, and the voltmeter will still tell you everything is hunky-dory.

The warning light tells you that there is a difference in the voltage between the alternator and the battery. It won't tell you directly which is higher, only that there is a difference. The greater the difference, the brighter the light will be.

When you first start your car, and after the engione has ran up to an rpm sufficient to get full output from the alternator, you should see a slight charge indication on the ammeter as the alternator replaces the energy used by the starter to spin the engine over. As the battery is charged, the ammeter indication should go to zero. If at a later time you should see a charge current, then you know you have a bad battery, a short somewhere, or you have turned on one of the loads that for some reason isn't wired correctly. If you get a discharge reading, then you know the alternator isn't keeping up with the car's electrical demands - either because the alternator isn't working properly or you have exceeded its capacity. In general, ALL electrical loads are suppied by the alternator, none of the current is supplied by the battery when the engine is running and everything is working as it should. There are exceptions. For example, on an ammeter equipped TR6, the horns are wired on the battery side of the alternator, so they will show up as a charge current. There are valid reasons for this, and it is not a problem because you seldom are watching the ammeter when you need to blow the horns.

When you first turn on the ignition, but before you start the car, you should see about 12.6 volts on a voltmeter. If you see less, you have a bad battery (if you see more, find out why and patent it!). After the engine comes up to speed and the alternator is working, you should see about 14.6 volts. If you see more or less, there is a problem with the alternator. 14.6 volts means the alternator is working as it should, but it doesn't mean that the battery is being charged. It only means that the battery will be charged if it is capable of taking a charge, and there are no other problems. For example, if there is a lot of corrosion around the battery terminals, or the terminals are loose, you might not be charging the battery even if the voltage is correct. Normally, a problem like this will show up when you try to start the car, but the terrminals may work loose while driving. Less voltage means the alternator is either overloaded or not working properly. The alternator isn't going to be overloaded accidently (unless there is a short somewhere), so if you haven't deliberately turned on a lot of high powered loads, you know the alternator is bad. More voltage means the regulator is bad, and if the voltage is high enough, you will soon boil the battery dry.

When you first turn on the ignition key, the alternator warning light (sometimes called the "ignition" warning light for some odd reason) should come on. At this time, the alternator output voltage is zero, and the battery voltage should be 12.6v. With 12.6 volts across the light, it will glow brightly. It will also glow brightly with only 10 volts across it, If you battery voltage should be that low, so it won't tell you that you have a bad battery. Once the engine is running and the alternator comes up to speed, the 14.6 volts of the alternator will also be seen on the battery terminals, so the light goes out - 14.6 volts on each side equals zero volts across the lamp. If the light comes on again (or stays on)after the car is running, there is a problem. If the lamp gets dimmer with engine speed, it is an indication that there is a problem with the alternator. If it gets brighter with engine speed, it is an indication that there is a problem with the battery. If it stays at the same brilliance, it may indicate that there is a blown fuse (depending on what kind of car you are driving).

What about wiring problems with an ammeter? It has been pointed out that an ammeter adds extra connections in the wiring to the car's electrical loads. Maybe it does and maybe it doesn't, depending on how it and the rest of the car is wired. Typically, it adds no extra connection at all to the wiring feeding the electrical system, and adds only two connections to the wiring to the battery. The fewer the connections, the better, but if they are well implemented and maintained, two connections shouldn't be a real problem. As long as you are using the stock alternator with an output of 30 amps or less, you should have no problem with the wiring. When you go to a 60-0-60 ammeter, though, you will find it very difficult to wire. The wire size needed for 60 amps is much larger than required for 30 amps, and can be quite stiff to route. The connections on the back of the 60 amp meter are not any bigger than for 30 amps, so you must excercise extreme caution to insure there is no stress on the wiring at the back of the meter.

So, ya pays yer money and ya takes yer choice! As Paul pointed out, you can get by quite well with only the warning light. You can also get by quite well with only an idiot light for the oil pressure too - most people do on modern cars. You don't really need a tachometer either. About all you really need is a speedometer and a fuel gauge. Oil pressure, water temp, tachometer, voltmeter, ammeter - they are all nice to have, though, and I still want them on my cars. If you want an ammeter, by all means put one in, and don't worry about it - just do it right, like you would anything else you do to your car. On the last car I wired, a TR6 with a Chevy 350 V8, I installed both a voltmeter and an ammeter, and I really liked it that way. If you REALLY want to know what's going on with your charging system, I recomend that you do too.
Dan Masters

Dan - as Ford advertised many years ago "Cortina comes with a rev counter to help you time your gear changes to perfection" YUK! Oil pressure, temperature, they just give you more things to worry about.
Paul Hunt

Dan,

Thanks for all that useful information. I guess the same applies pretty much for a dynamo as opposed to an alternator?

This all reminds me of an embarrasing story of when I first got my B. It was in great condition but the battery boxes were rotten and full of dirty. I took the batteries out, cleaned them up, cleaned all the battery boxes, etc. I put the batteries back and wired things up. I started up the car and all was fine - however, the ingnition light was on and the rev counter didn't work? I drove it back to the garage that I bought the car from (about 10-15 miles) and they were puzzelled. I lef the car with them and returned a few hours later. It was very embarrasing to find out I'd either (I can't remember now, it was a long time ago) put the batteries back in as negative earth (and it's a postive earth car) or I had put one in the right way round and the other the wrong way round. Oops.

I'm surprised the car started and drove OK!

Jamie
jamieo

Dan

Thanks for the in-depth discussion. I guess every decision is based on a careful consideration of risk vs. benefit. Your explanation gives me a lot to chew on in both respects. I appreciate you taking the time to explain things so clearly.

On a first read though, it does sound like it's kind of silly to put one gauge in without the other.

Terrence
Terrence Goodell

Dan, that's an impressive and thorough explanation!
Once the engine is running then the alternator is actually powering the electrical system, so yes, the voltage you see on the gauge is the voltage the alternator puts through the system, not the battery voltage. So yes, the voltmeter is not telling you if it's charging the battery. But with the switch in the on position but engine not running, that has to be the battery voltage. I believe that part of the function of the voltage regulator is to run the starting system off the battery, then switch the system to run off the alternator once the engine is running, and only supply a charge to the battery. Anyway, once those batteries fall through the rusted battery boxes, the gauges are going to show a change, even if it's not the 'charge' to the battery ;o)
Ken T
Ken Thompson

Ken wrote:

"I believe that part of the function of the voltage regulator is to run the starting system off the battery, then switch the system to run off the alternator once the engine is running, and only supply a charge to the battery."

Actually, the function of the regulator - alternator or dynamo - is to maintain a constant output voltage regardless of the load. I wrote an article on this for the Triumph club that you might find interesting, at: http://www.vtr.org/maintain/alternator-overview.html

The changeover from battery to alternator (dynamo) is automatic. Current will always flow from the higher voltage to the lower voltage. Before the engine gets up to speed, the battery voltage is higher than the alternator voltage, so the battery will supply the current. As soon as the alternator output voltage exceeds the battery voltage, the battery will not be able to supply current - the 12.6 volts from the battery will not be able to buck the 14.6 volts from the alternator - so all the current to the car's electrical system is supplied by the alternator.

You are absolutely right - when you first turn the key on, the voltmeter will read the battery voltage, and it is a good idea to hesitate a moment before starting the car to check the battery voltage. Most of us flip the key from off to on to start in one quick motion. Naturally, if the battery voltage is too low, you'll find out soon enough when the starter only groans, rather than spinning the engine, making the voltmeter reading redundant.

If everything is workling as it should be when you are operating the car, the ammeter will read "zero" charge, and the voltmeter will read the alternator's 14.6 volts. If the alternator isn't damaged by disconnecting the battery - which is may be - the gauge readings will stay the same: ie, zero charge to the battery and 14.6 volts from the alternator. You will have no indication that the battery is gone!

Take a look at the article mentioned above, and the one I referenced previously on ammeters, and I think this will become clear.
Dan Masters

Just one thing, when the engine is running the 14.6v +-0.2v you should see at the alternator terminals you will also see at the fusebox i.e. car electrics *and* the battery terminals - subject to any losses due to bad connections en-route - so in that sense what you see on the voltmeter is the battery voltage at that time.

My car, like most others, has a sticker saying 'do not disconnect battery when engine is running' or words to that affect. I don't really know why, since the voltage regulator should maintain the alternator output voltage, and I inadvertanly once ran with the alternator unplugged for 30 miles. But someone recently claimed 'someone he knew' (always second-hand, never first) had a battery come loose while underway at night and every light bulb that was lit on the vehicle vapourised. With the load of a full set of lights and the coil (albeit intermittently for the coil) I just can't see that happening.
Paul Hunt

Paul,

I'm with you - I don't see how pulling the battery should cause damage to the alternator. In fact, I know many folks who "test" their alternators by lifting the battery leads with the engine running; if everything works without the battery, then the alternator is working.

However, all alternator manufacturers that I know of say that the alternator can be damaged by doing this. Assuming that they know more than I do (which wouldn't be hard to do), I think it's best to recommend NOT doing this. My assumption is that under most conditions, this would not cause a problem, but there must be certain specific situations where it could, so they take the "scattergun" approach and warn aganst it in all conditions.

Maybe one of these days I can find the time to do some research on this and get a definitive answer.
Dan Masters

The definative answer is : removing the battery cable , engine running , to test alternator function works and causes _no_ damage , period . re-attaching that very same cable can and ofttimes does , cause the alternator to " spike " (go to max output) and that is what burns the lamps and
blows the diodes .

Better to get a carbon pile and test it correctly by loading down , remember to never load below 12 volts ! .

BTDT , took the time to learn why and never had a problem since .

-Nate
Nate

Does anybody know a source for a shunt-driven ammeter, either analog or electronic? If I was to run all my car's current through a couple of connections, I'd prefer them to be bolted to a shunt, where you could insulate everything thoroughly with some shrink, and then just run a couple light gauge wires to the meter. Has anyone seen a source for something like this for automotive use? Joe
Joe Ullman

Nate,

That makes sense, thanks!

Joe,

The problem with using a shunt is getting a gauge to match our cars. The resistance of the shunt has to be very small to prevent voltage drop to the car's electrical system. A 0.03 ohm shunt will drop 0.9 volts at 30 amps, more than you would really want to allow. That means you would need a +/-0.9V voltmeter, with a center zero, to monitor a 30-0-30 amp range, calibrated to show the +/-0.9 volts as +/-30 amps on the scale. Getting a voltmeter with that characteristic would be difficult at best, impossible at worst. There may be gauges of that type used on some modern cars or electronic equipment, but they sure would look out of place in an MGB. If you have the skills needed to convert a Lucas or Smiths gauge for that purpose, you could probably pick up a good chunk of change doing so. I dare say there would be a pretty good market for it.
Dan Masters

Thanks, Dan. I guess it's back to the drawing board on that idea; something to think about in our spare time, though! Best, Joe
Joe Ullman

Nate - still don't see it. The alternator voltage regulator shouldn't allow that to happen, and any spike occuring *after* the battery is reconnected will be damped by the battery. A conventional ignition system generates spikes of a couple of hundred volts throughout the cars electrics with no problems. However I can see that flashing the battery on and off *might* cause problems.

Looking for 'remote shunt ammeter' in Google comes up with 1250 pages of which the first few at least seem to be relevant. It is a *remote* shunt ammeter you want, all the auto ammeters I have seen have a shunt, but it is attached to the gauge. I have also seen systems that measure the volt-drop between engine and chassis and use this as a measure of alternator output current, whereas battery current is what centre-zero ammeters traditionally measure.
Paul Hunt

I hear you Paul but I also trust the Delco and Bosch training plus I have been doing road side quick tests this way for 30 + years and never had a problem once I learned to _always_ disconnect the ground cable and tuck it down behind the batt where it cannot touch the battery post whilst I am walking 'round to shut it off by the key . I do quite a bit of road side service calls plus of course all these damned salvage - reconstructs .

I wonder if an old military surplus aircraft gauge would maybe have the correct shunts - I worked with a guy some years back who shoe horned an ammeter into the fuel gauge of his 1967 Chevy II , it looked a bit odd having two nedles and two gauge faces in one housing but worked really well and pleased him to no end .

-Nate
Nate

"it looked a bit odd having two nedles and two gauge faces in one housing"

Hmmm, now a temp gauge and an oil pressure gauge in a single housing *would* be useful ;)
Paul Hunt

Last night the charging system went out on my (pardon this word) Miata, and as I was reading my Haynes manual, it occurred to me that we didnt' take into account that the battery is DC and the alternator is AC. So when the starter system is cranking, the car is powered by direct current. When the alternator kicks in, the car (bridge rectifier in the regulator?) switches the car to alternating current. Dan, or Paul, I'd like to hear you comments on this.
Thanks,
Ken T
Ken Thompson

Ken wrote:

"...battery is DC and the alternator is AC"

Ken,

The term "alternator" is a marketing term only, and has no real bearing on the function of the alternator. At the fundamental level, an alternator is no different than the old generators.

Alternators and generators both "generate" AC voltage. Inside each unit, alternator or generator, the AC voltage is rectified to produce DC voltage. The voltage at the output of either is a fluctuating DC voltage which looks like a standard AC voltage wave riding on top of the DC voltage. For more on this, see:

http://www.vtr.org/maintain/alternator-overview.html

It is physically impossible to produce DC voltage with a rotating machine. The simple fact that the machine rotates means that the output is positive on one half rotation, negative on the other, and passes through zero in between. If you look at the output of a generator and an alternator on an oscilloscope, you'll see they look the same.

One of the primary differences between a generator and an alternator is in the way the AC is converted to DC. In a generator, it is converted by the use of segmented slip rings - a mechanical conversion. In an alternator, it is converted by the use of solid state diodes.

Another difference is that the output windings rotate in a generator, whereas the output windings are stationary, with the field windings rotating, in an alternator. Windings large enough to produce a large amount of current can be quite heavy, so by spinning the lighter field windings, an alternator can produce more current for a given size as compared to a generator.

By spinning the output windings and keeping the field winding stationary, permanent magnets can be placed around a generator, making is possible for a generator to produce output without having a battery present. The magnetic field from the permanent magnets get the generator started until it's own output can create the needed field current. Magnets would be too heavy to spin if located with the field windings in an alternator, so an alternator needs a battery to produce the field current needed to provide the initial magnetic field required to get an alternator started until it can produce it's own field current.

Probably more than you wanted to know, but I hope this helps.
Dan Masters

Yep, varying DC it is. The battery acts as a damper to smooth out the variations, so what you see is 14.6v DC when charging.

But generators or dynamos as used on our cars don't have permanent magnets as such, but rely on residual magnetism in the core to prime the field. Dynamos have to be polarised before they will produce an output and this is achieved by momentarily connecting 12v to the field winding (or the output, can't remember which or if it matters) to magnetise the core. If they had permanent magnets this would not be necessary, and neither would it be possible to repolarise the dynamo when converting from +ve ground to -ve ground. Although alternators do have a priming current via the ignition warning light they also have residual magnetism and in my experience will start charging without the light, but have to be revved to 3k or so (instead of about 900 rpm) before they start. Once started they will charge down to 600rpm or so as normal.
Paul Hunt

Paul ; I'm surprised you forgot the field terminal on the _generator_ typ voltage regulator is directly connected to the idiot light in the dash so it's self exciting (or polarizing) .

Interestingly , you can use the external regulator from an alternator on an " A " circut generator in most cases .

BTW : motorcycles have rotating permanent magnets in their alternators , works well but not the same high amperage out put of course .

-Nate
Nate

thanks, guys, that explanation makes a lot more sense. It dawned on me that radios always require 12VDC and there's never any kind of "ac adaptor" so the car has to be DC.
One more question: the MGB alternator has a brown wire (8 or 6 gauge?) that goes from the alternator back to the solenoid. Is that how the alternator provides the trickle charge back to the battery?
Ken T
Ken Thompson

Nate - in the diags in the Workshop Manual the warning light is connected to the generator output or D terminal, but does eventually reach the field winding through various windings and contacts in the control box. However it is self-exiting irrespective of the warning light, but not self-polarising. One of the tests of a dynamo is to disconnect it from the control box and connect the F and D terminals together and run the engine, which should generate 20v before 1000 rpm is reached. When changing the polarity of a cars electrics you do have to repolarise the dynamo, if not its output will be opposite to that of the battery and having pos to neg and vice-versa causes a large current to flow.

Ken - when the engine is running the alternator (or dynamo) is powering the whole car plus charging the battery and the thick brown wire is doing all this. The thin brown wire used on some eras of alternator as a 'sense' wire used to feed the solenoid voltage back to the voltage regulator in the alternator. A bit pointless really, and in 72 or so it was done away with. Subsequently two thick wires from the alt to the solenoid were used, but this was to increase the current carrying capacity/lower the volt-drop with higher output alternators.
Paul Hunt

Paul , of course you were right , I'm holding a Lucas voltage regulator in my hand and it is the armature terminal not the field , it is _directly_ connected to the idiot light , no windings nor contacts , this is how the generator gets excited .

Apart from changing the polarity of the car (I do that a lot) I find no need to polarize generators even ones sitting on the shelf for years .

-Nate
Nate

Paul,

I stand corrected - you're right, there are no permanent magnets in a dynamo. More correctly, there are iron pole pieces that are "magnatizable" and will hold that magnetism for a long time. I guess this makes them "semi" permanent. As you said, flashing the field sets the north-south pole direction according the the prefered output polarity - negative or positive earth.

That's my understanding, anyway, but I've been wrong before!
Dan Masters

This thread was discussed between 28/09/2002 and 12/10/2002

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