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MG MGB Technical - voltmeter ?

After having a number of electrical/charging problems solved, I am thinking of adding a voltmeter rather than an ammeter as I don't want all that juice under the dash looking to fry the car and or myself. How hard is it to wire in and is there something to look for in the guage.
Thanks in advance.
By the way this will be going into a 79 B'. I have also looked at the archives. These were a little vauge(sp) to me.
Scot
Peachtree City, GA.
Scot Macdonald

A voltmeter doesn't really tell you if the battery is discharged or if the alternator is working properly. Yes, its easier to install, but it doesn't perform the task you really need it for. You already have the heavy brown circuit under the dash, so adding two more wires shouldn't be a deterrent. Just my thoughts.
Jeff
Jeff Schlemmer

The main problem with voltmeters is the scale is marked every two or three volts so reading the meter requires interpolation. No big deal hooking it up a Jeff mentioned. I'm going to an auto fair tomorrow and that is one item I will be looking for because a voltmeter provides more useful information than the warning light.

FWIW, Clifton
Clifton Gordon

I made a number of electrical changes on my car. Halogen headlights (with relay), A/C, additional cooling fans for the condenser, etc. To cope with all that, I swapped my trusty Lucas for a 105-amp Saturn (Delco) alternator. I wanted to monitor my system a little more closely than whatever information the idiot light could impart, so I added a voltmeter. It was very easy to wire, and it has taught me a lot about what's going on. I heartily recommend it. No doubt the ammeter provides more information regarding the condition of the battery, but since the main feed for your entire electrical system runs through it, if your wiring fails, or if the gauge internals fail, you're totally shut down. Not that it couldn't be pretty quickly fixed, but it could be a nuisance, at the least.

FWIW,
Allen
Allen Bachelder

You do NOT want to add an ammeter to a '79 MGB. It is a lot more work than it may seem, involving quite a lot of rewiring work. I would much prefer an ammeter, but the difficulty in installing one puts it out of the question for me. In fact, that's the reason car makers dropped the ammeter and went to a voltmeter (or idiot light). Back in the old days, when 30 amps was a pretty healthy output, adding an ammeter wasn't so bad, but when you get into 50-60 amps, the size of the wiring alone creates problems with an ammeter. Using an ammeter with a 100 amp or more alternator is out of the question.

To add an ammeter to a '79 MGB, you must remove 5 wires from the solenoid and connect them into one wire to the ammeter, and then run another wire from the ammeter back to the solenoid - not an easy task, and a real fire hazard if not done perfectly.

I have a writeup on ammeters and voltmeters that should be pretty interesting for anyone contemplating either an ammeter or a voltmeter. E-mail me if you'd like a copy (it's an excerpt from my TR6 electrical maintenance handbook).

Dan Masters

Scot
I had a standard 2" Smiths voltmeter fitted in mine for several years, I found that although the info is not as good as an ammeter with habit over a period of time you could tell by small increments of the needle what was going on electrically.
Ron
R. Algie

An expanded scale voltmeter is a very useful addition to a car that is not in everyday use. For cars not used daily the battery will slowly discharge and when it gets below 12.5V will loose some of its capacity. After a number of these events the battery voltage will be lower than a new battery causing the alternator to continuously try to charge the battery.

At this point it puts extra load on the alternator and shows up as slightly dim lights. The actual voltage the battery should read at no load is dependent on temperature. A properly charged/maintained battery will read more than 13V with the ignition off on a nice day (70ºF). A Deteriorated battery might only read 12V

The battery condition can best be monitored with an EXPANDED scale voltmeter. It must be wired directly to the battery terminals. Do not wire it to the ignition wires. Or at least wire it to the heavy wire on the starter.

Werner
werner haussmann

"It must be wired directly to the battery terminals."

If you wire it directly to the battery terminals, the voltmeter will be on all the time. Not a heavy drain on the battery, but an unneccessary drain nevertheless. Acceptable, perhaps, on a car that is driven regularly, but not for a car that is seldom driven.

If the voltmeter is connected to a convenient green wire (hot only when the key is on) and it reads significantly different than it does at the battery terminals, it has already given you a valuable indication - your wiring/connections need to be improved.

Most auto voltmeters do have an expanded scale, usually reading from 8-18 volts, or there abouts. see http://www.autometer.com/cat_gaugedetail.aspx?gid=2627&sid=7
Dan Masters

Dan you are correct of course. It's not a good idea to wire to the battery directly unless the drain of the voltmeter is 5ma or less.

But you do need to take care of where it is wired. The voltage drop of the wiring can easily be .5 volt on a good car depending on the location. Be sure to wire it at the ignition switch or install an ignition relay that takes its voltage from the battery.
werner haussmann

There are ways of measuring current without breaking into the circuit. My bench voltmeter has a plug-in device that will clamp around a DC wire to sample the magnetic field. The meter displays the current as an equivalent voltage. We've also used Hall-effect sensors in the aircraft industry to do the same. This seems especially attractive for the applications in this thread, because of the hazard of re-routing heavy current wiring. Aren't there such devices for use in automobiles?

Dan? Paul? Others?
Fred Doyen

Can anyone elaborate on what extra information the ammeter conveys over a voltmeter?
Steve Postins

Steve,

A voltmeter is a device connected in parallel, whereas, an ammeter is a series device. If connected at the alternator, a voltmeter will indicate only whether the alternator is delivering the power requested of it. An ammeter will indicate how much power is being delivered (assuming constant voltage). A voltmeter is more likely to show problems with the alternator, and a ammeter will show everything else.

An alternator, in theory, will output a constant voltage, with a current whose magnitude varies depending on the load. A voltmeter at a correctly functioning alternator should always indicate a constant voltage, unless the capacity of the alternator is taxed. For a 105 amp alternator, it could be dangerous to rely on a voltmeter to indicate electrical system problems. The first indication of a problem may be smoke.

Though I am an electrical engineer, I am not an auto specialist. Paul Hunt may be able to give more useful information than I am able to provide.
Fred Doyen

An ammeter tells you if you are drawing power from the battery (Discharging) or returning power to the battery (charging). If everything is working correctly, when the car is running you should see the ammeter indicating a charge condition. Immediately after starting you will see an increase in the charging current which will diminish as the battery becomes fully charged, at that point your ammeter should indicate a slight charging condition. The ammeter will only tell you if you are charging the battery or not, it will not tell you the condition of anything else that may be wrong in your wiring except that if you don't get a charging condition when the engine is running, your alternator is bad, the battery is defective, or you have too much load on the electrical system. A short in the system may also cause a discharging condition, likely accompanied by smoke. If you have a voltmeter, and everything is correct you should read 13.8 to 14.6 volts with the engine running and a good battery. If you have no change in the voltmeter reading,between running and not running, it is likely your alternator has died. A discharged battery will reduce the charging voltage some until it is charged by the alternator. In the end, either one will indicate if your charging system is working correctly, the voltmeter will be much easier to install. Neither one can tell you everything that may be wrong in your electrical system.
John H

"There are ways of measuring current without breaking into the circuit. <snip> Aren't there such devices for use in automobiles?"

Fred,

It could be done, but as far as I konw, it hasn't been. The problem with adapting this approach to an MGB would be getting a matching gauge for the dash. It would require a (-)x-0-(+)x gauge, where X is the maximum voltage supplied by the sensor and which corresponds to the maximum expected current, discharge or charge.

If you were working in Detroit, designing a dash layout from scratch, and money is of no real concern, you could easily design such a gauge, but it would be dang nigh impossible to make one at home and have it look the same as the other gauges in the dash. And you will for sure not be able to find an existing gauge that will work.

If you're really sharp, and good with your hands, you might be able to modify the guts of an existing ammeter to make it work, but even then I'm not sure you'd find one that would be a good match.

If anyone wants to take on such a project, I'd sure like to see the results.

Steve,

You have mail!
Dan Masters

Not to muddy this thing up further but an ammeter is really only a voltmeter that measures a voltage across a known resistance. The current can then be calculated by using Ohm's law I = V/R

So it is very feasible to put the known resistance(where the high current is flowing) next to the starter and just run the voltage sensing leads(small wire gauge) back to the drivers area where the gauge is mounted.
CDD Dewey

Dan and CCD are correct, You can buy electronic current sensors that will feed a volt meter or you can use a DC shunt. You buy the sensor or shunt in the correct current range (based on your alternator capacity) and connect a voltmeter to it. Typical shunt output voltage is 50 or 100 mV. The solid state sensors cost more. You can install a voltmeter for $20 or $30 or you can go this route for an ammeter, cost about $50 to $100 and a lot of screwing around for what really isn't much of an improvment over the voltmeter. If you are really worried about burnt wiring and smoke, fuses and circut breakers are what you really want. For the heavier loads you can buy small circut breakers that look like a flasher unit at NAPA, cheap insurance.
John H

CCD,

Yes and no. In general electrical applications, an ammeter is sometimes just as you said - a voltmeter across a resistance. However, that's not the way they work in a car.

An ideal ammeter has zero resistance (an ideal voltmeter has infinite resistance). The ammeters used in a car come very close to zero resistance, as they are nothing but a short loop of large gauge wire between the two terminals. The ammeter needle is magnetically coupled to the loop of wire, and swings right or left depending on the amount and direction of the current flow.

To use a shunt, you would either have to have a large resistance shunt or a very sensitive voltmeter. Let's assume a 30 amp alternator and a 0.1 ohm shunt. At 30 amps, a 0.1 ohm shunt would drop 3 volts, an unacceptably large drop for a car, and would require a -3 - 0 - +3 volt meter. Dropping to a 0.01 ohm shunt would give a voltage drop of .3 volts, much more acceptable, but would require a -0.3 - 0 - +0.3 volt meter. As I said earlier, if you're designing an instrument from scratch, you may be able to do this, but you won't find a ready made gauge that fits the interior decor of a car in this range.

The only real reason for not using an ammeter in a car is the difficulty of connecting the required large gauge wires to the terminals on a fairly small, delicate instrument.

In the days of 15 amp generators, a 14 gaue wire was good enough, but by the time you get to the 50, 60, 70 or more amp alternators used in a modern car, the required wire size jumps to at least a 10 gauge, or even 8 ga. A 14ga wire is pretty pliable, but trying to connect an 8 gauge wire to the ammeter is a bit difficult. I've had to do it for a couple of customers with 60 amp ammeters, but it wasn't easy, nor was it much fun getting the gauge/dash assembly back in the car without damaging the gauge - the gauge bends easier than the wire!

If you'll send me your e-mail address, I'll send you a picture of the internals of an ammeter showing the loop of wire.
Dan Masters

If you must fit an an extra instrument then the voltmeter is better. With an analogue ammeter capable of registering up to 60 amps charge or discharge it is very difficult to see which side of zero the trembling needle really is when the alternator is taking all the load and just trickle charging the battery, and won't go into negative territory until the battery is being significantly discharged. By contrast a voltmeter clearly shows when it is dropping below the ideal of 14.5v, i.e. the alternator has reached maximum output and beginning to reduce the trickle-charge to the battery, but it *is* still being charged. However where you connect the voltmeter can have a significant affect on its reading - the green (fused ignition) will generally give a lower reading the white (unfused ignition) and should have its own in-line fuse, which will be lower than the battery voltage which is probably the most importnt, which will be lower than the alternator output voltage. I'm talking practical terms here with 30 year old wiring and connections, and not theoretical with zero resistance wiring and connections.

Neither tell you much more than the ignition warning light (as John Twist says "buy yourself a nice pair of driving gloves instead"), and *none* of them tell you whether the battery will have enough charge to start the car next time!
Paul Hunt 2

Hi all.

While I agree with nearly all of the remarks in this thread, I would offer a couple of thoughts.

In my experience of vehicles with 30A alternators the actual 'running' current indicated by an ammeter is usually quite low, less that 5A.
You can make it go higher, of course, by having lots of 'electrics' switched on without the engine running, but a thoughtful driver isn't likely to do this.

A 30A ammeter should be quite adequate for MGB's unless thay have lots of 'extras'. These instruments are very simple and unlikely to fail.

Wiring in a 30A ammeter isn't that difficult, I have always run the wires in a separate sleeve for protection against chafing and been careful to pass wires through holes in the bulkhead that have been carefully grommeted. I also put an insulated sleeve over the ammeter's clamping bar, and insulate the connection studs with those hard-to-find angled covers. There are other high-current wires that are not particularly well protected, for example the wiring clip above the starter motor is a potential cause of chafing.

Whnever you look at promotional material for a 'sporty' car, there is always a pic of the instruments, I believe that instruments are highly valued for cosmetic reasons, even if they aren't actually useful, which I agree they very often aren't.

Just my personal thoughts.. Don

Don

Perhaps Don is right. Perhaps the main point for having any of these ~ dash lamp, voltmeter, or ammeter ~ is to satisfy some need other than a purely technical one. I would wager that most owners are content with a dash lamp, while others need a more dynamic graphic representation of what is happening, choosing either the ammeter or voltmeter depending on their own notions of which aspect of electrical system performance is either important or at least sufficient to their understanding of how it is working at the moment. Even then, any of these monitors is pretty much, to me, either a gauge (no pun intended) of reassurance that all is well or impending doom. As I recall, I saw and smelled the smoke of wiring being fried before I noticed the ammeter in my MGA being pegged.

I will stick with the voltmeter in the GT as it and the dash lamp are probably as much as I will ever really pay attention to. To my mind, it would be far better to have a flashing dash lamp when Lucas, God of smoke and fire, comes to call, to grab your attention than a gauge that fades into the clutter of the dash while you are focused on other things while driving. A trigger for flashing the lamp (more than simply having it come on) would seem a good project.
Robert Muenchausen

Hi Robert.

It would be certainly be possible to detect the ignition warning light coming on, but I can't immediately think of a simple way of doing it.
An opto-isolator would be a neat solution, these inexpensive devices have a pair of connections for input current and another pair of terminals which provide an electrically isolated transistor switched output.

I believe that best way of indicating an urgent problem (eg overdrive engaged and reverse gear selected [inhibit switch not working]) is to have some sort of acoustic alarm.

I do believe that both an ammeter and voltmeter can be educational for those who are interested in that sort of thing, for example if you are stuck in traffic with lots of electrical loads switched on they might indicate the need to switch something off before the battery goes flat.
They are occasionally useful for spotting and tracking down electrical faults.

Don
Don

Don and Bob - I did the sensor/audible alarm on our TD for oil pressure (after I had lost all the oil in the car while watching traffic instead of the gauge and the first indication of the problem was an audible knocking from the engine). I just used a oil pressure switch from the later MGBs, ran it through an inverter to get a trigger voltage when the pressure dropped to zero and then had made a time delay so the alarm woul only sound after about a 30 second delay after the ignition was turned on (so I didn't have to listen to the alarm while starting the car). This really amounts to lot of fussing around for a minimal gain (other than peace of mind). Cheers - Dave
David DuBois

Don - surely a correctly wired ammeter will show no increase in reading with increasing load. However as the load approaches what the alternator is capable of supplying it will show a decrease in reading, and go into negative territory when it exceeds what the alternator is capable of supplying. At that point the battery is carrying part of the load, and hence being discharged.
Paul Hunt 2

Hi Paul.

Agreed, an ammeter will spend most of it's time showing a small charging current while the engine is running. It will usually show a slightly higher than usual charging current for a minute or so after starting the engine as the charge used for cranking is replenished.

The most obvious circumstance for an ammeter to show battery discharge is with the alternator producing a relatively low output current due to low engine revs combined with a high electrical load.

An example of this might be sitting in traffic with headlights, HRW, heater blower and wipers all on.

The alternator would probably be able to supply all of these loads with the engine running at speed, but at tickover it is likely that the battery would be called on to provide current and so start to discharge. This can cause the engine to cut out if it goes on for long enough.
I recall an occasion when there was a spate of this problem when a lot of cars were driving very slowly down a road for people to view Christmas lights.

Don

Don

This thread was discussed between 11/11/2005 and 20/11/2005

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