VOLTMETER v AMMETER

[Lee Dutton]

I have a 66 4a Neg earth, fitted with an alternator, it also has a voltmeter fitted in place of the ammeter.

The voltmeter seems to have stopped working and I suspect it may be faulty.

My question is can I refit an ammeter or does it have to be a new voltmeter as I have an alternator & Neg earth set up.

What is the best solution?

Best regards Lee

[GUY BENNETT]

Quite a bit of wiring alteration involved as an ammeter is wired in series, voltmeter in parallel.

You will need to fit an ammeter to match your alternator, so if its a 66amp alternator then you need a suitable ammeter to cope. I don't think any of the earlier TR's were over 45amp. You will need a very big cable running running behind the dash board and instruments. I would get the voltmeter replaced. May only be a loose connection on the back of it.

[Mike Collins]

I would start by removing the voltmeter and connecting it to a 12V source and if it gives no reading, take out the movement to see why it's not working.

 

If it's old style Smiths, it's probably a bimetalic strip with a heater coil wound around it, mechanically similar to the fuel and temperature gauges but different resistance.. It might be as simple as a broken wire to the coil.

 

Mike.

[88V8]

Ammeter = big unfused cable running behind the dash with loads of current going through it. Bad enough with a dynamo, factor in the output of an alternator it's a good way of setting the car alight.

Ammeters were relevant in the days of dynamos that couldn't cope, when alternators came along they were replaced with voltmeters because it's the voltage that tells you about the health of the alternator, not the amperage.

And a voltmeter only needs an itsy bitsy connection to any part of the live circuit.

Unless you're dead keen on originality, stick to a voltmeter.

 

Ivor

Edited August 13, 2010 by 88V8

[RogerH]

The ampmeter shows the current flow to and from the battery.

The biggest amp value would be with the engine stopped and all the services switched on.

This would give a big discharge but no more than the original spec when new (assuming no extra fancy things)

The next biggest current flow would be after starting the engine. The alternator tries to charge the battery.

This often gives 20-25 amps for a few seconds running down to 10amps and so on - a bit more than a dynamo but still within the capacity of the wiring.

 

Assuming that the car/wiring is standard then the wires for the ampmeter are still behind the dash and shorted together with the voltmeter connected to this shorted junction.

I would have thought tracing the wires, undoing the shorted connection and installing a standard 30+/30- ampmeter would suffice.

 

Problems can arise (rarely) with a big alternator if the battery throws an almighty wobbly and tries to suck the alternator dry (lots of amps).

 

Somebody had a post the other day whereby they had a 40amp fuse inline with the battery hidden in a regulator box.

This would protect things quite nicely

 

Roger

[littlejim]

was there ever a Triumph voltmeter?

Despite the mental anguish of working out the circuit, I wanted to keep the ammeter after putting an alternator in.

Some of the 'guru' articles don't actually give you all the answers.

Might buy a little fire extinguisher just in case the brown faecal material hits the rotary cooling device.

Rather than discharge I was more worried about the alternator putting 65 amps through the charge side and bending the needle or burning a winding. In practice the sky hasn't fallen in yet.

I now wonder whether some of our correspondents speak from experience or theory.

Edited August 13, 2010 by littlejim

[Dr. Hugo]

Hello Lee,

 

Early dynamo charging systems and the electromechanical regulators had poor voltage regulation and the the output voltage altered with engine rpm, and was low at low engine rpm and the voltage sagged under high current loads and a number of factors affected the charge currents. From the driver's perspective the charging current, rather than the voltage was the important parameter to have displayed on the dash instrumentation. Also, historically, the charge currents were usually monitored when charging batteries. The amp meter indicates whether charge delivery in Coulombs per second (amps) was either being delivered to, or extracted from, the battery, so the driver can see right away whether the battery is charging or discharging at a glance. The charge current is always higher when the battery is flatter giving some indication of the battery's overall state of charge.

 

On the other hand, once Alternators came of age the charging system dynamics were quite different. The alternator output is regulated to a fixed and stable voltage, usually around 14.2V at the alternator output terminals (but some alternators have sensing wires to stabilise the voltage at another location).Their output is better sustained at low rpm and is stable at high rpm. The charge current then ends up being whatever the battery requires. This is equal to the alternator output voltage minus the battery voltage divided by the sum of the resistance of the wiring between the alternator and the battery and the battery's internal resistance values.

At this point in history the charge current was regarded as a less important parameter, due to the effective "auto-regulation" of the charging system. Also manufacturers probably assumed that the average driver had a limited knowledge of physics and wouldn't be interested in charging currents much anyway. On top of that the more modern system charge and discharge currents where getting much higher (due to additional electrics and higher output capacity alternators) requiring thicker wire to the amp meter behind the dash, and 60 amp rated amp meters too. So part of the reasoning of moving away from ammeters was to keep all the high current pathways under the bonnet, not behind the dash.

 

So manufacturers went over to voltage gauges to monitor the battery condition in alternator charging systems.These give no exact indication of whether or not charge is entering or leaving the battery, but they indicate the battery terminal voltage which is a guide to the battery's state of charge. Sometimes the volt gauges in cars were badly connected to the electrical system wiring to a handy 12V connection point where the voltage sagged under load, like on the feed to the IGN wiring somewhere behind the dash, and the indicated voltage drops down with the current to the external load, reducing the utility of the idea of a voltage gauge as a "useful instrument". The voltage gauge is really a "gross battery condition indicator" and if it's say 11v or below then clearly the battery is discharged, if 12.6V or more the battery is "probably" in a reasonable state of charge and if its around 14V its most likely fully charged.

 

The bottom line is (in my view) that voltage gauges are not that wonderful and really only indicate fairly gross anomalies like a badly flat battery and often the calibration on their faces is pretty coarse. The amp meter is the superior way to go, and a "bi-directional meter" conveying useful data and it is original for your car. It is connected in the same way as it was with the generator, eg between the dynamo(or alternator) output and the battery, and the alternator output side connects directly to the car auxillary circuits(lighting etc).This way you can see the battery's charge and discharge current. With the alternator you will notice that switching on your headlamps(large load)will barely effect the battery's charge current on an amp meter, unlike with the dynamo system with the electromechanical regulator, where due to the compensation current coil on the voltage regulator relay bobbin, the charge current drops with the lights on, a protection used to reduce dynamo burnout/overload.

 

Also as some alternators age the regulator packs go out of calibration and their output voltage drops a little ,say 0.6v to 1V for example is more difficult to see on the volt gauge face, but detected as reduced charging current on the amp gauge if you had one. Also if your battery gets sick with a higher internal resistance, the voltage will look fine, but the charge currents will drop off. Usually in that case though a slow starter motor would give that away but the ammeter still provides better diagnostic information than a voltage gauge.

 

If I were you I would go back to the amp meter as the voltage gauge is only one level above the proverbial idiot light in terms of a useful piece of dash instrumentation. That remark might provoke a response from someone else I would think.

 

As it happens this weekend I'm uploading some data on lucas charging systems for TR's and some info that might help. I can show you how to wire the amp gauge back in if there is any problem.

 

Best regards,

Dr. Hugo.

[Dr. Hugo]

I agree with Roger, the charge current will unlikely exceed 25 amps, so use the original meter, if you are concerned simply add a 30 to 4 amp fuse as suggested to protect the wiring/meter. The charge current could possibly go higher with a good battery in very discharged condition, but it would be for a short time as the terminal voltage under high current charge comes up quickly and the charge current drops off just like Roger says.

Hugo.

[Dr. Hugo]

supposed to say 30 to 40 amp fuse!

[88V8]

A most instructive reply.

 

There was an ammeter in my first Landrover. As it had an alternator, I installed a voltmeter instead. It is true that some of them have poor scaling on the dial, this is a matter of selection.

 

I have a voltmeter in my present Landrover, also with an alternator. It told me when the alternator was overcharging, before the battery was damaged.

 

And I have one in my TR6. On this, although the scale is quite crude, I can see when the lazy alternator diodes have not yet clicked in, and when the electric fan is on but the warning lamp is invisible due to sunlight.

 

I can also see a voltage change if I switch on the heater fan or the headlamps, which allows me to infer a rate of charge.

 

So I find it useful, even though these voltage changes may indicate that it is in one sense connected sub-optimally in that it is recording a sagging voltage against load. The fact that the voltage is sagging tells me something, it warned me for example when the fan was running continually in traffic and unknown to me my fan belt was slipping, it warned me that the fuel pump was about to pack up so I was able to pull off the road before the voltage dropped below 12V.

 

It under-reads about 0.4V but I know that because I have checked it against my DVM.

 

So I'll stick to my voltmeter. It may not be perfect but it has its merits.

 

Ivor

[Dr. Hugo]

HI Ivor,

 

Well the utility of a volt meter in a car would be an awful lot better if say it was a meter that displayed with 0.1V resolution and if it was connected directly across the battery terminals. This is because the battery's state of charge from flat to fully discharged is only indicated by a small range of voltage, say 11 to 12.6 V off charge and 14V on charge at the battery terminals. This would require something like a digital meter with a relay conected to the IGN circuit, so the meter only got connected with the IGN on.

I once saw some LED digital volt meters in the same size body as a standard gauge which would be ok as an option. However LED digital meters are poor in sulight, LCD would be better there and they would need to be back lit at night. Some very nice ones have been made for aircraft. The thermal movements in most car volt meters are by their nature sluggish and often poor in calibration, however as you say, if you have one, it is a matter of selecting a good one and perhaps knowing as you do what its calibration error is.

 

Hugo.

[Alec Pringle]

I'd suggest is easy to be over-critical of voltmeters. Agreed they have their limitations, but a half-decent unit is useful once you are accustomed to its behaviour. It may not be that accurately calibrated in absolute terms, but if you're used to the instrument reading then anomalies ring warning bells ! At least that's my experience, maybe I've been lucky.

 

As for ammeters, the bigger (than standard) alternators on our V8s are rated at somewhere around 70-80 amps, but on a 50 amp ammeter I can't recall having seen as much as a 30 amp charge, and the inline fuses (40 or 35 amp) haven't ever blown. Again, that may be just good fortune, I'm not clever enough with electrickery to understand all the theory, I stick with what works for me !

 

One thing is for sure, when it comes to an endurance competition car I'd rate the extra weight of both instruments as a justifiable luxury. That's saved the bacon before now . . . .

 

Cheers,

 

Alec

[Badfrog]

Hi,

 

I have installed one of those NipponDenso light-weight alternators for the sake of reliabilty. I kept the ammeter in the dash board and installed a voltmeter in the engine bay for regular checking of the alternator, with the classic 11.5, 12.5 and 14.8V rule of thumb.

During my recent problems with L.A., I appreciated greatly having the ammeter inside as it gave me a hint that something was wrong, simply by not moving. Once given the idea by someone on the forum that a bad earth could be part of my problems, I understood why the ammeter was dead on the car while OK on the bench. Sanding the neg. battery cable attachment to the car shell immediately solved related problems and the ammeter hit the +30 mark for a moment to reload the battery. The voltmeter in the same period was logically indicating a normal situation !!

My dashboard ammeter is the original one, mounted in serie with the big wires. Loom is new: 5 yrs-old only.

 

As far as I'm concerned, with an old car always prone to weird breakdowns, both gauges are no luxury because they show different informations.

 

Badfrog

Edited August 15, 2010 by Badfrog

[Lee Dutton]

THANKS TO ALL FOR ALL YOUR CONTRIBUTIONS AND ADVICES,I MUST SAY THIS TOPIC REALLY GOT ME THINKING SO MUCH INFO ESPECIALLY FROM THE DOCTOR!!

BEST REGARDS LEE

[Tony Millward]

Hi Lee,

 

A fully charged battery will read 12.6V to 12.8V with no load. a battery in need of charge will read between 11v - 12.0V. If you have a battery that reads 14.0V (no load), bin it and buy a new one. Remember never to take voltage measurements until at least 30 mins after the engine has been run, allow the voltage to settle first.

 

Tony