Ammeter reading

Robert Scarlett · Post by **Robert Scarlett** » Wed Oct 01, 2014 11:48 pm

Hi,

I have a 1964 series one 3.8 FHC running a reconditioned but standard dynamo.

Could someone please advise what my ammeter should read under normal driving conditions?

When starting cold, the needle points 3/4 of the way up the scale to the left or towards 'C'. After a short time it settles to the left above the centre line or '30'. It largely stays in this position while driving unless I have the lights on at tick-over etc. obviously it then falls towards below the centre mark towards the 'D' a blip on the throttle brings it back up again.

Is this correct or should the needle stay about 3/4 up the scale towards 'C' ?

My Coolcat radiator fan that is actuated by the standard sender unit usually kicks in at about 70deg on the temperature gauge. I thought this a little early but convinced myself it better than being late. I'm suspicious that the fan might be running all the time, I could quickly check by disconnecting the fan and seeing what the ammeter reads.

I just wondered what was 'normal' for reference.

Thanks

Gfhug · Post by **Gfhug** » Thu Oct 02, 2014 8:13 am

Hello Robert, don't have an ammeter on my car but do on my aeroplane so similar system. After start the charge is quite high to re-charge the battery, then after it has recovered the normal position is to show a slight charge. (Aero-engines tend to mainly run at high enough revs to keep the charge showing, unlike the up-down revs of car driving.) So yours sounds fine by my reckoning. High charge to replenish the battery or when high load, lots of lights etc. then normally a low to very low charge under regular driving.

Hope this helps
Geoff

Robert Scarlett · Post by **Robert Scarlett** » Wed Oct 08, 2014 9:39 pm

Thank you Geoff, it looks like all is well then.

Does anyone else have a ammeter on their cars? if so what do your cars read on the scale?

Thanks,

Robert.

christopher storey · Post by **christopher storey** » Wed Oct 08, 2014 9:59 pm

Robert: the problem is that many ammeters are wired in a somewhat eccentric way and do not fully reflect either a. all the demands on the electrical system and/or b. the attempts of the charging system to respond to demand. If the ammeter is correctly wired , it should first of all reflect all ( except that of the starter ) the demand on the battery. So with the engine not running , the ignition should demand perhaps 3 amps ( and you should be able to see the flicker of the needle as the points open and close, and also the tick of the fuel pump ) . Similarly, if you then turn on the sidelights, you should see another 2 to 3 amps demand. Headlights should produce a further 8 amps demand on dipped, and of course heater fan etc will increase it further. This will tell you whether all demand is wired through your ammeter. Then start the engine . You should see with anything more than about 1500 - 1700 rpm a significant degree of charge, which should then taper off over a period of minutes as the level of charge in the battery rises, until at reasonable mid rpm, say 2000 or more, there is just a trickle of charge ( perhaps 2 to 3 amps ) going into the battery even after taking into account the loads on it . If at that stage you allow the engine to go back to idle, you should see the needle move into the debit side of the scale, but then recover once the engine is speeded up again

Tony · Post by **Tony** » Wed Oct 08, 2014 11:27 pm

My car is a 62 roadster with a dynamo and ampmeter. It does exactly as yours. When you first start the car it will go to the left at 3/4 and stay there untill the battery has had it fill. It will then drop to say 1/4 to the left as the battery regains it full charge.

Obviously it will drop as you put more things on and the Dynamo tries to keep up. On tick over it wiil go negative to the right.
Your car sounds normal to me. If the needle starts jumping all over the place and maybe jambs on fully charge I would suspect the regulator is bust. As I found out on a French trip. Not a happy experience.

Tony

malcolm · Post by **malcolm** » Fri Oct 10, 2014 9:11 am

I had a boat with dynamo and ammeter; what is described is exactly what I would expect. Large charge on starting, tailing off as battery is charged, but staying on charge while above tickover to replace the permanent currrent useage while running.

Robert Scarlett · Post by **Robert Scarlett** » Sat Oct 11, 2014 6:52 pm

Thank you for your replies they have all been a great help.

It would seem that my ammeter is wired correctly but before I got the chance to disconnect my radiator fan and give the car a run, I got side tracked by a small oil leak which is now rectified so I will have a play again next weekend.

Robert.

paydase · Post by **paydase** » Mon Oct 20, 2014 11:50 am

Does anybody know what is the scale of reading of the ammeter : e.g. when it goes one division towards the right (half way between middle and D), are we talking of 15 amps draw or more?
That would be useful to have an idea of the rate of discharge of the battery and the possible existing time to continue driving before engine collapses in case the generator/alternator fails. Also allows to see at which rate the alternator is charging the battery when the engine is revving.

Btw I remain confused on the relative interest to swap the ammeter with the voltmeter when you have an alternator instead of a generator.
Actually, having an ammeter allows to know the discharge current being drawn from the battery, so allows to know the remaining time or charge in Ah, in case.
Having a voltmeter allows in principle to know the soundness of the battery before cranking (should be in the range 12.8-13.2 V when charged) and also allows knowing if the alternator is charging the battery when revving as then voltage should be in the 14V range. But if that reading is ca. 13V during driving, it would not say much about the battery status...

Best would be of course to have both an alternator and an ammeter :D
But curious to know the arguments

Post by **Heuer** » Mon Oct 20, 2014 11:53 am

viewtopic.php?t=166

paydase · Post by **paydase** » Mon Oct 20, 2014 11:57 am

David,

Thanks for the link, but I had already read it and it didn't enlighten me. It explains how to swap the ammeter with a voltmeter but does not say why it should be better...

Post by **Heuer** » Mon Oct 20, 2014 12:02 pm

Sorry the link to this article is broken so here you go:

"Which is better for monitoring a vehicle's electrical system - a Voltmeter or an Ammeter?

Short answer: A voltmete, by far.

Electrical guru Mark Hamilton of M.A.D. Enterprises points out that amperage is a measure of current flow, so an ammeter is actually a "flow meter" that's intended to measure current flow to the battery (under normal conditions) or discharge from the battery (in the case of alternator system failure). On a typical flow meter, all output must be directed through the device to obtain an accurate reading. In the ammeter's case, that means all the alternator output used to recharge the battery must first be routed through the ammeter under the dash. Which requires a heavy-gauge cable and presents a possible fire hazard. And the ammeter itself must be able to handle all this current flow, so it must have a higher current rating than the alternator's maximum rated output.

All this might be worth the hassle if the ammeter produced reliable information. But the ammeter can only measure the amount of current output to the battery for recharging purposes: When the alternator recharges a "low" battery, the ammeter indicates a high charge rate; with a fully charged battery the voltage regulator reduces alternator output, and the ammeter is supposed to indicate a very low charge rate. But how can you really tell the regulator has reduced alternator output because the battery is fully charged? Maybe a diode in the alternator rectifier failed, or the alternator belt slipped after it warmed up, just as if the battery were fully charged. Or maybe the meter indicates a medium charge rate most of the time-does the battery want this much or could the voltage regulator be overcharging the battery?

On the other hand, a voltmeter works like a fuel pressure gauge-but instead of measuring fluid in psi, the voltmeter measures electrical system pressure in volts. Just like a fuel pressure gauge, a voltmeter only needs to tap into a circuit; all the fuel (or electricity) does not have to detour through the gauge itself. Voltmeter installation is easy, quick, and safe: It hooks up to a fused, ignition-switched "off/on" source and does not require any modification of the circuit used to recharge the battery or any part of the alternator/regulator system. In short, the voltmeter installed at the dash will be a stand-alone circuit.

The voltmeter directly measures the result of charging-system performance. With normal alternator/voltage-regulator function, battery voltage is maintained at 14.0 to 14.5 volts-and this is reported directly by the voltmeter. In the event of alternator-system failure, voltage will be low and continue to drop as the battery discharges. In the event of an "overcharge" condition, the voltmeter will climb above its normal zone. In summary, there is no chance for misinterpreting a voltmeter's readings as can happen with an ammeter."

paydase · Post by **paydase** » Mon Oct 20, 2014 12:08 pm

Thanks David, that is now very clear, and emphasises again the fire hazard.
And also, a voltmeter is also preferable when a generator (instead of an alternator) is the charging system.

malcolm · Post by **malcolm** » Mon Oct 20, 2014 1:39 pm

In my view, both ammeter and voltmeter can be viewed to demonstrate the same end result.
If you have an ammeter, you can ensure that the charging system is ok while driving; should always be charging when engine is running at, say, 1500 revs or more. If so, then you can check the battery voltage when stopped with a normal multi-meter. It should be c. 13.7 when alternator is charging with no load, and more than 12.1 when everything electrical is switched on.
With a voltmeter, you can see the battery state whilst driving. It should be about "central" on the E type voltmeter while driving at above 1500 revs. At tickover, with cooling fan on, it will drop into the red zone.
While stationary, you can check with a multimeter again that the charging syatem is ok. Read the battery voltage with engine and all electrical appliances off. Should be about 12.5. Start up, run at 1500 revs or more, and voltage should immediately go to 13.7ish. With fans on (won't take long when you're stationary!) and all other electrical things on, lights etc, it should not be less than 0.5 volts lower than the first reading (before starting)
I'm sure other more knowledgeable electricians wil tell you more, but the above tests tend to re-assure me that all is ok.

christopher storey · Post by **christopher storey** » Mon Oct 20, 2014 9:39 pm

I am not a lover of voltmeters. They are all very well in the long term, but unlike ammeters , they are in real terms hopelessly behind the game. An ammeter will tell you immediately when the charging system has gone on strike: a single glance every now and then is sufficient . A voltmeter, however, only tells you when you have already used a significant proportion of the available power reserves - that is, unless you monitor it continuously, during which process you will get much more up to date information from the rending noises as you drive into an immovable obstacle at speed

Sarthe72 · Post by **Sarthe72** » Sun Oct 26, 2014 10:43 am

I have the same set up - dynamo and Cool Cat on a 3.8 - and mine also cuts in a little early in my opinion. I bought the Cool Cat otter switch and changed back to the old one, but the fan still cuts in at just under 70 degrees although a tad later than the CoolCat did, so I reckon, but haven't yet tested, the actual water temperature as yet but think the gauge is not reading pcorrectly . Anyway, with the fan not running the standard charge is just past zero, and doesn't change much with the fan running. If I stop and put the car in neutral, the discharge isn't great, far less than the old Kenlowe I used to have, so I don't worry too much about it. I appreciate that this isn't exactly a scientific answer, but maybe it'll reassure you anyway.

malcolm · Post by **malcolm** » Sun Oct 26, 2014 10:52 am

Quote Christopher " An ammeter will tell you immediately when the charging system has gone on strike: a single glance every now and then is sufficient ."

That is true, and I do like ammeters for that, BUT, if the ammeter is showing that it's charging at, say, a few amps when the fan is on (as with Sarthe 72) it does show that the dynamo/alternator is charging, but not if it's charging enough; a voltmeter will show you if the system is charging with, say, a fan on, by virtue of the voltage staying up, and will also show if that charge is sufficient (maintaining an adequate voltage) Also, if the system isn't charging at all, the ignition light should come on and show that.
So as I said before, I do think there are pros and cons for both. In fact, on one boat I had in the meddy for longish term live aboard, I had both voltmeters and ammeters for those reasons.

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