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Ammeter readings with an Alternator

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Hi all

 

Quick question - just had an alternator conversion and my ammeter is showing pretty permanent negative values, (over a long run and a short town drive)

 

should I ignore this now I have the alternator is is it still a sign that there is a net "loss" in the charging circuit?

 

Thanks

 

Nick

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It depends on how you have the ammeter connected Nick . On the face of it a constant negative reading would show that the battery is not being charged ....but...

If your alternator conversion also included a swap from positive earth to negative earth - and swapping the ammeter polarity was missed - it could be showing either a constant charge or that some other load is connected on the battery side of the ammeter.

 

What polarity does the ammeter show with the ignition on but the engine not running?

How big is the constant 'drain'?

Was anything else on at the same time e.g. lights, cooling fan, etc?

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Hi Nick,

with the ignition off, turn the headlights on - does the ammeter indicate positive if so then the wiring to the ammeter is back to front.

If it still indicated negative then you have a problem with your Alt wiring.

 

After start up is there a big positive charge for a few seconds (rapid re-charge).

 

Roger

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I am probably wrong but someone will dive in and correct me! But I was always told you can't wire a conventional +30-30 Ammeter in with an Alternator, they were designed for use with dynamo's that virtually never exceeded 30 amps output, whereas an Alternator can be bunging out 40-60 amps and more!. Again I thought that was why there was a switch to voltmeters to show the state of the battery as for an Ammeter to work it has to be carrying ALL the current (apart from starter motor) and the wiring designed to carry max 30 amps is more than likely to get fried.

 

Of course this could just be an old wives tales that I read once and believed.

 

Alan

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It's such a crude instrument that it won't hurt it to be used on an alternator conversion. The higher charge current will only be experienced for a short time after starting. It's only carrying the balance between charging current and load current.

Edited by peejay4A

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Alan - if wired correctly, most of the time there is little to no current flowing through the ammeter. As Pete says for the brief time immediately after starting when there may be significant current to the battery, there will be no harm to the instrument. I don't think my ammeter has ever shown more than about 20 amps anyway even after considerabled cranking. Just because an alternator is capable of supplying 60 amps does not mean it ever has to do so - and even if it is, the wiring should not route that load via the ammeter. The meter is only intended to show what current is flowing into or out of the battery.

Edited by RobH

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Alan - if wired correctly, most of the time there is little to no current flowing through the ammeter. As Pete says for the brief time immediately after starting when there may be significant current to the battery, there will be no harm to the instrument. I don't think my ammeter has ever shown more than about 20 amps anyway even after considerabled cranking. Just because an alternator is capable of supplying 60 amps does not mean it ever has to do so - and even if it is, the wiring should not route that load via the ammeter. The meter is only intended to show what current is flowing into or out of the battery.

 

Ah so does the Ammeter wiring need to alter, with a dynamo and voltage regulator I think the Ammeter is just inserted into the heavy brown feed cable from the regulator.

 

Alan

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No Alan - there should be no need to change the wiring (other than perhaps to swap the meter polarity). As standard the only load connected to the battery side of the ammeter is the horn. The leads to the A and A1 terminals on the dynamo regulator are connected together and go to the output lead from the alternator. That means that all the large loads are taken direct from the alternator, not though the ammeter.

 

http://www.advanceautowire.com/tr24a.pdf

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If the ammeter alternator (silly me!) has a modest rating (mine is 34 amps), continued use of the ammeter is not a problem.

I have never seen, in 23 years of use with an alternator, my ammeter go higher than 30 amps charge, and that was only for a matter of seconds.

Ian Cornish

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Did you mean if the alternator has a modest rating Ian?

 

I used to have a 60 amp alternator on my rally Imp, and the needle would bounce off the stop sometimes, but car ammeters are robust devices and can withstand such treatment occasionally!

 

Pete

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The standard 30A ammeter will be fine with an Alternator conversion - 'onest !

 

Bob.

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Ammeters were fitted to the TR5 and early TR6s which also had alternators as standard so yes they can be fitted. The later 6s had voltmeters.

 

The ammeters can be tricked into false reading if a current is drawn from the battery side as the flow through/past the ammeter will read as if the battery is charging.

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Hi All

 

The meter is definitely the right way as it does register "negative" with the ignition on and lights on.

 

what checks can I do on the charging circuit to see where there is a problem? How do I test the Alternator to see what it is outputting?

 

Thanks

 

Nick

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Hi Nick,

the first simple test is to measure the battery volts with the ignition off 12 volts (it would be a little bit more than this.)

Then measure the battery volts with the engine running at 2000rpm = 13.7volts

 

Roger

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Ask whoever did the conversion how they connected it up.

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Nick - you don't say the detail of your conversion, but if you are using the old voltage regulator as a junction box for the wiring, you do need to disable/break the old internal connections to the external spade connectors - this isn't made clear in some of the instructions I've seen and as I found to my cost - there are earlier threads on this forum as to how to do that.

 

cheers

 

Rich

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Where in the circuit is the ammeter connected? it must be on the load side of the ammeter, i.e. not the battery side.

 

Bob.

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Hi Bob,

you have a typo

 

Where in the circuit is the ammeter connected? it must be on the load side of the ammeter, i.e. not the battery side.

 

Roger

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Oops ! what I meant to say was :

 

Where in the circuit is the alternator connected? it must be on the load side of the ammeter, i.e. not the battery side.

 

Bob.

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Hi Rich

 

thanks for the heads up about the voltage regulator - though I can't find the previous threads - could you point me to one - thanks!

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Nick - I'm not very good at searching the forum either but if you paste the following into Google.... "using a TR voltage regulator as a junction box for an alternator" it will bring up a number of links to posts on this forum - the one dated 12th Jan 2015 headed "Dynamator Alternator Conversion" should do the trick - the others might but I haven't checked them

 

Cheers

Rich

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Might be worth perusing the article which Jim Christie and i concocted about Moss alternator kits, and which was published in TR Action 248 (March 2011). If you don't have that TR Action, email me (see page 82 of TR Action) and I can send a MS-Word version.

Also, my earlier article about conversion to alternator, which appears in Section J4 of the Technicalities CD might prove helpful.

Ian Cornish

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Hi All

 

Thanks for the help - the answer was that the Alternator had not been fitted properly by the well respected garage I used (I'm not going to name names as I hope they will put it right)

 

all the mountings were various shades of loose meaning under load it just didn't spin the alternator enough.

 

2 new bolts and general tightening and all sorted

 

Nick

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