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MG TD TF 1500 - Alternator Modulated Charge?

Can an electronics guru please help with a query about the TF's charging system. I have converted to a 45A alternator. If I switch all the lamps on, including two spots and the heater, without the engine running the ampmeter needle goes about half way into the discharge section. Start the engine and just above tickover the needle swings about half way into the charge section and then almost immediately drops to just charging. If I leave the lights on for a while with the engine off and then start it exactly the same thing happens. I think if I had the dynamo fitted I would see a healthy charge for some time until the battery had been topped up. In fact I have never seen the needle way up the charging section even after repeated starts of the engine. Is the more sophisticated electronics in the alternator providing a smoother charge to the battery? I must admit I was concerned when I fitted the alternator that the ampmeter would have to take a full 45A current and burn out but this just hasn't been the case.


Jan T
J Targosz

Jan,
The folks who can help you out are here:

http://www.vintagemecha-tronics.com/

I'm sure that Dave will lead you in the right direction!!!

Steve
Steve Wincze

Hi Jan, I have an alternator system on my TD and it does similar to what you are describing and it works great. From your description its working as it should. Most alternators have the voltage regulator built into them.
ECS Stanfield

Very difficult to diagnose without knowing how the alternator is wired into the car circuitry. First question would be has the original voltage regulator been bi-passed.

Not all alternators have an internal regulator.
G Evans

I have to agree that its normal behaviour for an alternator with the external voltage regulator correctly bypassed. The alternator doesn't have to break sweat to make up for the battery discharge - providing the battery is in good condition and was fully charged to start with. Mine does exactly the same.
Dave H
Dave Hill

You have to understand that, when the charge current drops a short time after starting the car, it's not because the battery is fully charged. It's because the battery voltage has come up to a normal value, and the regulator has switched to "float charge" mode. That's when the battery charges slowly at constant voltage, because it's nearly fully charged.

So, yes, the behavior you describe is quite normal.

It's almost pointless to have an ammeter on a car that uses an alternator, as it will indicate close to zero the whole time. The regulator in an alternator is much better than the old, generator/mechanical regulator combination. I guess it might tell you if something has broken, but that's all. I think a voltmeter might be more useful.
S Maas

Thanks for the help. I know my alternator is effective since it will still push the ammeter needle to the charge side even with all the lamps, heater and wipers etc. are switched on. I would have liked to have fitted a voltmeter but couldn't find anything that would fit into the TFs triple gauge. I don't want to mount one under the dash so am relying on the red tell tale lamp going out as the engine is reved.

Jan T
J Targosz

I couldn't find a voltmeter that would fit into the ammeter recess either, so I just fitted a round Smiths type into a chrome bracket under the dash. However, you are right, its not ideal and is very close to my left knee when driving. I think a better solution is to find / make a bracket that fits to the steering column. I haven't done it yet but taking the ammeter out of circuit would be a sensible additional move as having potentially 45Amp under the dash is a bad idea. In comparison a Voltmeter takes hardly any current at all.
Dave H
Dave Hill

JT do a search on the internets... There is a company that makes a voltmeter that is a single LED that changes color depending on whether it is under, over, or at the correct voltage... A nice unobtrusive single bulb easily mounted most anywhere.
Geoffrey M Baker

Head's Upô Voltage Monitor
http://www.signaldynamics.com/heads-up-voltage-monitor/

I don't think this is the one I saw, but it's the same idea....
Geoffrey M Baker

Seems like a very good solution that gives all the information needed.
Dave H
Dave Hill

I wonder with some judicial face trimming or adaption whether this would fit in the TF Ammeter slot, be an interesting exercise.

http://www.ebay.com.au/itm/12V-BATTERY-STATUS-CHARGE-INDICATOR-MONITOR-LED-DIGITAL-METER-GAUGE-CONDITION-/121785575746
G Evans

It might, but it looks too modern!
Dave H
Dave Hill

This thread was discussed between 26/03/2016 and 29/03/2016

MG TD TF 1500 index

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