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Dave C

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Apologies if this has been covered before.

What reading would you expect from an ammeter when driving at night with headlamps on, assuming a brand new and fully charged battery?

Mine is reading slightly negative/discharge most of the time, in a variety of driving conditions including when driving on the motorway at 60-70 mph.

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Hi Dave, 

Dynamo or alternator? and if alternator how is the ammeter wired in?

Before I changed from a dynamo to an alternator I had an ammeter in my Spitfire and it would show a negative reading with the headlights on only at engine idling speeds, once on the move it would go back into the positive. Does your battery go flat? if not I would suspect that the ammeter is giving a false reading. 


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Sorry, should have said. Alternator. The car is a TR5.

The story is I was driving home the other night and started to lose electrics. The battery had been fully charged before I left home as I use a CTEK battery charger when the car isnt being used for a while.

First I noticed the headlamps were pretty useless, then the indicators wouldn't flash, then I lost overdrive.

I got home and parked the car in the garage. The following morning the battery was completely flat. I started the car using jump leads. My multi meter indicated the alternator was putting out about 13.5 - 13.8 volts at 2500 RPM.

I bought a new battery and all is OK for now. But the ammeter reading suggests to me that the alternator isn't charging sufficiently.

But I've not long had the car and never had a car with an ammeter before, so I'm looking for confirmation of what I should be seeing.

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An alternator should charge at idle.  (Sometimes it will need a little rev to start the excitation, but unless the idle is very low should continue to charge.)

As Dave has already said, you need to have a look and see how the ammeter is wired up - it will either have a shunt resistor (So is effectively a volt meter) or whole current. The latter will have very thick wires going to it as it will have the load going through it (except starting current)

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As on another post:

Alternator Health check

Tick-over - ignition only                                 13 v - 13.5 v

Tick-over - Headlights on                               11.5 v - 12.5 v

2000 rpm - ignition only                                  13.5 v - 14.5 v

2000 rpm - Headlights on                                13 v - 13.5 v


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