Zendervision Posted June 26, 2012 Share Posted June 26, 2012 What are the symptoms of a faulty voltage regulator in an alternator? I have a Lucas ACR which is putting out 13.3v at idle (across the battery) and flickering between 13.3 and 13.5 while revving. My in dash voltmeter reads just over 13 while driving, but plummets to 11 if I use the indicators and falls off the scale if the headlights are turned on. I replaced the worn brushes but that only perked it up a little bit. Belt tension is fine, red light is working as normal. After ten miles yesterday (during daylight!) the battery read 12.4v so it is charging. Is this a voltage regulator thing or would I be wasting ten quid replacing it? Quote Link to comment Share on other sites More sharing options...
bodders1 Posted June 27, 2012 Share Posted June 27, 2012 Hi ZendervisionAt first glance, I would say you have nothing to worry about.It all depends where you have your dashboard voltmeter connected - how far away is it from the battery?In all electrical circuits, the wires themselves and the joints between those wires have a small resistance. Passing a current through a resistor will result in a voltage drop across the resistor. In low-current applications, this resistance is too small to make any appreciable difference, BUT... ...the voltage drop across the resistance is directly proportional to the current, and in a high-current circuit it will lead to a measurable voltage drop.Your indicators are 2 x 21W bulbs. 42W @ 12 volts will draw a current of 3.5 Amps (Amps = Watts / Volts). You are seeing a voltage drop of around 2 volts, so this means that the wiring behind your dashboard has an internal resistance of 0.57 Ohms (Resistance = Volts/Amps). This is not unreasonable.The usual cause of increased resistance in vehicle wiring is corrosion in push-fit connections. You may get some improvement by unplugging and cleaning every connector in the circuit from the battery to wherever your voltmeter is joined to the loom. If you have a fuse-box, clean and reseat the fuses in their holders. Or just live with it.The voltage that matters is the one measured on the + terminal of the battery, or on the large bolt on the starting solenoid where the thick brown wire from the alternator is terminated.If you want to have your dashboard voltmeter give a true reading of battery voltage at all times, you will have to rig it up with its own feed, taken directly from the battery. If you want to make the voltmeter come on with the ignition, put a relay in the new feed line, and drive the relay from the old voltmeter feed. Quote Link to comment Share on other sites More sharing options...
Zendervision Posted June 27, 2012 Author Share Posted June 27, 2012 Thanks for that, Bodders. The voltmeter's connected to the ignition switch, and only concerned me because I don't remember it moving about so much. The needle only used to fall a little when the lights were turned on, it certainly never fell off the scale. Doesn't this mean I'm running off battery power when driving with the lights on?I'm planning on turning my attention to the rats nest of wiring and adding relays to the lights, sounds like a properly wired in voltmeter would be a sensible addition while I'm doing it. Quote Link to comment Share on other sites More sharing options...
piman Posted June 27, 2012 Share Posted June 27, 2012 Hello Zendervision,you should see more than 14 volts from an ACR when revving the engine. (I'm assuming you were using a hand held meter for the first readings in your post).Regulators are cheap and easily changed so I would give it a go. Alec Quote Link to comment Share on other sites More sharing options...
bodders1 Posted June 27, 2012 Share Posted June 27, 2012 As I said above, the only voltage you need to worry about (when assessing the health of your alternator) is the terminal voltage on the battery. If this is showing 13-14v when the lights are on, then there is nothing to worry about. You are not running off the battery. Quote Link to comment Share on other sites More sharing options...
Zendervision Posted June 27, 2012 Author Share Posted June 27, 2012 Just been out with my multimeter, and getting 13.3v across the battery with the headlights on. So you're right, I should worry less!piman wrote:Hello Zendervision,you should see more than 14 volts from an ACR when revving the engine. (I'm assuming you were using a hand held meter for the first readings in your post).Regulators are cheap and easily changed so I would give it a go. AlecI couldn't get more than 13.6 today revving the engine so I suppose it couldn't hurt to change the regulator, especially as it's pretty much the cheapest and easiest thing to swap. Thanks for your help. Quote Link to comment Share on other sites More sharing options...
Nick Jones Posted June 27, 2012 Share Posted June 27, 2012 13.6v should be fine. 13.8v is the "official" voltage IIRC and you're within .2v! I've not had much luck with the more recently available Lucas ACR parts (regardless of the colour of the box they come in - this is why I fit newer Bosch alternators) so I wouldn't change unless I had too - the new one might not be a forward step!Nick Quote Link to comment Share on other sites More sharing options...
piman Posted June 27, 2012 Share Posted June 27, 2012 Hello Nick, my workshop manual gives 14V at full load for an ACR alternator, so off load you'd expect a .3v to .5v higher voltage? I think that a check is required across the battery with some reasonable load?Alec Quote Link to comment Share on other sites More sharing options...
Zendervision Posted June 27, 2012 Author Share Posted June 27, 2012 I went out again and put as much load on it as possible. Main beam, indicators, brake lights, heater blower and CD player. I got 12.8 across the battery, occasionally flickering to 12.9 when revved vigourously. Quote Link to comment Share on other sites More sharing options...
bodders1 Posted June 27, 2012 Share Posted June 27, 2012 You forgot the wipers ;-)I wouldn't worry at all with those readings. In a fully loaded condition, your alternator is still putting out enough current to charge the battery. The nominal terminal voltage of the battery is 12v. If it falls below this value, it is discharging. If it rises above this value it is charging. The absolute voltages you are measuring (13.6 vs 13.8, etcc) are not that important. The book values assume perfect conditions with clean connections in the circuits - the wiring on even the newest Spitfire has now had at least 31 years in a less than perfect environment and push-fit connections inevitably suffer from corrosion.By the way, the job of the voltage regulator in an alternator is to stop the output voltage rising too high, which would lead to over-charging of the batttery (which, in a worst case, could cause it to boil the electrolite and potentially explode) and popping of headlamp bulbs. Quote Link to comment Share on other sites More sharing options...
piman Posted June 27, 2012 Share Posted June 27, 2012 Hello Bodders, yes a battery is nominally 12v but a fully charged battery will be over 13v open circuit. There may well be other problems with the wiring as you suggested earlier but I would not be happy with an alternator giving that output.Alec Quote Link to comment Share on other sites More sharing options...
Zendervision Posted June 27, 2012 Author Share Posted June 27, 2012 bodders1 wrote:You forgot the wipers ;-)I knew there was something else! No bulb in my courtesy light either. ;DOkay, well I'll leave it as it is for now. I will proceed but with caution. Quote Link to comment Share on other sites More sharing options...
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