Grahamgl Posted September 17, 2015 Report Share Posted September 17, 2015 (edited) Good morning, A question to the electrical experts on here please. My car has a Lucas ACR17 alternator fitted, which I understand has a 35 amp output. I want to buy another alternator to keep as a spare. I went to a local alternator supplier who offered me an alternator that would be a straight fit to my car but it had a 45 amp output. When I questioned the chap about the higher output he told me that it would be suitable for my car even though it had a 45 amp rating. He said that it would not necessarily be putting out the full amperage and would only generate the amps required by the car when it was running. The only additional electrical items I have on the car over how it came out of the factory is a pair of period fog lights and modern radio. I have seen posts on here expressing concern about the old wiring on our cars and the risk of over loading it. My question is would it be ok to fit a 45 amp alternator, or, would I need to increase the size of wiring to and from the ammeter and fit an ammeter with a greater capacity? Graham Edited September 17, 2015 by Grahamgl Quote Link to post Share on other sites
RogerH Posted September 17, 2015 Report Share Posted September 17, 2015 Hi Graham, your man is correct. The Alt will only supply what is asked for. However if there is a serious short circuit then a few more amps will be available to do a proper job of melting the wires. At present I'm sure you are not getting anywhere near the original 35amps so your wiring is having an easy life. The ammeter should be wired so that only the charging current is shown. After a difficult start with a flatish battery you may get a big swing to charge the battery - more than 20amps. This will only last for a few seconds and then taper off to 10 then 5 amps until charged. keep the ammeter as is and get the 45 amp Alt if it is a good price. Roger Quote Link to post Share on other sites
stillp Posted September 17, 2015 Report Share Posted September 17, 2015 Actually the higher output would be less likely to melt the wires, as it will blow the fuse quicker. Pete Quote Link to post Share on other sites
peejay4A Posted September 17, 2015 Report Share Posted September 17, 2015 ...but the main charging circuit isn't fused. Quote Link to post Share on other sites
stillp Posted September 17, 2015 Report Share Posted September 17, 2015 (edited) True... Edited September 17, 2015 by stillp Quote Link to post Share on other sites
Superaquarama Posted September 19, 2015 Report Share Posted September 19, 2015 keep the ammeter as is and get the 45 amp Alt if it is a good price Don't you need to shunt the ammeter, Roger ? Would have thought the higher output would be end-stopping it a lot of the time. Regards, Anthony Quote Link to post Share on other sites
RobH Posted September 19, 2015 Report Share Posted September 19, 2015 No Anthony - the new alternator may have higher current capability but it will only supply as much as it has been 'asked' for. There will be no difference from before unless additional loads are installed. As Roger says the most likely difference may be seen just after starting if the battery has run down a lot, though even then if the old alternator never reached 30 amps charge, neither will the new one. Quote Link to post Share on other sites
MRG1965 Posted September 19, 2015 Report Share Posted September 19, 2015 Hi just to add bit more. I upgraded to an alternator earlier in the year and the ammeter goes up to 20amps for a few minutes after starting if its been sitting for a few days or 10amps upon restart after a short run, but goes down to a few amps after a few minutes. Unless you add loads of spot lights or very high wattage headlamps or other loads, I can't see the output getting close to its maximum. Even brought a 45-45 nos lucas ammeter to replace the 30-30 standard, but did not need it (it's going on ebay unless anybody wants it). Think the main benefit of the alternator in normal use is higher output at low engine speeds which helps keep the battery charged over a dynamo in similar conditions. Mark Quote Link to post Share on other sites
Superaquarama Posted September 19, 2015 Report Share Posted September 19, 2015 just after starting if the battery has run down a lot, though even then if the old alternator never reached 30 amps charge Mmm, not sure I agree Rob. If for example you've been churning away on the starter for 30 secs whilst the fuel pumps through, I'd imagine the charge rate would be at or near max initially until it settles down. Still, end-stopping the ammeter for a short time may not matter - don't really know. Did a conversion on a 3.4 Jaguar, albeit with a 75A alternator (off a Transit as we needed to mount the p/steering pump on the back), and this was certainly end-stopping the 20A ammeter for a while after start-up until we shunted it with a paperclip. A very worthwhile conversion anyway, keeps the battery at a higher level of charge. A night stop at a garage or service area was always a bit fraught in case the car wouldn't restart, or if left standing for 2 or 3 weeks the battery won't be flat ! Quote Link to post Share on other sites
peejay4A Posted September 20, 2015 Report Share Posted September 20, 2015 It's a very crude moving magnet design. End stopping it is unlikely to do any damage. If it were a moving coil hairspring movement that would be a different matter, but it isn't. Quote Link to post Share on other sites
MichaelH Posted September 20, 2015 Report Share Posted September 20, 2015 60-0-60 ammeters are available put one in when I put the original alternator in as all the current goes thru the ammeter I just use a voltmeter now depends if you want originality (? with an alternator?) or function Michael Quote Link to post Share on other sites
RogerH Posted September 21, 2015 Report Share Posted September 21, 2015 Hi Anthony, the charging current through the battery is dependent on the battery impedance - not necessarily its low state of charge. The general battery for a TR will take 20+ amps for less that a couple of minutes (more like a lot of seconds). I have never seen one get near the end stop even with serious cranking. Your Jag 20 amp gauge will end stop because it is 20 amps not 30 amps. It doesn't need a shunt. The car electrics should not be going through the ammeter - only the charging circuit. Roger Quote Link to post Share on other sites
cotswold Posted September 28, 2015 Report Share Posted September 28, 2015 I fitted a Dynamotor with a 45amp output to my 4A and inserted a 35amp fuse. No problems at all the Amp meter never goes anywhere near max charge so no need to change that. I was told that even if the battery was flat and the circuit called for more than that it would go off the dial and then return ok when charged . Quote Link to post Share on other sites
Ade-TR4 Posted October 2, 2015 Report Share Posted October 2, 2015 (edited) You could just wire it back to the battery (at the solenoid connection is easy) The ammeter will only show discharge, but at least you'll know it isn't having the charging current shoved through it. Shhimplez. Adey Edited October 2, 2015 by Ade-TR4 Quote Link to post Share on other sites
Lebro Posted October 2, 2015 Report Share Posted October 2, 2015 Just leave it wired as normal, it will be fine. Bob. Quote Link to post Share on other sites
cotswold Posted October 2, 2015 Report Share Posted October 2, 2015 Bob +1 Quote Link to post Share on other sites
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