The Triumph Dolomite Club - Discussion Forum

This is what mine looks like in situ.


It was pretty much a straight swap and the pulley on the new alternator is a bit smaller than the old one. I'm getting 14.5v when driving which goes down when I switch the headlights on but the ignition light doesn't come on no matter what the voltmeter is reading.

All this talk of alternators, has everybody uprated the wiring to suit?
The original alternators were what? 30A and now we are sticking 50/75 or even more amp alternators on.
That means if the alternator is charging at max current, you are shoving at least double or getting towards triple the current down the loom. It is potentially rather hazardous.

Best bet when upgrading an alternator is to add an extra wire from it (ACR types have a spare terminal, very handy) direct to the battery. 40-50A wire is ideal.

Of course, there will be many who are running oblivious to all this, and you may be OK as long as the alternator doesn't have to work hard. But there is no downside to the extra wire.

Me, I am running one of the Brise motorsport Denso 40A jobbies. Actually sold as a kuboto digger alternator. They are small, light and will easily outlast the car. (this is after I had several bad experiences with genuine Lucas recon alternators a few years ago. Won't touch one again)

Well, that extra current only flows to the battery to charge it quicker, and, perhaps more importantly, keep the charging voltage over 13.8 (ish) at lower rpms and with more stuff on. The rest of the loom, even the cable to the starter, is carrying the same current as before. That is, unless you've added more stuff or increased the power of what's there.

Looking at the feed and return to the battery, that looks well up to carrying the 65 amps this new alternator is claimed to give.

Graham

Indeed, normally it won't be booting out the full current. However, it may only need to do it once and for a shortish time (few minutes) to cause an issue. Besides, an extra cable direct to battery can only improve matters.

And I very much doubt the original cable is rated at anything much over 30A continuous. Cable was expensive even back then!

As a precaution I would only change the positive and negative battery leads, if they are the originals on the car. They do corrode down the ends.

Shouldn't need to really as, the battery once even patially charged won't draw that much, and the whole car only can draw up to the max amperage of the fuses installed.

If you have cable (preferably black and red) I have 50mm ring terminals and a hydraulic crimper. Will bring to Stonleigh on Sunday.

I know I'm putting in a large stereo and lots of electrical in my car, so for me it's a must as I'll have a running 60 amp draw, 10 in idle.

"Shouldn't need to really as, the battery once even patially charged won't draw that much, and the whole car only can draw up to the max amperage of the fuses installed. "

Err...apart from the circuits that aren't fused, like all the lights.

While an alternator can in theory put out 65 amps you have to work out if it ever will.....

Max load is all your headlights, side lights wipers, heater fan, heated rear window and one set of indicators plus your ICE.
You could add some peaks for brake lights too, but you shouldnt have them on all the time, plus overdrive and fog lights..

I'll let you do the maths....see what you come up with...


Jonners

The very most extra current a charging battery could ever possibly draw would be about one third of the extra capacity of the alternator - if the alternator has an open circuit output of 15 volts and the lowest voltage for a chargeable battery is a very, very pessimistic (probably ridiculous) 10 volts. So in going from a 45 to a 65 amp alternator, the extra current flow into the battery is not 20 amps, but definitely (much) less than 6.7 amps. And that, as a fraction of the maximum current the system and charging battery could draw, isn't much at all.

Graham

As an approximation, I'd say about an extra 30 amps, but the alternater will be supplimented with the battery, so 30 + the 2 17amp fuses.

Please do tell your thoughts Jon, I am doing some mental working out before I collect my car tomorrow and start sorting the electrics during the week.

I thought the o/drive was fused? and the fogs through the fuse in the pull switch?.

Re the lights mine will be fused and possibly relayed within the month, stressing old wiring and possibility of a electrical fire if shorted is stuff our of nightmares.

Remember lead acid batteries have internal resistance, sometimes quite a bit of it, and this resistance varies according to battery design, condition, charge state etc.

it's a different mounting position all together than the 1500 nice work though for the other half of the group.

Similar alternators/mounting points/plugs though

There is an internal resistance in the battery, but don't forget it also has an internal voltage in series with that internal resistance that can't be zero, unless the battery has had its chips. And it's because of the internal resistance of the alternator and the battery's infernal voltage that the battery can't ever draw anything like the alternators short circuit current when charging.

I know that the charging voltage has to be greater than 10.5 v, because if the internal voltage of the battery is discharged below that, it's had its chips. But as Bill said, there's an internal resistance, and when you charge, the voltage caused by the charging current through that resistor adds to the internal voltage of the battery. So the charging voltage has to be a lot greater than the internal voltage of the battery if its flat. And if it's not flat, then the internal voltage is higher than that of a flat battery. So, if the internal voltage of the alternator isn't much over 15 volts (which it had better not be if you aren't to risk over-charging), that figure of 1/3 is very definitely a maximum. Probably more like 1/4.

As an aside, if you're thinking, I've measured a discharged battery at much less than 10.5 v and it re-charged. That's because most ways of measuring it take current. And because the internal resistance in series with the voltage becomes huge in a discharged battery, the current into a multimeter or AVO creates a voltage across the internal resistance that is subtracted from the internal voltage you're trying to measure. If the internal resistance is big enough, much bigger than the internal resistance of the multimeter, you'll measure virtually zero. And as the internal resistance of the multimeter goes down when you switch to a lower voltage scale, the lower the scale, the more current the meter draws, and the lower the voltage you measure. It can be measured, of course, but it's a little complicated.

Graham

There is a good reason why lights aren't fused if you think about it...

If the filament breaks they go open circuit....if the filament breaks in such a way that it reconnects and draws a higher
current than usual, it burns out pretty quickly, like a fuse. Also all the individual light spurs are in parallel, thus one failed
bulb leaves the others intact. If you had one fuse for the lot, one failure could take out the whole lighting circuit.

The best thing to do with lights is fit relays, because the major weakness in dolomite lights is that the main/dip switch can and does
burn out if higher wattage lights are used.

Some time ago I wrote a calculation on a typical dolomite load electrically speaking...

I'll see if I can find it.

Jonners

Thanks Jonners,

That would be interesting and informative reading.
I take aboard what you say about fusing the lights, I will be
putting in relays for the fronts, which should lower the load,
the brake lights may be changed to leds once I get round to
suitable replacement selection, the rest of the lights are pretty
minor, again leds.

I think the loads are pretty easy. Very few of them, unlike a modern car.
The following are rounded up in the interests of getting some safety margin.
Side and tail lights, about 2A
Headlights (assume max is 4x55W) 20A
Heater motor 3A (?)
HRW 3A (?)
Ignition, probably sub 2A as an average.

Sound system, most are about 1-2A unless a megawatt, ear bleeding, heart stopping, window shattering system is installed)

On our cars it is the headlights that eat the juice. Even so a 25A alternator should be largely adequate, but a 45A will give a wide safety margin.



Now LED's. Has anybody actually found any that are as good (or indeed better) as normal bulbs, notably for stop/tail?? I tried some that were rubbish despite being described as brighter and longer lasting. I got a refund.