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Old 3rd October 2019, 08:48   #1
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Default Diesel alternator amp rating

Hi. The battery warning light is on dash. I checked battery voltage. It's 12.3 when engine not running and 11.8 when running. Called garage for a new alternator and came back with 2 amp versions. 100amp or 120amp. Which is correct one? My car is 04 cdti
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Old 3rd October 2019, 10:05   #2
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The 2003 version of RAVE gives 115 amps for the diesel, but I would be inclined to choose the highest maximum output possible, in your case 120 amps.

Make sure that you're being offered an exchange unit. There's no point in paying the very high price for a brand new one outright.

Bear in mind that at normal diesel engine speeds, the output will be nothing like 120 A anyway.

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Old 3rd October 2019, 10:37   #3
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The 2003 version of RAVE gives 115 amps for the diesel, but I would be inclined to choose the highest maximum output possible, in your case 120 amps.

Make sure that you're being offered an exchange unit. There's no point in paying the very high price for a brand new one outright.

Bear in mind that at normal diesel engine speeds, the output will be nothing like 120 A anyway.

Simon
Does other cars alternator fit or just rover 75?
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Old 4th October 2019, 11:20   #4
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It doesn't matter really which one you chose. The demand from the car's electrics is nowhere near the alternators maximum output.

For instance with the headlights and dip lights on together ,they take only around 20 amps.

Putting on all the other lights and heater etc won't double that.
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Old 4th October 2019, 11:29   #5
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It doesn't matter really which one you choose. The demand from the car's electrics is nowhere near the alternators maximum output.
Agreed Jon, but if we had access to the alternator output graphs they would probably show that the 120 amps version has a greater output at practical engine speeds than the 100 amps model. When you're running the headlamps, wipers, heated rear window and possibly air conditioning to demist the windscreen in the winter, that's worth having in my view.

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Old 4th October 2019, 20:13   #6
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Hi there Simon. I would like to post a bit of info. that you might know already.

The actual speed of the engine above lets say 1,500 rpm has little bearing on the alternators output.

The output is completely controlled by the regulator. It is designed to sense the voltage in the cars electrical system and to get it close to 14 to 14 4 volts most of the time.

There are mild variations dependent on batteries condition.

To obtain this pre-set voltage the regulator varies the supply of current to the alternators armature.

The armature is an electro-magnet. The power supply to it can vary the strength of this magnet. So at the revs I mention the alternator can put out a large range of amps into the system. Possibly up close to its maximum output.

As the demand of the whole system with everything switched on is unlikely to be more than half the maximum rated output of the alternator, the alternator will be able to cope at quite low revs.

To get the maximum output from the alternator requires the regulator to send around 6 amp to the armature.
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Old 4th October 2019, 20:22   #7
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Further to the above post. I am doing this in two goes as sometimes when I try to post a reply I lose the whole post.---

When they designed the car's electrical system I would guess ( and this is only a guess. Lol. ) they would not have used a system that rapidly depleted the amount of electricity in the battery at lower revs. Like folk stuck in traffic, or they would have ended up with lots of very disillusioned customers in the cars they had managed to sell them.

Nearest taxi rank just over there Sir.
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Old 4th October 2019, 21:11   #8
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Hello Jon,

I am looking at a graph of the output current of Lucas A127 alternators. One is rated at 35 amps maximum output and the other at 70 amps (yes, it's quite an old reference book, but the principle is the same).

At an alternator speed of 2,000 rpm the output of the smaller alternator is 20 amps and the larger 47 amps. That was the basis of my advice to peelaaa.

All the best,

Simon
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Old 5th October 2019, 19:48   #9
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Simon.---How old was that book ??

The trouble with electronic developments means that things that worked well yesterday might not be quite up to it today.

A wee bit like comparing the old cathode ray tubes with todays almost flat televisions. Things like colour and definition. ( Mind you, the sound output has gone rapidly downhill. )

The old electronic controls on the first alternators were not a patch on what we have today. The design of the alternators themselves is far superior also. higher output at lower revs.

I clearly remember playing around with dynamos. A word rarely seen today unless you are talking about that magician.---Screwing in and out little screws to try and vary the dynamos output without burning out all the copper segments in the split ring commutator.--


It seems to have got to the stage today with the speed of change that if you are over thirty you are a spent force as far as understand or developing any new electronic designs.

Us old codgers have no chance. Evolution has left us far behind.

Soon AI will be doing all new developments and NOBODY will know what's going on.------------------- EXIT that way.---
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Old 6th October 2019, 08:42   #10
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A wee bit like comparing the old cathode ray tubes with todays almost flat televisions. Things like colour and definition.
Now there's a subject for another thread! The facts about colour and definition will surprise you. Like most of television, it's all an illusion.

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