Alternator charging fault

Hi all.

I've done some searching on this topic, however I haven't found what I needed.

I have a charging issue with my 1995 defender.

When the engine is idling. The voltage on the battery terminals starts at about 13.5 volts. And takes a good while to eventually read 14.1 volts

With the headlights on, it drops to about 13.8

Headlights main beam and heater about 13.0
And with the radio on it drops to 12.9 or 12.8.

Now from the very little I know about alternators and charging circuits. I beleive the alternator should maintain about 14 volts regardless of the load placed on it. Is this correct?

I've cleaned the main earth's on the chassis a transfer box about 6 months ago due to having an earth fault. And haven't had any problems since. However I'm not sure if this could be earth related? My headlights always appear fairly bright, although I have upgraded the loom and running the positive feed off the alternator.

Anyone got any suggestions? Thanks alot

You're not just measuring the alternator output at the battery terminals. The battery load is dropping the voltage. IMHO.

zeaphod said:

You're not just measuring the alternator output at the battery terminals. The battery load is dropping the voltage. IMHO.

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Isn't the alternator supposed to maintain a stable voltage at the battery posts though?

It was the same coming off the alternator too, give or take 0.1 volts.

is voltage regulator in good shape? if it isn't that could cause the alternator to not work properly

Test at the alternator with a multimeter connecting positive on the output terminal of the alternator and the negative terminal to a good earth point. Start the engine the reading should be roughly between 13.8v and 14.2v. You could also test at the battery terminals, with the engine off take a reading and then turn the car on and take another reading. If the battery and alternator are ok you should have a reading of around 12v-13v with the car off. If you’ve run your car for a long time and with the car still running the voltage reading is in the 13v-14v range that should be OK as well, but if you switch the ignition off and the battery voltage instantly drops to well below 12 volts (10 volts or under; 9 volts or under in freezing weather conditions), it is likely that your battery needs maintenance or replaced. Zeaphod is right, you will have a voltage drop when measuring at the battery due to firstly the huge drain on the battery caused by the starter motor which will have to be replenished by the alternator. Switching on other things like lights will also cause a voltage drop to be replenished by the alternator. Your lights or indeed anything else should not be connected directly to the alternator.

Better way of putting it is - the alternator will TRY to maintain the charging voltage. It can only do so much however. As the drain on the battery increases and it discharges, it's voltage drops - and the alternator is connected across it. I guess if you had a massive fully charged battery, the voltage drop would be much less. If you are in any doubt about the condition of your battery, it's time to change it - before the winter starts, as it will be adversely affected by the winter drop in temperature.

ONE LIFE LIVE IT. D90 said:

When the engine is idling. The voltage on the battery terminals starts at about 13.5 volts. And takes a good while to eventually read 14.1 volts

With the headlights on, it drops to about 13.8

Headlights main beam and heater about 13.0
And with the radio on it drops to 12.9 or 12.8.

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According to these readings I would think that all is in order. IMHO

Think of the alternator like a pump filling a big tank - the alternator is maybe putting out 65 amps, but the pond/battery is capable of holding 700amps. When the pond is full, the pressure (voltage) will be high, but when you are letting water out somewhere else (lights) or have just used a lot (starter motor) the pressure will drop. I think. Nearly 40 years since my ONC, and the memory isn't what it was!

chainsaw said:

According to these readings I would think that all is in order. IMHO

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I would tend to agree, my TD5 isnt far off that. Even with everything on, heated seats, screen, lights, aux lights etc... never drops below 13v. Usually 12.4 - 11.9 when the car off and everything else off.

Cheers for the help everyone.

The battery sits at about 12.57 volts after being left for a few days.

The voltage at the positive post on the alternator was virtually the same as the battery posts. Just a very small amount higher 0.1 volts roughly.

The battery has been tested at a motor factors, which they said it was in good health. However, when I come to first start it of a morning. It sounds like there's not enough juice in the battery to crank the engine. It's a fairly big battery too! Anytime after the first start, it flicks over perfectly and very fast.

From the voltages you stated it looks like your alternator is working as it should. It also looks like the battery is OK as well so that leaves the starter motor, I would check the power and earth leads to the starter and if it still persists get the starter checked.

ONE LIFE LIVE IT. D90 said:

Hi all.

I've done some searching on this topic, however I haven't found what I needed.

I have a charging issue with my 1995 defender.

When the engine is idling. The voltage on the battery terminals starts at about 13.5 volts. And takes a good while to eventually read 14.1 volts

With the headlights on, it drops to about 13.8

Headlights main beam and heater about 13.0
And with the radio on it drops to 12.9 or 12.8.

Now from the very little I know about alternators and charging circuits. I beleive the alternator should maintain about 14 volts regardless of the load placed on it. Is this correct?

I've cleaned the main earth's on the chassis a transfer box about 6 months ago due to having an earth fault. And haven't had any problems since. However I'm not sure if this could be earth related? My headlights always appear fairly bright, although I have upgraded the loom and running the positive feed off the alternator.

Anyone got any suggestions? Thanks alot

Click to expand...

I know some have said this sounds OK but I don't think it does, well to clarify, it will work, but I don't think it's right it sounds a bit low at higher loading and I would not be happy, is your meter good and calibrated?

It could be that your alternator is simply not coping well, having said that, it's enough that you would probably work fine unless you always drove about with full load and the battery would begin to suffer from never being fully charged.

It could be that the battery is a bit shot and it's sitting low dragging the whole system down and it is the whole system, you can measure at the posts, the rear light circuit or at the alternator it should all be the same voltage (small variations for high impedance corroded connections and earths etc.).

Side lights and headlights is about 12A at a nominal 12V - or 11.07A at 13V.

Then add the heater (we will skip the main beams as it's only a variance of 10W) which is not actually that high a load, it will run about 4.5A (54W @ 12V)

Radio is negligible - 10-20W so 1.6A.

Total 12V load is: 18.1A - lets call it 20A, now when do you take these readings, after a cold start, or after it's been running for a while? You need to allow charge current for the battery to charge too, now I will need to guess a little here but lets assume after that shortly after start and the initial battery replenishment it needs about 5A tapering off to less than 1A to bring it back up to around 14.1V at idle (unless the battery is not holding much charge at all and gets a hammering during startup - which the slow climb from 13.5V to 14.1V could suggest). So now you are sitting at 14.1V at idle after a while (how long?) then loading it up with the side and headlights is as we know about 11-12A and the system voltage drops back to 13.8V - assuming at this point at idle the alternator had the system up to 14.1V then putting worst case 12A on it should not be an issue at all, it should still sit at 14.1V to be honest - maybe 0.1V of a drop.

Really it could go on and work just fine for plenty more time, but it does sound like there is something not quite right.

Your starting issue just sounds like a lazy starter.

If it is at around 12.5 to 12.6 after a few days but you still get a slow crank. Get the battery shunt tested it maybe dropping a cell under load. If that checks out ok look at the starter and connections if one is getting warm on cranking it is a bad contact.

ONE LIFE LIVE IT. D90 said:

The battery has been tested at a motor factors, which they said it was in good health.

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chainsaw said:

get the starter checked

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