Alternator/Battery Light

[fastrack1966]

After three months idle my car's battery light is very faintly on nearly all the time. The garage that finally got me mobile said the alternator could be overcharging, which is no short term worry until the battery fries up. The lamp is barely noticeable unless daylight is bad. Can anyone advise on this please? I don't want to pay out for an alternator overhaul if the remedy is simpler. I have a multimeter,

BrianJ

[H536NOG]

Hi, dimly glowing warning light usually signifies undercharging rather than overcharging & may be nothing more serious than worn out brushes.
Try removing the brushbox/ regulator pack (presuming your alternator is a Lucas A127/A133) & see how much length is left on the brushes, also its worth taking a look at the slip rings in the alternator (the copper parts that the brushes bear on) as they may be badly worn.
If its worn out brushes they can be replaced far cheaper than the complete alternator.
Whilst inside the alternator, compare meter readings that you obtain with those specified in the manual (Haynes has a section on it IIRC)
Nige

[H48HPE]

I'm presuming the alternator drive belt is tight? If it is: -

There’s no way the alternators over charging, the light is coming on dim because there’s a slight potential difference between the battery voltage and the output voltage from the alternator, this means that the battery volts are higher than the alternator output voltage, as stated above this could be because the brushes are worn out or the regulator is broken, I would just replace the alternator/brush assembly with a new one from rover. Dirty connections aren’t an issue here I don’t think as the offending connection is internal (should it be at fault) and therefore I wouldn’t assume this to be the problem.

An explanation may be necessary here so I'll give you one as I see it.

operation

The alternator you have will probably be an A127/65 or so it should be. This type of alternator is self-excited and has a machine-sensing regulator.

An alternator has a stator winding and a rotor winding. The stator winding is the stationary one and consists of three separate windings (which produce three alternating currents i.e. one in each winding, this is called three phase) insulated from each other. The rotor winding is one single winding and is the one which spins round in the centre of the alternator, this is surrounded by two pole pieces these look like two halves of a cylinder with a zig zag cutting the two pieces apart, once fitted on the coil the two halves don’t contact each other but the zig zags almost interlock (this causes the magnetic field which is made when the coil is energised to be concentrated across the gap between the two halves. because the rotor is spinning the current cant go into it through fixed wiring, it has to go through brushes , these are carbon blocks that run on copper rings (slip rings) the electricity goes through the brushes and can pass through to the slip rings which are connected to the winding.

This is how it works then:

When the ignition is switched on a power is sent through the ignition warning lamp (which should be wired in parallel with a resistor in case it fails) this is connected to the rotor winding. This initial energisation is required so the rotor becomes an electromagnet, once the engine starts the rotor spins round and the magnetism causes current to be induced in the stator windings. Once this is happening the alternator is producing current, as well as supplying the car battery and the wiring inside the car, the alternator produces current to power the rotor winding (this connection is internal to the alternator), this means that there will be no voltage difference between the circuit through the ignition warning lamp (ultimately from the battery) and the voltage from the alternator. So even though the circuit from the warning lamp is still connected to the rotor and is live, the alternators already putting power into the winding at the same value, the only way the bulb can come back on is if there is less power going into the rotor directly from the alternator than is available in from the battery. For example if the battery was at 12V and the alternator was at 8V then 4V would flow through the warning lamp circuit causing it to glow slightly.

The above process is fine but the problem is that if the alternator throws loads of current into the rotor winding the magnetic field would be bigger and therefore more current than was good would be produced in the stator winding (i.e. overcharging) this problem is dealt with by the regulator, on the Lucas A127 this is a solid state device which regulates the amount of current going into the rotor, it stops the magnetic field getting too big, and keeps it at a constant voltage of about 14.2V (this is dictated by a zenner diode which switches two transistors on and off independently to control another transistor that delivers the power current to the rotor. if this set-up failed too much current would get into the rotor winding or not enough. (The regulator is housed in the plastic unit at the end of the alternator and also incorporates the brush housing)

The other device that can fail is the diode pack, this is there to ensure that the current which leaves the stator winding as alternating current is rectified to direct current. If one of the diodes fails it will cause poor operation and can cause the alternator to discharge the battery through the windings.

I’m bored so people who didn’t want to know about that will have to forgive me

testing

Test the stator windings, you need to test the resistance between the three wires an each other, each pair when tested should give a reading of about 0.1 ohms.

Test the resistance between the stator windings and the iron frame they are held in, you will need to do this in megohms (it should be greater than 1 megohm), a normal multimeter cant do this so if you can get an electrician to lend you his megger. They are rather expensive pieces of kit!

To test the rotor check the resistance between each slip ring, this should be 3 to 4 ohms, if it’s a high resistance the winding has a break.

Check the resistance between the rotor slip ring and the iron frame. It should be high; a normal ohmmeter on a high range will be ok for this

To check the regulator, start the engine and run it at 3000 rpm for 3 to 5 minutes and measure the battery voltage over this period, it should rise up to 13.6 to 14.4 volts, if the voltage measured is high, low or continually changing the regulator needs changing.

I think your problem lies with the regulator

This is the longest post I’ve ever done, most of its probably irrelevant to most so sorry for that, I have tried to keep my English up to a level certain members are striving for and I can only apologise if this is not up to their standard

Andy

[SimonR]

Originally posted by fastrack1966
My car's battery light is very faintly on nearly all the time.

F153's battery charge light has glowed faintly ever since I replaced the alternator with a recon Lucas unit. I've done about 20K since then with no problems. I'd wager that either there is nothing wrong or your alternator is on its way home.

Cheers,

SR

[E_T_V]

If the alternator light is glowing faintly ALL the time (even with the ignition off) Then check for a stuck relay or the diodes in the alternator might be fried.

If it is lit when running and the belt is tight, replace the regulator/rectifier/brush unit, it'll cost under a tenner and you don't even have to remove the alt to fit it (just 3 screws on the back).

Fixed my problem which was undercharging but no warning light.

An alt can overcharge when the battery light is on by shoving AC voltage through the system rather than DC which knackers your battery at best!

[D87 SMW]

The regulator and brush box are the part that fits into the end of the alternator, I've seen illustrations which show the rectifier to be internal and therefore you would need to strip the alternator to get to it.

;)

:rolleyes:

[H48HPE]

yes you certainly do have to take the alternator apart to access the rectifier, it would seem that Dans problem was a faulty regulator or brush assembly not the rectifier as when he fitted the new regulator the problem was sorted. Nice clear illustration that.

Andy

[E_T_V]

Yup nice clear piccie that. I thought the "black box" also contained the rectifier as it does on some models, (the ones I've blown up before ;) :banghead: ). The smell of burning silicon isn't pleasent I can tell you, and neither is boiling a battery dry and butting 30V through all your electrical systems which is what happened to mine when it all packed up.

[fastrack1966]

Thanks everyone, I guess I'll make a start at the weekend, the easy ideas first,

BrianJ

[E_T_V]

If you've not got a voltmeter - I can thoroughly recommend everyone to have one in their toolbox. They used to be expensive but now you can get one for under a fiver and are worth 10 times that in the time saved diagnosing electrical faults.

Good luck

[fastrack1966]

Well I said I'd start with the easy ideas first:
I wonder how many forum readers here are on their backs in all weathers because they're fed up with professional repairers botching things? The garage that got my "Diesel Head Woes" running did a great job, but I think they're a bit lost on electrics. When a garage says my alternator is not charging properly the last thing I ask them is "Are there any loose connections?", after all they're a garage!
Rang Lucas re regulator/brush box and they wanted £24, I can get a recon alternator for £44. Decided to sit and wait, which was a good idea because coming out of a shop and restarting the electrics were all dead.
Aha I've had this before, I know what that is! The positive battery clamp is slightly too big for it's terminal and comes loose twice a year. Clean terminal, wrap cooking foil around it, clamp is now tight, car starts, I've not seen the red light since........
BrianJ

[fastrack1966]

Spoke too soon! The lights back on again. Rang Lucas and they say their regulator/brush box is about £24. No other dealer round here does alternator bits. ETV mentioned a reg/brush box for "a tenner". It may be worth my trying mail order if anyone can suggest a source for a tenner please?

BJ

[talkingcars]

And how on earth (without the use of an air wrench) does one get the pulley wheel nut undone?

I have tried several methods but to no avail.

James

[tony]

you need a vice and a fan belt,put the fan belt round the pully and then tighten the belt in the vice ,nice and tight turn the alternator 1 full turn to twist the belt even tighter then try to undo the nut :) ,good luck

[talkingcars]

or lay the alternator on its side on the ground, hold the nut with a long 22mm spanner resting on the ground, insert a 7mm (or was it 8) allan key into yhe end of the spindle, wack the allan key with a 16oz club hammer twice. The nut comes loose.

I tried it wacking the spanner but the alternator just twisted, I tried holding it still in the workmate but the 2 halves just twisted, I tried holding the fan still using a screwdriver, the first bent, the second bent the vanes on the fan.

Next problem - the alternator case on the 65amp diesel item is diffrent to the 55 amp O series petrol in that one of the mounting holes, the top one, is threaded, on the petrol it is slightly larger and not threaded - out with the drill I guess.

James