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MG MGF Technical - Alternator Light (Battery Charging)

I was travelling down the motorway tonight in semi darkness (side lights and instrument lights on) 60mph in heavyish traffic and my ABS light came on - I've had this problem for years, cannot seem to trace the fault - Usually it will stay on until I next switch off the igntion and re-start - Then it can be off for months - Certainly happens more often in wet weather.

Anyway, I pulled into the fast lane and accelerated to pass the traffic - While the engine was under load I noticed a red warning light and thought I'd left the handbrake on (I pulled the handbrake slightly and then lowered it and to my horror, realised that the handbrake light was on the other side of the dash). At this point I must have eased up on the gas (in fright) and saw the red light fade out (from the corner of my eye). By the time my eyes fixed on where the light had been, I couldn't be sure if it was OIL or Alternator light! I assume the way it faded would indicate alternator (belt slipping under load maybe??). Then I noticed that the ABS light had extinguished also - Really odd, as it normally won't do this unless you switch off the ignition. I tried accelerating again a few times after that but could not get the red warning light to come on again.

Anyway, got home and checked:

Oil: Clean,just on the max - Perfect
Water: On the seam - Perfect

Switch the ignition on: Oil Light, Handbrake, ABS, Engine bay overheat, SRS lights come on (but NO ALTERNATOR light) - All lights then extinguish except for the handbrake and oil lights. Turn the engine over, the alternator light comes on while the engine cranks and immediately extinguishes when the engine fires.

Is this normal? I thought the alternator light was supposed to light as a bulb test when the ignition was switched on? To be honest, I've never paid enough attention before today.

ABS - This has made me wonder about the ABS problem - I have checked each wheel sensor with a meter and they are all fine, cleaned each plug in connector to the sensors and pulled and cleaned the ABS ECU connector but nothing has cured this intermittent probelm - I know ABS is sensitive to elecrical changes and fails safe if there is any problems - Does anyone know if a dodgy alternator could be causing my intermittent problem?




Mark Clayton

OK, so things just got really weird - Driving to work this morning, ABS light comes on again, then alternator (battery charging light), followed by power steering and SRS lights - then the dash ilumination lights went dim - Turned every electical item off in the car and waited for car to conk out - Then illumination came back to normal and all warning lights extinguished - Car carried on as normal? Alternator surely?
Mark Clayton

Had the same on a boxer, lots of dash faults for a few days, one after the other then it died completely on the M25 one very wet, cold dark night and that turned out to be the alternator.
Alex Sturgeon

Hi Mark I would put a volt meter on to the battery to see around 12.5 volts start up turn on all electrics see if the volts stay up over the 12 volts.

With nothing switches on, it should not be more that 14.5 Volts.

If you think of taking off the alternator buy a new carbon brush for the slip rings before you start as this is the only service item worth changing.

But this brush normaly lasts 150k or more.

Hope this is of some help Regards Roger
RJ Rowlinson

The alternator light does not function as you expect!

Basically the alternator is an electric motor, except you drive the spindle and voltage comes out rather than the other way around.

If you remember back to school days, an electric motor uses big magnets and a coil of wire on a stick. Big magnets are heavy and expensive, so for an alternator the big magnet is replaced by an electromagnet (another big coil of wire).

When the alternator is running it provides power to the electromagnet, but until it powers up it needs to start the electromagnet - This is the purpose of the bulb.


I.e.

The bulb feeds the electromagnet, so the light _must_ come on with the ignition or the alternator is dead.

When the alternator starts generating current the light _must_ go off or the alternator is dead.


Of course, it might just be that the wires have worked loose, which would give you the same effects and explain why in some circs it comes back to life.

Will Munns

Thanks Will, that clinches it for me.

Just been out and checked a friends car, their light does indeed come on with the ignition - Mine still doesn't light.

Also, a few other 'coincidences' perhaps make sense (or still could be coincidences?) - I have had a belt noise (faint whine) for a while now and initially put it down to a cam belt change, more noticible when engine hot. However, when I changed the alternator belt in the summer (I have aircon, so I also have a tensioner and aircon pump which could have been suspects), the noise went completely for about a week and then gradually returned. If I opened the drivers door and listened to the engine through the air intake as I switched it off, there was a faint grating, gritty sound (bearing?) as the engine stopped turning - I thought then that the noise might be linked to the alternator.

In the last week or so, the ABS light problem has been more obvious, the alternator belt has started to slip occasionally, even though it is correctly tensioned (and newish - diffculty turning the alternator perhaps?) and the belt noise has become more noticable - Plus my active sub inside the car has lost volume (which I couldn't understand). Other strange things have been that I had the feeling that my dash illumination didn't seem as bright as normal, even with the dimmer switch up to maximum - and here's a weird one - My temperature gauge is reading 2 notches below centre instead of 1 (sometimes 1.5 notches below but always a bit below the normal position) - I'd never seen this happen before even in extreme cold weather.

After googling the symptoms, I found lots of reports of this strange behaviour with various dash lights performing their startup routine (explains why the ABS light extinguished) on all sorts of cars (many people noticing that the radio volume died down??) and in all cases the alternator was the culprit.

Then I read your response and that clinched it for me - Thanks Will and to everyone who contributed.
Mark Clayton

yup, low power shuts down a lot of things. The ABS may be assuming the engine is not running due to seeing a lower voltage than it is expecting, and hence shutting down.
Will Munns

Thanks all for an interesting and very informative post. My first thoughts were the engine earth strap; but while you are in there, may as well check it too....!
Charless

Charles, that's really sound advice - The number of problems which can be traced back the earth strap are seemingly endless. I had what I thought was starter motor trouble but turned out to be a poor earth - Everything else looked fine and the car would run but the starter motor seemed dead - Tested the live connection, fine - EARTH STRAP!! (engine to body)Allowed enough juice to flow to keep the engine running but couldn't handle the load the starter motor needed.

Anyway, back to my alternator saga... I got a new unit yesterday and tackled the job - I mentioned that I had aircon, so I have a belt tensioner pulley on the alternator belt. The first job was just to slacken off the pulley (done it a couple of times before when changing and tensioning the belt - This time SNAP! oh dear... Sheared the spindle. Why does every job have a hitch??

Sourced a new part today and replaced the broken piece, then moved on to the alternator (it's situated really high up on the engine, right by the exhaust manifold on my car, not sure if it's a little further down on the non-aircon model? Anyway, the top bolt was easy enough but the bottom one was a nightmare! just to get to and then to crack.. but I prevailed and eventually won the battle........ or so I thought!

Got the nut off the on the manifold side and then started to withdraw the bolt from the other side - Uh oh - it won't pull fully out, it's fouling on the raised section of the subframe (the alloy bit which carries the top engine mount). It's not by much, almost clears but I can't get it past. Feels as though I need to tip the engine slightly forward to get clearance (I'm going to ask advice on this in a separate thread) - Lost the light now and I've got work in the morning, so hopefully I'll have some answers by the time the weekend comes.

I will write a closing to this thread, once everything is completed and I can confirm that all my woes where down to the alternator....
Mark Clayton

You quit with the light?

You havn't really lived until youve changed the headgasket using just a domestic garage overnight in mid feburary, where the car has to stick out of the open doorway because you can't fit youself and the car in the garage with the door open :)

The F is a car where you really need to be able to feel the location of everything - not an easy task when you can't even feel your own fingers!
Will Munns

LOL - I had one of those once (a domestic garage) but the wife insisted we convert it into a dinig room, so now I'm relegated to the drive... Seem to remember reading about someone (Rob Bell?) setting the valve timing under a canvas gazeebo in mid winter... Now that's dedication! (mind you your head gasket change takes some beating too!!)
Mark Clayton

The real life saver is the 500W spotlamp, light and heat in one small package.

Just remember to move it out of the way before you drive off the ramps :(
Will Munns

....and then there was light!! A beautiful bright red one!!

Just finished - Didn't need to tip the engine in the end, even though the bolt fouled the subframe it had actually cleared the alternator - I thought that the bolt was still holding it but it turned out that the alternator was just held by the jaws of the bottom bracket (not surprising after 10 years I suppose). Rubbed down all the mating surfaces and then gave a spray of electrical contact cleaner for good measure - All went back together in about a 10th of the time taken to get the old one off!

I now have my alternator light back when I switch on the ignition - I put a meter on the battery as Roger suggested: 12v, started the engine: 14v, switched a few thirsty electrical items on: 13.5v - Nice one!!

Went for a spin - Dash illumination is back as it should be, the temp gauge now reads 1 notch below centre instead of 2 (that was a weird one?) - Flicked my CD player on and my sub kicked back into life (and to think I'd had that thing in and out of the car and checked all the wiring to no avail!!). No sign of the ABS light, although it's a bit soon to be certain on that one. Here's one last thing - I've always had a sort of murmor in the tickover, especially when warm (the rev needle tends to dip every now and then, like a skipped heartbeat) and that would appear to have gone - Again, a little early to be certain.

Well, another saga laid to rest - Hope this is of some use to someone in the future, once it works it's way into the archives. Just shows you though how a failing alternator can give such obscure symptoms. The irony of it is that I identified that the alternator wasn't supplying much over 12v months ago but failed to change it after I discovered that I had a poor earth connection - Just shows, you should always go back and re-test even when you think you have solved the problem!!
Mark Clayton

Hi Mark,
Glad to see you have fixed your problem. I suspect a diode failure or corrosion around the diodes causing poor output. The alternator must have been churning out some current as if it had failed totally you would have only driven a few miles before coming to a complete stop.
Had this happen to me on a hire car in the US of A some years back. Found a slip-road and just got off the freeway before the car came to a complete standstill. The tacho went beserk during the last few seconds of power.

Missed this thread as I was away on hols.

Just catching up on F news.

willyphixitt
W A Nixson

Thanks Willy, I'm sure you're right - I think it's been giving a sub standard output for a while but this recent cold snap, rain and darker evenings has meant I have been running with lights, wipers, blower, HRW and sound system - I must have been slowly discharging the battery and because the alternator light wasn't getting a signal there was no warning that the battery was being drained - Until it was barely charged. When I think about it, each time the lighting show began I slowed down and turned all electics off which must have allowed the alternator to trickle enough current to keep me going - Well, you live and learn - It was certainly new ground for me.

Thanks once again to you all...
Mark Clayton

This thread was discussed between 28/10/2008 and 31/10/2008

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