Alternator fail - no battery warning light?

[Gavin207]

I've been using the TF a lot recently without any problems until today.  Last week on the way home the battery charging light came on.  Pulled over to check the alternator belt was OK, left the engine running, and when I got back into the car the light had gone out.

Used the car over the weekend for a short journey,  no issues.

Went to use the car today, all appeared OK, but got a mile down the road and the car started shutting down, warning lights coming on and the power steering stopped working, but not the battery charging light. Coaxed the car home.  So I'm assuming the alternator has packed up  but if so why did the battery warning light not come on again, after the initial time last week?

[Roverdrive]

Does it start OK?

[markvrs]

Could be a few things. I am assuming charging light comes when you turn ignition on and goes off when you start car. (If not, bulb blown easy fix) On some cars you need a working warning light to excite the alternator field to start charging, so no warning light when ignition is on may mean no charge when car starts, and no warning light if bulb is blown.

The diode pack in alternator may have gone bad, current not flowing to battery but enough volts to turn light out.

Poor connection on alternator big +ve cable so little charge to battery but enough to turn light out.

Poor earth , battery and/ or alternator.

Internal fault in alternator.

Duff Battery can cause weird things.

Go for cheapest first, check all your cable connections are clean and tight. Then time to get a multimeter out.

Cheers, Mark

[Gavin207]

Battery is fairly new ( and a replacement from ECP for a duff battery). So, battery charged sufficiently to get all lamps illuminated and start the car. However, with ignition ON the battery warning lamp isn't illuminated, so next stage us to check the lamp. Also, with engine running only 11.26v across the battery, so I assume alternator not charging.

[ArntyR]

In that case of new battery/11.26V I'd be inclined to check the battery earth strap is connecting well at the front first, then take out the alternator (check + clean connections in the m/plug too) and replace it with a used part from one of our excellent parts recyclers.

When that finally wears out - and it may be a long time - replace with new. Substitution will give a quick result and may even be an immediate fix for a minimal outlay.

I've found our recyclers are concerned to offer the very best tested parts and are very mindful of their reputation.

ps I discovered that the m/plug had melted out of shape somehow on my previous '05 TF so I reckon the alternator's got to come out, at least...

[Gavin207]

So, checked battery warning lamp, appears ok, continuity through it. Next check the voltage again with the engine running and only getting 11.55v with engine running and revving it. Alternator off, anyone know any checks i can make?

[Geoff.F]

Disconnect the battery and charge it then recheck the voltage.
Geoff.F

[mgtfnut]

Disconnect the battery and charge it then recheck the voltage.
Geoff.F

Agree, 1st step.

Then, have you checked connections as ArntyR said above at the earthing strap at the battery end and alternator? Don't forget the engine and battery are at at opposite ends of the car and "dodgy" connections can only add to an existing voltage drop. Don't forget earth connections from the engine block to the body as well. If all ok, it is worth checking from time to time anyway as the charging system is vulnerable to cable connection "ageing" with such a long separation of battery to alternator

If the battery checks out ok, and all the extra connection checks are made, then you can assume it is the alternator. Drive belt ok?

[Gavin207]

Battery charged, alternator taken off and inspected, no obvious damage etc. Alternator back on, connector sprayed with switch cleaner, all back together, additional earth run from battery to Alternator body using a set of jump leads, car started, 11.5v at battery, 11.5v between alternator feed terminal and body of alternator, so can only assume alternator duff.

Two questions:

Why isn't the battery warning light illuminating- is this fed from the alternator diode or something?

Where to get a replacement alternator, Rimmer do an original mg one but it looks cheap compared with a bosch from eurocarparts?

An alternative suggestion I've had is to change the alternator regulator as it is likely this is the damaged part, and I suspect if the damage is any more than this then I may not get the deposit back on an excganger unit? https://www.ebay.co.uk/itm/252421838608

[Chris Tideswell]

If the battery warning light doesn’t come on when the ignition is turned on just before starting the engine then the issue is with the alternator regulator/diode pack.

The one on my TF failed a few days before moving house so I needed to get the car mobile and exchanged the alternator at a local motor factors.

However if it happened now I would check that the bearings on the alternator are OK and if they are I would buy a new pack as listed. You may find it cheaper to go directly to MG Mania. http://mgmanialtd.com/index.php?id_prod ... er=product

[Geoff.F]

A good charged battery should give a voltage of 12.5 + Volts. At 11.5 then you have a failed cell
Geoff.F

Note. That is not to say that the Alternator has not been damaged due to the battery. Your local vehicle Electrics shop (not Halfords) should have a rig to run the alternator to test it and perhaps fix it.
My local shop is in Earlsdon, Coventry.

[Gavin207]

So, tested the ignition warning light by turning the ignition on, no warning light, earthed the brown and yellow wire on the alternator plug and it came on, so all OK at that end. Also checked the 40A fuse on the alternator feed which is also OK. So pretty sure the problem is the alternator regulator so got one on the way.

Battery is OK, the voltage was only low because I hadn't fully charged it ( I reckon I did 25 miles at least after the alternator failed before it became apparent it wasn't charging).

[nigelandjo]

I have a brand new alternator that I bought when I thought mine had failed, but turned out it wasn't faulty it was the ignition ring. Happy to sell as it's just sat in the box on the shelf if you do need a new alternator.

[Gavin207]

Nigel thanks for the offer, regulator arrived from MG Mania today, fitted to Alternator, all back together and working fine, ignition light coming on with ignition and going out when running, battery charging.

All's right with the world

[ArntyR]

Now that's a good result - and made more so by your feedback

[nigelandjo]

Nigel thanks for the offer, regulator arrived from MG Mania today, fitted to Alternator, all back together and working fine, ignition light coming on with ignition and going out when running, battery charging.

All's right with the world

Glad it's sorted

[Gavin207]

To summarise what I've learnt, if your alternator isn't charging and your ignition light isn't coming on, these are the steps:

1.  check the 40A fuse on the bulkhead 
2. Check fuse 25
3. Disconnect plug from alternator and with ignition 'on' ground the brown and yellow wire in the plug, if the light doesn't come on check the lamp in the dash, if the battery charging light does comes on when you do this the fault is with the alternator,

The most likely fault on the alternator is a failed regulator and these can be bought for around £35 and are easy to fit.