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MG MGB GT V8 Factory Originals Technical - Alternator

Well, I finally got my V8 on the road for the first time.... straight from it's first service, 5 miles down the road and the battery gave up....

AA engineer diagnosed a faulty alternator. It seems to be working somewhat irregularly.

Is this a common fault?

Car details - 1973 factory original, ~50k miles (allegedly....)

Guy Harper

The alternators are unique to the model, being AC_Delco (yours might have been changed ???).
Therefore they tend to be expensive. When mine stopped working a couple of years back I took it to a local auto electrician (BRacknell, Berks) who repaired it by fittinga new diode pack (or possibly even a component) all for 15. Remember if it's original it is over 25 years old, and getting back on the road is quite a shock for the poor thing !! !
David Smith

I'm not sure if its a common problem but it certainly happened to me. Battery was dead so I called the RAC who duly jump started the car and it worked fine again for a while. Then the alternator was not charging the battery sufficiently and it cut out. I had to be towed home. I ended up replacing the alternator with a new one and have had no problems since. I am told that the cooling fans take up quite a bit of power from the battery, not to mention the headlights, wipers, stereo etc. so if the alternator is old then chances are its not up to the job.

James Read

Looks that way, eh..... remind me not to choose a hot day to do initial testing.....

The car has suffered some considerable disuse recently, so I guess I'm in for numerous glitches. Next in line is those noisy tappets....... and not forgetting the perennially sqeaky brakes.


Vaughan, South Tyneside

I recently bought a MGB GT V8 and I have recently fitted a new alternator to the car.
On examining the service history the car has had two alternators fitted within the last eight years - one at 92,000 miles, one at 118,000 miles and this recent one at 120,000 miles.

Is this an electrical problem with the car or is it the norm?
V.A.V. Wilson

Shouldn't be the norm, the alt on my V8 came with the car and I have done over 55k on it up to now (and I hope that isn't tempting fate). Could have been caused by the PO welding without unplugging the alt, or jumping with the leads the wrong way round, or possibly flattening the battery from time to time (how many times has that been changed?). If you are careful how you do those, and attend to any high-resistance connections in the heavy current cable you should be OK.

Paul Hunt

What I should also have said is Hunt's 4th Law clauses 3 and 4: "'new' parts will sometimes fail soon after fitting; 'new' parts almost certainly won't last as long as the originals".
Paul Hunt


I just got a v8 (w/a rover 4.2) and I'm not at all up to speed on it ... but can't we use essentially practically any AC Delco alternator? What's the constraint? Or are you just talking about factory V8s and trying to identify the OE part?
Matthew White

Probably no constraint for you, but apparently Delcos are as hard to find "over there" as Lucas is in North America. Does Vauxall, however you spell it, use Lucas?
George B.

Let me add Kile's Corollary to Hunt's 4th Law, Clauses 3 and 4:

"New parts will have a statistically significant chance of being defective right out of the box"

My Factory V-8 has the Delco alternator which was working fine in service. However, I had the chance to buy a NOS replacement a year or so ago, which I did. I figured the NOS unit would look better on my restored car, since my in-service unit was somewhat pitted and corroded, I could keep the original one for a spare.

Well, I hooked up the NOS unit to my engine and ran it on the test bed - no output. I've simulated all the proper wiring connections, even the correct wiring colors, but no luck. Soon it will be off to the electrical shop to get the unit checked and serviced. The new one sure looks good, though. Maybe I should just keep it to install for shows!

Paul Kile
Paul Kile

Paul - that is my second clause, the first being "Many break-downs occur soon after a car has been worked on".

Paul Hunt

Paul, some old/new altenators have to be polirize, yes! There is a tool for this task. In the rear of the alt you will notice a small hole were the tool is inserted and polirize, check with your books or contact a GM dealer. Mac, and Snap On tools sell this tool. Some new stock alts have this ommitted.
Also the AC Delco Alt. needs a 12 V to energize the Alt. If you notice there is a Hot wire to the alt, that comes from the starter to the alt. then the two wires comming from the top, one goes to the Hot wire from the Bat. and the other goes to the IGN. SW
The samll wire needs to be in good condition and a fully charge battery when install, if not you will have problems with the alt of not charging properly. specialy when a new alt is install or sitting for long time. One easy check to see if the putting out current, is to see if the rear of alt. (bearing cup) is magnetize check this with a screw driver or wrench (spanner) do not ground the bat wire. If it is then is OK. if not replace the regulator, that is the only thing that goes bad with them. Ofcourse there are exeptions.

You could also modified the alt if replacing the regualtor. Replace it with a one wire type.
This will omitt the two wires from the field (two samall wires) leaving only the Bat. wire.

The cost is about $30. from Summitt.

One commom problem is the hot wire not getting 12v to the alt from the ign sw or the Bat. and a bad ground, usually altenators go bad in a short time if this precoutions are no taken.

r/ Bill
Bill Guzman

Never heard of 'polarising' an alternator - a dynamo, yes. Never heard of a switch (is that what the tool does) on an alt to allow it to run on either +ve or -ve ground but I suppose there could be. MGB alternators are 'primed' or 'energised' by receiving 12v on the brown/yellow via the ignition warning light from the ignition. If this is missing then used, at least, alternators will still charge but may have to be revved up to about 3k before they start, once started they will charge as normal. Don't know about 1-wire alts, what powers the ignition warning light then? Without an ignition warning light you lose some useful diagnostics. Not for nothing is the ignition warning light also called the 'idiot' light - "only an idiot ignores a change in the functioning of that light".

Paul Hunt

Paul, believe it or not. The tool does exist I do have one. The voltage regulator circut takes care of the energizing etc... very simple, and yes!! if faulty wire the alt needs to rev up so.... at idle or low speed cruising it won't charge = LOW BAT.

MGB-V8 Alt is a AC Delco (GM not MG)
What I meant is that one wire goes to the IGNITION SWITCH a 12v source. Not a separate SW.

I am not an Idiot so I don't need a light to tell me if it works, I like the smart gauge the one that shows with numbers iether Volt meter or Amp meter, these are very effective and tells you how much is charging from the moment you start your engine, specially when lights are on etc.., The light does not. But if you like the "idiot light" then there are ways to install one with a one wire alt. a very simple task, really.


Bill G.
Bill Guzman

Since AC Delco alts. are impossible to find in Oz, I'm using Bosch 80 amp alts. on my cars. They're small enough to fit in front of the rocker cover & have a stud conn. for the brown lead & a spade conn. for the brn/yellow lead. One's been in use for the last nine years, & the other for two. I don't know if they're available in the US or UK, if they are ,I thoroughly recomend them if you're not worried about originality. To me, efficiency is more important. Barrie E



1. I have a spare AC Delco alt for the V8 if you need it.
2. The same alternator is fitted to Vauxhall Cavaliers.
3. There are much better later ones from Lucas available. The item fitted as standard to the Maestro 1600 with aircon is a perfect example. I got one from Halfords. I forget the number but give me a call if you are interested in info or the original.
Thomas Hardy

If anyone in Australia or UK need parts or complete AC altenators, let me know. I am not doing this for bus purposes, but rather help those of you who needs them.

You pay for Alt and shipping, and I get the satisfaction of helping you.

63 Amp Alt goes for $30 to $45 plus shipping.
The must common part to go bad is the regulator and brushes, they can be shipped via Air Mail The cost is of regulator about $15 to $20 plus postage.

Bill Guzman

This thread was discussed between 12/06/2000 and 03/07/2000

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