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MG MGB Technical - Ignition light on slow indicators

Hi, I wonder if you can help me please chaps - I have been out this afternoon in my 1974 RB MGBGT. It has not been driven for two weeks so I thought it would be good to give it a run. After about 20 miles the indicators became very slow to flash and the igngition light kept coming on at 2000rpm. Above that it (th ignition light)did not come on but the indicators were still slow. Everything thing else works okay - I've looked at one or two threads about electrical matters but nothing seems to fit. Any ideas? Regards Edd.
E McGee

Will you do a few more checks?

You say everything else is fine - do the hazard lights blink at a reasonable rate - or are they too, slow?

Is the blink rate of the indicators the same with engine off as they are with it running?

Do you have a volt meter of any type? If so, will you:
a. Check the battery voltage across the terminals with engine off and no lights on?
b. Check the battery voltage with the engine running at 2000rpm with the ignition light on?
c. Check the battery voltage with engine running at 3000rpm?
Hal Adams

Try the simplest first, loose fan belt?

Herb
Herb Adler

Hi, I will do the other checks and reply back on this thread. I did check the fanbelt and it is nice and tight. Thanks for helping me out on this. I'll do it tomorrow and reply back.
E McGee

Sounds like the rectifier pack, inside the alternator, is on its way out. One, or more, of the diode trio components may have failed. An inexpensive part to purchase and fairly easy to replace. RAY
rjm RAY

I agree - the alternator is giving up. With the engine running you should have over 13 volts across the battery, even when idling. I bet you haven't.
Mike Howlett

You could also check the carbon brushes in the alternator, if worn they could be the problem. There are 2 brushed one for the ring contact and the other for the centre contact, the "ring contact" one wears faster. A simple swap of the brushes and a clean up worked for me.
R. E Bester

HI, I did not have chance today to do anything as it was very heavy dew and thick fog all day. I have to work outside so did not fancy doing any work where damp could get in. I'll try again another day and see what I get. I think you are probably right the alternator is on its way out - it is 34 yrs old after all. Edd
E McGee

It is easy to forget that come October in the UK, it is unpleasant to work on the car!

99% certain it is the alternator. Just nice to have the voltage readings to be sure. Classic symptom though of the ign light acting erratically.

You could buy a recon kit of bearings, brushes and diodes, but if it were me, I'd pop along to Halford's with the car, get them to ascertain there is no charge on the battery, and they can bung on an exchange alternator in no time at all. And you will get a warranty..!

I doubt your flasher unit is faulty, although it may be a bit tired. Try it again when the battery is charged and compare it with the flash rate of the hazards - they should be near enough the same.
Hal Adams

Hi I have finally managed to find a day when it has not been raining too much...

I went and checked the voltages as requested and found the following:

(checked after not running for three weeks)

across Bat = 12.32v
2000rpm = 14.21v
3000rpm = 14.21v

On the above the ignition warning light went out on tickover.

..................................................

Engine allowed to warm up:

across Bat = 11.94v
1000rpm = 11.94v
1500rpm = 11.94v
2000rpm = 11.94v
2500rpm = 12.48v
3000rpm = 13.35v

on the above the ignition warning light went out at 2000rpm

.............................................

Engine switched off for short while - then re started.

Across Bat = 12.08v
1000rpm = 11.92v
1500rpm = 11.93v
2000rpm = 11.93v
2500rpm = 11.93v
3000rpm = 12.03v

On the above the igntion warning light did not go out until after 2500rpm.

So, what do you think please - is the altenator toast?

Regards

Edd
E McGee

I would have thought you should have more than 12v at tickover. I think mine is also going the same way as it was giving the same symptoms as I drove it to York the other week for some bodywork repairs. I'll check it out when I collect it later this week. I don't know how to fathom if its the rectifier or the the bushes, I think I'll just replace the whole thing as they are only around £50.
Paul.
Paul1970

Midland Sports and Classics are selling new, uprated alternators for £34 + VAT outright.

http://www.mgpartsuk.co.uk/prods/3558.html

Dave O'Neill2

Had all the Ed symptoms the other day, so bought the Midland Sports Alt for £43.50 inc VAT and Postage off E-Bay. Just the job!

I thought about overhauling the existing one, but at this sort of price for a new one, it wasn't worth it.

Peter
P A Allen

It sure sounds like the rectifier pack is on its way out. RAY
rjm RAY

Edd - yes, your alternator is breaking down. lets not go into the whys and wherefores, as the others said, just put a new one in as my earlier suggestion.

If you install it yourself, then measure again and you should get the first set of readings you had - 12V with engine off and 14oddV with engine running, but when hot as well !
Hal Adams

Measure the voltages at the alternator output terminal before splashing out on a new alt, it could be wiring or connections. Check the alt spades and connector sockets are clean and a good fit.
PaulH Solihull

battery voltage with key oof are 12+ with ebgine running about 13.3 idle or more

RE HALLE

Well got my car back from the bodyshop today. As before indicators slow / stationary in the case of left one.
Found my tester. It says the battery is ok - measuring 12.5 volts. Measured the output at battery with engine running, at tickover no change (12.5v) if I rev the engine I get 13.5 volts and revving very hard can get the 14v light just flickering. Measuring at alternator terminals I get 14v so surely some voltage getting 'lost' somewhere. Checked all connections and all seem OK. The other thing I'm confused about is the alternator has 3 terminals 1 'normal sized' terminal and 2 bigger ones. Currently the big one is connected to a large connector with 2 brown wires the smaller is connected to 2 brown and yellow wires. Currently not connected is a further brown/yellow wire and also a small brown wire not connected what has happened as there is nowhere for these to go!! At all engine speeds the ignition light glows
Paul.
Paul1970

IGNORE LAST POST. just retested and got same result from alternator terminals and battery. Yes its not charging unless I rev the engine quite fast. I'll order a new one tonight. I have found on another site that the 1970 was fitted with 5 wires originally this will have been rewired on replacement so there will be some redundant wires. Earlier had battery sensing wires.
Paul.
Paul1970

It does sound like an alt problem, although I'd check the brushes (new ones are pretty cheap) and clean the slip-rings before buying a new alt (but then I'm so tight I squeak when I walk). Mine had the ignition warning light flickering, but doing that seems to have fixed it. When replacing or refitting brushes you have to poke a bit of stiff wire through a hole in the cover and both brushes to compress the springs so the cover can be fitted over the slip-rings, then withdraw the wire. New brushes come with a bit of wire, but it's too short and too thick to go through the cover, so you have to find your own bit anyway.

The two big terminals are both output terminals, and it is an machine sensing alternator. Where there is one big and two standard sized, that is a battery sensing alternator. The factory switched between both types a couple of times.

Five-pin, five-wire alternators were used from about 68 to 71 and had two plugs, one with two wires and the other with three. That had three brown/yellows, two of those were simply a loop. It also had two browns, one thick which was the output and the other thinner which was the battery sensing wire, both go back to the solenoid terminal that carries the thick battery cable. If you have a machine sensing alt with the large output spades you can connect the thinner brown to the other output spade to get a bit more current carrying capacity and less volt-drop. Make sure the spare brown/yellow can't short to anything.
PaulH Solihull

Thanks for that, that now really makes sense. Good idea about the small brown wire I think I'll order a replacement plug rather than have separate spades as currently to make a neater job. The large brown wire needs trimming back a bit anyway as the insulation is dammaged. Just for clarity are you saying that one of the wires on the brown/yellow spade with 2 wires simply runs to the brown/yellow on the single wire spade. If so I'll disconnect the one which is not required when I replace the terminals then the single spade will be dead.
Regards
Paul.
Paul1970

"Just for clarity are you saying that one of the wires on the brown/yellow spade with 2 wires simply runs to the brown/yellow on the single wire spade"

That should be the case, the trick is in chopping off the right one :o)
PaulH Solihull

Thanks for your help, I'll try to sort it next week (working nights this weekend). I have been bought the haynes manual for my birthday today. It is really easy to follow as its now in colour. It's a bit easier than following wiring diagrams in black and white!
Regards
Paul.
Paul1970

Happy birthday. Someone understands what a great present we boys like ;0) Bummer you're working nights and not in the pub...
MGmike
M McAndrew

This thread was discussed between 20/10/2012 and 10/11/2012

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