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MG MGB Technical - Does your ALT light stay on initially?

Hello All,
When I first start up my '72 B in the mornings, the alternator light stays on until I get rolling (10-20 mph), or until I give rev it up.

I have a new wiring harness, etc. My old alternator did this and when I replaced it last spring I thought the new one would be better, but it's the same.

Does your's do this?

Regards, Dave
David Steverson

Dave; On my 74 it comes on when I start the car, my idle speed is around 900-1000 rpm. It's Lucas alternator.

Clifton Gordon

Light for ignition is on as long as current flow from battery towards consumers (starter motor, coil, ...), and it goes off when current start to flow to battery, what mean that is alternator produced more current then is used, so battery is charged. It is normal that is at low RPMs light on, but above 1000 rpm should switch off.
I have 1966 B that has originally dynamo. Since we have stupid law from year 2000 that all cars have to drive day and night with lights on (and I have some additional consumers, as additional cooling fan, radio) happened to me very often that I was not able to starts car in the morning. So first I thought that battery with bigger capacity would solve problem, but wrong! At the end I installed 55 A alternator and now works fine.
If I return to your problem, try to charge your battery over night and then observe ignition light. It could be also that is your battery bad

My '74 B does the same thing.

My 74 also does the same. I am concern though about the idle. I usuall let the engine run until the temp gauge begins to move. At that time my rpm's are about 1200. Hit the gas peddle, light goes off and down the driveway I go
GG Ginty

Probably normal. The revs do have to rise to about 900 or 1000 rpm for the initial exitation to kick-start it into full charge, thereafter it should stay off (i.e. stay on charge) down to about 600 rpm, then have to be raised to about 1000 rpm to switch it off again etc.

If you start 'feet off' or the merest throttle when warm (i.e. no choke) then the light won't go out when started. When starting on choke, or when warm if you give it enough throttle to blip the engine it should go off straight away. If not starting off straight away you should always blip the throttle if required to extinguish the light, or it is dischraging the battery. I like to get mine on charge straight away.

If the battery was bad it probably wouldn't start the engine. Problems with the battery will cause cranking problems long before they start showing funnies with the warning light.

BTW, if is far better practice to drive off straight away and keep the choke to a minimum by driving gently, than leaving it to warm up especially if it needs some choke. 1200 is about right for fast idle i.e. first 1/4" of choke knob movement. And like I say, if you let it warm up with the warning light on, it is discharging the battery.
Paul Hunt 2

Interesting Paul. Just get in it and go is better than warming up? I used to notice folks doing that when I lived in Ireland, but I put it down to the cost of petrol.
David Steverson

I usually let my engine idle for about 30s before driving off, apparently any more "warmup" than this is merely a waste of petrol.
I find that if I do not give the car a chance to warm up alittle, the oil pressure drops to nearly nil when I rev up the first few times. I put it down to 20w50 oil on a cool winter morning, the oil flow in the pan is slow, and the oil pump sucks a "hole" in the oil, until the oil is warm enough to flow more freely.

Erick Vesterback

David - yes that's about it.

Erick - from your location if the ambient is down to 15F you should be using 10W/30 to 10W/50, below that 5W/20 to 5W/30.
Paul Hunt 2

Paul - in most Canadian locations, we see very cold temperatures,and switching to thinner oils makes alot of sense. In Victoria, being moderated by the Pacific Ocean, we don't see temperatures below 0C (32F) very often. Usually we are at around 5 to 10 C during the winter at the coldest. I don't drive my car when it snows, and rarely when temperatures are below freezing even if the roads are dry. I haven't felt much need for changing to thinner oils as a result,especially since idling for 30 seconds seems to prevent the oil pressure drop when first driving away.
Erick Vesterback

Hi Erick - in that case I'd suspect a marginal problem with the pump, or possibly a small air leak in the pickup pipe, or possibly simply low oil level due to an incorrect dip-stick?. The strainer/pickup should be very near the bottom of the sump and hence several inches of oil above it. Difficult to imagine a 'hole' being sucked in that, I've driven both my roadster and V8 down to freezing - the latter well below and it was kept outside, and never experienced what you describe.
Paul Hunt 2

Paul - I haven't given this too much thought, but maybe there is more to this than I had really thought. This symptom has existed for me in my 72 B that I had 10+ yrs ago, and in my current 73 B both before and after a complete rebuild done last winter. Interesting idea about the dipstick, the rubber piece that is supposed to plug the dipstick tube really doesn't plug it, in fact its quite loose. I suspect that rubber has worn away from it over the years, and the 72 B that I had was the same. It may be that on both cars, the dipstick sits deeper into the tube than is intended, and maybe I am not filling the sump fully with oil. I've never looked at how deep the oil pump pickup is in the oil sump, but it would only make sense that the engineers would make it as low in the sump as possible to make sure oil is always available to it. Then again, a little slack in the dipstick seal wouldn't account for 2+ inches of oil level, and I find that oil changes consume 3.5 L of oil (the remaining 0.5L I figured was being held in the oil cooler), so I wouldn't think I am starving the engine of oil that much. Puzzling.
Thanks for your replies Paul, I'll have to check into this more.

Erick Vesterback

I note this on probably 50-70% of the Lucas alternator powered cars that come through our doors. A slight blip of the throttle above 1,200 RPM (just off idle) usually shuts it off or waiting 5 to 60 seconds. This just tells you that the system is not charging 'yet'. Some other older vehicles do this as well along with Dynamo powered cars which tend to do this at idle if everything isn't adjusted perfectly.

BMC Brian McCullough

This thread was discussed between 12/12/2006 and 17/12/2006

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