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MG MGB Technical - Alternator & dim headlights

Octogonians,
The headlights on my 1977 MGB are brighter when the engine revs, dimming noticeably when idling, and I suspect it's the alternator. The first step is determining the cause...I would guess it's the brushes. Has anyone experienced this?
Regards,
Randy Olson
1977 MGB
randy olson

My '77 B has always done this perceptibly and it's the one with the high-output alternator as well (the one where, when you replace it, you always get given the wrong one, also costs more). I think that it's just the way that they are. The alternators don't kick out enough chuff below 1000 rpm (engine) to run Halogens at max.
Brushes are obvious. The red light on the dash comes on, intermittently at first and then solid. How do I know this?..............:-X
Tim Cuthill

my 76 did this when my belt was ready to go out and when I didn't have enough tension on the belt.
Josh

When i had my '77 it did it as well. Came stock w/ halogens. I recently uprated my '69 to halogens all around and changed my alt to a Bosch as per Bob T's web page. Great results (after getting a second alt and rebuilding it). Most discount auto stores have crappy rebuilt units, so be careful if you go that route. I also needed to uprate the headlights w/ relays as the switch tended to burn out from the highour output.

Good Luck
Luis

Good comments so far. So, do owner's of '77's live with the dimming headlights, or is there a fix? It's not like I need bright headlights when the engine is idling, but...
I don't want to go to a Bosch alternator, and have to add a relay to keep the switch from melting.
Is there another solution to this obviously shared problem?
Cheers,
Randy Olson
1977 MGB
randy olson

It's quite normal with just about any alternator -- e.g., I have a 75A Motorola unit attached to a Rover motor. Granted, modern cars don't really do this. But, it's kind of nice -- you don't need screaming bright lights when stopped, and presumably the rest of the world is a little happier.
Ted

Yep, it's normal for the Lucas. It's not normal for the Bosch. My car did the same thing, till one cold and rainy, night when I was stuck in bumper to bumper traffic, heater on, cd player on,wipers on, the lights just got dimmer and dimmer and then went out all together. At that point, the battery was too low to even turn over the starter. After I got it jumped off in the dark and rain, I got home and checked out the alt and it was normal. It just would not put out enough under those conditions to keep the battery charged. The Bosch I replaced it with starts charging at 500RPM, the Lucas did not start until 1200-1500RPM. Do yourself a favor and replace the Lucas with a Bosch. It's a direct, bolt on, plug in swap. You will not regret it!
gerry masterman

Some dimming/brightening is normal although too much can indicate a problem. Check the voltages. You should see no more than 14.8v running at a fast idle and it should still be charging i.e. above 12.8v when idling. It should continue to charge down to about 600rpm at which time the warning light should suddenly come on and the voltage will drop to battery voltage i.e. the 12.8v mentioned above. If you idle for a long time in this state with a heavy load then you will discharge the battery, but this is below the recommened idle speed for most alternator-equipped cars. Slowly increasing the revs by 1000rpm the light should have suddenly gone out and be charging at the higher voltage again.
Paul Hunt

A quick and dirty way to test your alternator is to park in front of a wall, with your lights on, at night. Rev the engine up to around 1500 rpm or so and then kill the engine. If the lights don't dim appreciably when the engine comes to a stop, then the alternator was not working properly. The lights are noticibly brighter with the 14.6 volts from the alternator than from the 12.6 volts of the battery. If the lights don't dim, that means you were working off the battery voltage instead of the alternator.

I do a lot of travel at night in the winter, which means I have the headlights, the heater fan, and the radio on. With the stock Lucas alternator, my lights dimmed appreciably at stop lights, even to the point that the warning light would glow dimly. I replaced it with a 65 amp GM alternator, and now I have no problems. No dimming of the lights at all, with every thing in the car switched on. In fact, I even recharged a friend's battery whose alternator had failed on the road by jumpering from my battery to his and sitting there with the engine idling - that much spare capacity with the GM, even at idle.
Dan Masters

I tried the quick and not-so-dirty test by bringing the RPM's up to around 1500 with the headlights on and then shutting off the engine. The lights dimmed a little, but not a lot. In other words, it was perceptible but not dramatic. So I guess that means that the alternator is working (?). I may try more sophisticated tests later.
If I were to go with the Bosch or GM alternator what are the part numbers? Or, what do I need to tell the auto parts guy in order to get the correct one, assuming it's available locally?
It sounds like the consensus is that some dimming of the lights at idle is expected and quite common with the Lucas alternator, and something most drivers accept. I'm not opposed to this. Since I don't drive the car in the winter, or in the rain, I normally don't have a lot of load from fans, or wipers; I use the radio occasionally, the 8-track tape player rarely.
Just got back from an early morning, 2-cup, pre-dawn, top-down, no traffic, warm Fall drive, with no moon and zillions of stars...absolutely beautiful in a great running MG.
Cheers,
Randy Olson
1977 MGB
randy olson

http://hometown.aol.com/idbritcars/boschAlt.html
gerry masterman

Randy,

I have nothing against the Bosch alternator, but I believe you'll find the GM to be cheaper, and easier to find if you're ever stuck in the boonies and have to replace it. Virtually every parts store in the country will have them in stock, even in some very small towns. I've been told the Bosch is an easier installation, but I haven't tried it so I can't say for sure.

The model no for the GM is 7127. For instructions on installing it, see:

http://pages.prodigy.net/larryhoy/MGBDelco.htm

Whichever you choose, you'll be glad you did.
Dan Masters

Dan makes a real good point in favor of the Delco conversion. The downside is that you must make up mounting brackets for it, no big deal if you have the tools and abilities to do it. The Bosch is more expensive, but a direct bolt on swap. Mine even used the same electrical plug and belt as the Lucas. I did have to scrounge up a pulley and fan because some Bosch alts come without these, but the Delco pulley and fan are perfect fit and common as dirt. If keeping you car as original as possible is important to you, consider that the original Lucas plug must me removed from your harness and a Delco plug patched in. With the Bosch, you or the next owner can easily swap back in a matter of minutes for a concours show, if you want to. Either way, you will be pleased with the upgrade. BTW, I have done both conversions and recently removed the Delco and replaced it with the Bosch to keep both of my cars the same.
gerry masterman

If you haven't got a wall, just look at the interior light.
Paul Hunt

This thread was discussed between 03/10/2002 and 06/10/2002

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