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MG MGB Technical - Lights brighten with engine revs

1977 MGB, finishing restoration and now getting the lighting to be reliable.

This is the situation:
- Halogen headlamps brighten and dim with engine revs
- turn signals flash in sync with the speed of the engine revs
- brake lights dim when headlamps are on
- with the headlamps and/or the brake lights on directionals don't flash much if at all
- side markers occasionally don't work when headlamps are on
- ignition light only comes on during starting

What I've done:
- replaced light switch
- replaced gauge voltage stabilizer
- replaced directional switch

I've been through the archives and will check the grounds.

I'm looking for other suggestions and any recommendations of locations of the likely ground suspects.

Thanks for your time and assistance.
Brian
Brian Denis

Sounds normal to me. You might try cleaning all bullets and replacing all the connectors. These things only have a 34 amp alternator and put out much less at idle.
Bob Ekstrand

Brian - Even with a 34 amp alternator, you should not get that much variation in voltage at idle and with a 77 MGB the alternator should be an 18ACR which has a much higher output. Also, with what you have listed in your posting, unles the halagon headlights are of the special megawatt variety and unless you are running a stereo system rated in megawatts, you don't have that much of an electricla load on the alternator. One of the advantages of a alternator is that they put out a relative even voltage across a wide range of rpm. I would suggest taking your alternator in and have it tested, it sounds a lot like a bad internal regulator. The other thing to check is the connector at the alternator and the individual wires in the connector. The wires can develop corrosion under the insulation that will cause a high resistance and cause the voltage to drop considerable. Good luck - Dave
David DuBois

It does not sound like an alternator problem at all - the lights get brighter with engine revs.

Check the battery voltage with the engine idling - if it is under 12.5 volts with the headlights on then have the battery tested on a discharge meter by your local battery supplier - you may have a high internal resistance.

Also undo the battery earth/ground strap form the car and thoroughly clean the car and the end of the strap, coat with Vaseline/petroleum jelly and bolt up with a new bolt and washer.
Chris at Octarine Services

Brian
Sounds like the internal control pack in the alternator could have failed. This causes the output volts to rise with engine revs, with only the battery acting as a "regulator". This can also often mean there's ac volts getting to the battery, which will kill it very quickly and get it very hot. Check the battery volts as you increase the engine revs - if the regulator is ok, the volts will stabilise between 13.8 and about 14.3 volts. If it continues to rise, that's your problem.
Anything over 15v will quickly blacken/blow lamps and boil (literally!) the battery........
HTH, John
John Marr

Thanks Bob, David, Chris & John for the input so far. Variety of opinions but all good stuff to occupy my weekend!

I'll start with the battery voltage test because I have a new regulator & rectifier anyway... and because I hate chasing grounds! Although the ones suggested by Chris & David are specific and sound like good preventative maintenance anyway. I'll do them at a minimum and hope it takes care of my problem.

Anyone have a suggestion on how to clean the inside some of the connectors? I wire brush all externals, but I haven't come up with a good way to clean some of the plug-in female sides.

Thanks again!
Brian
Brian Denis

Don't forget step #1, check the condition of and the tension on the alternator belt.
R Hill

Brian,

Almost impossible to clean the connectors - plus the metal fatigues and breaks off - just buy some new ones.
Chris at Octarine Services

I use a 'fiberglass pen' which was originally sold to remove rust to clean those types of connectors as well as other types.

I forgot who makes it but I've had mine for quite a long time and I have used it for many purposes.

http://www.electronix.com/catalog/product_info.php/cPath/3/products_id/1081?PHPSESSID=aea0e62de2cc776bcbbef4f7aefafede
Mike MaGee

Brian,
Around here, places like Auto Zone and Advance Auto Parts check your battery and alternator for free.
Dave
David Steverson

I agree with Chris, replace the female sleeves, they are not expensive and will eliminate the chance of a loose connection due to metal fatigue. You can get them from British Wiring at: http://www.britishwiring.com/ . The fiberglass brush that Mike suggests is good for cleaning blade type connectors like the one found on the alternator. Just wear gloves when using them or your fingers will itch for the rest of the day from getting bits of fiber glass on them. Cheers - Dave
David DuBois

Classic old battery symptoms.
mike!

This thread was discussed between 16/07/2004 and 19/07/2004

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