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MG MGB Technical - Electrical connections?

Have had a quick search on the forum but can't find an answer to my question so far.

My '77 B has always functioned perfectly adequately where the electrics (internal/external lights, wipers, fan), but last week as I was driving along the dash lights suddenly became brighter, the fan increased in speed, and the wipers began to function much more rapidly.
This lasted for a few minutes before returning to 'normal', and has happened on a few occasions since without obvious precipitant (not dependent on car speed/rev count/ancillaries in operation). The battery (modern 12v) has never run flat.
I tested the voltage at the fuses and with the engine off it's just over 12v, going up to just over 14v with the engine running - so the alternator appears to be working well.
With the radio/fan on the voltage drops very little, but it does drop right down to 1v with the lights on (this may reflect where I had placed by test probes).

Am I seeing an abnormal surge in voltage/current (none of the fuses have blown), or is my 'normal' running in fact sub-optimal and the result of a bad connection/dodgy alternator?
J Eastwell

It sounds like a voltage regulator that is not doing its job full time. Apparently, it is allowing the voltage to rise, on occasions, and eventually this could lead to blown bulbs due to excessive voltage. RAY
rjm RAY

As Ray says, it does sound as thought the voltage regulator in the alternator is malfunctioning. Ideally, you need a voltmeter attached when the problem occurs so you can see what is actually happening.

I'd be interested to know where you did put your probes to get a reading of 1v.
Dave O'Neill 2

12v changing to 1v when you turned the lights on sounds like a bad connection in the 12v feed to the lights in question. However if that were the case the lights in question would not be lit when they should be. Also the bottom fuse should show 12v all the time, the next one up 12v only when the ignition is on, and the top two 12v only when the lights (parking or headlights) are on. The lights aren't powered through the bottom two fuses so there is no reason for those to drop. However it's possible to see 1v on the top two when the lights were off, going *up* to 12v when they were turned on. This can be caused by a bad earth connection in a place depending on what else was powered at the time. If the cooling fan was running then it could be the earthing point by the fusebox for fan and front lights, or the 4-way bullet connectors in the black wires by the headlights.

The apparent rise in voltage is the important thing to investigate. It could be caused by the alternator voltage regulator going haywire has been mentioned, but also by the battery becoming disconnected. The former is likely to boil the battery dry if doing it for too long, less likely to blow bulbs. The latter can blow bulbs as well as electronics.
PaulH Solihull

Thanks for the suggestions so far - I will definitely investigate the alternator regulator as it seems the most likely culprit - difficult to get a reading when the change in voltage/current occurs as it has been intermittent and only occasional so far.

I was using the bottom of the four fuses to measure the voltage, will have to double-check from another point - I must admit electrics is not my strongest point! I do need to round all the bullet connectors with some emery paper in the near future.
Lights working without fault so far.

J Eastwell

In that case you do need to check again. The purple side of the bottom fuse is after the fuse, so a problem in the fuse or its holder would cause the voltage to drop that side when something was drawing current. But ordinarily the only things on the purple circuit are horns, headlamp flasher, interior lights, clock and time delay unit, and out of those only the horns or the headlamp flasher are likely to cause it to drop that much.
PaulH Solihull

Managed to catch it happening with a voltmeter just now (in my lunch break!), and the voltage was climbing up to over 17v - new voltage regulator on its way!

Oh, and no v-drop when checked using a different earthing spot - think I may have originally used something other than ground.
J Eastwell

Just one thought - although it shouldn't be the case on a 77. Some years had battery sensing alternators and two brown wires from the alternator down to the solenoid, one thick output wire and one thinner sensing wire. If the sense wire is making a bad connection at one end or the other you could get high output voltage. But as I say that shouldn't be the case on a 77. However the complication is that you *may* have two brown wires, but both thick ones. These are both output wires to double the current carrying capacity/halve the volt-drop. Loss of one of these wouldn't cause high voltage.
PaulH Solihull

New voltage regulator fitted, problem seems to be solved so far (though I haven't done much driving due to the snow!) - time will tell!
Change of job rotation means I will be cycling a lot more so the mileage should drop a fair bit over the winter.... more incentive to take her out for night-time country lane fun :-D.

PaulH - Two thick wires and both have connections that seem sound when checked, so as you say this is unlikely to cause a problem.

Thanks to all for the advice.

J Eastwell

Of course you could look at this another way, i.e., your "normal" is in fact poor and the "bright" periods are in fact normal. Either way cleaning, re-making or replacing the bullets and spades would seem to be worth doing before going down more expensive routes.
Allan Reeling

Not if the 'bright' periods were when the alternator was putting out 17v :o)
PaulH Solihull

That'll teachme to read aaaaaaaaaaaaaaallllll the entries!!!!!!!
Allan Reeling

This thread was discussed between 20/11/2010 and 02/12/2010

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