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MG MGB Technical - Over enthusiastic dynamo?

Hi everyone,

I am having what I believe to be weak batteries on my '67 BGT. Before commiting myself to spending £60 on batteries I carried out a few simple tests on the dynamo. It does charge, but thought that it's charging rate may have possibly been sufficient to maintain the batteries in good state of charge.

Anyway, I found that the field current ranged from 2 to 7 amps on a normal engine speed range. This is much greater than the 2 amps guideline in the 'MG Technical Tips' book. Is there a possible problem here? If so, could this be causing the batteries to be losing charge capacity? These were replaced in 1995.

Any comments or suggestions will be welcome.

Many thanks
Brian McIlvenna
Brian McIlvenna

Check out the regulator box.
Daniel

Check the voltage, under charge you should see around 15v. Greater than normal field current could be caused by either bad batteries or a bad control box. A bad control box would give higher voltage as well, whereas with bad batteries you would expect to see either normal or low voltage depending on how bad they were. But really bad batteries would probably cause starting problems. There again, a bad control box could have damaged the batteries.

PaulH.
Paul Hunt

Thank guys for the reply. I should have indicated that I did these tests with the dynamo leads disconnected as prescribed by the test procedure.

This therefore wouldn't say anything about the condidtion of either the batteries or the regulator.

Regarding the batteries, I suspect that they are poor. After a full trickle charge, any starting attempts following starting the car usually are unsuccessful. Using the lights while driving will also run the batteries down. This indicates that the charge capacity of the batteries has diminished. In addition, the performance of the electrical items is very much dependent on the engine speed.

This detioration has happened over only a few months. The car is only used at weekends where distances of a minimum of 40 - 60km are covered.

Any further comments welcome.

Many thanks
Brian McIlvenna
Brian McIlvenna

In that case the correct test is to disconnect all wiring from the dynamo, link the F and D terminals and connect a voltmeter to the D (or F). Start the engine and keeping careful watch of the voltmeter slowly increase the engine revs. The voltage should rise steadily with the engine revs. Do not exceed 20v, don't race the engine in an attempt to reach 20v, you should see 20v by the time the engine reaches 1000rpm.

The dynamo should certainly output enough power to run the lights whilst driving i.e. the engine and lights alone should not flatten the batteries, if they are losing charge while driving that is more an indictment of the dynamo/control box than the batteries. A problem with batteries usually shows itself as an inability to hold a charge when left parked for a few days, but first make sure you have no drains while parked e.g. boot light not switching off. Whilst the dynamo does require a greater engine speed than an alternator to get up to maximum charge the difference is more apparent at or near idle than anything else.

PaulH.
Paul Hunt

Hi,

For a car that gets used so little you may like to convert it to negative earth and fit an alternator. I did this to my '67 BGT and was very pleased, although I will be upgrading to the beefier Bosch alternator (from a 1980 fiesta) as soon as I can find one.

Reversing polarity may or may not be a big job depending on what electronics you have in the car... if you have nothing (not even a radio) then just swap the battery leads and the wires to the coil and you're away... You can also re-polarise a dynamo to -ve earth but it sounds like yours is kaput so maybe an alternator is the best bet.

Cheers,
--
Oliver Stephenson
Oliver Stephenson

I tend to agree with Oliver. Switching to an alternator, if it does not screw up an expensive radio/sound system or a show car, is a great idea. I converted my 68 GT (with the original 16AC alternator) to the later 18ACR alternator. This did away with the external, solid state, voltage regulator. With a similar conversion, you could do away with the mechanical regulator (control box) and make a simpler system. Paul Hunt was kind enough to send directions which basically amounted to disconnecting the wiring to the control box and running two brown wires to the starter and the line to the ignition warning light. It has worked very well for two years. I suspect that, if you ask him politely, he will provide similar assistance. Les
Les Bengtson

With a dynamo discard the brown/green between dynamo and control box and run a new heavy gauge brown from the alt to the solenoid terminal. Tape up the brown and the brown/yellow (seperately, not together) at the control box and that should be it.

PaulH.
Paul Hunt


Is there an issue with the postive earthed tachometer? I will do a thorough check on the batteries as I have trickled charged them a few days ago. The starting performance will indicate their charge retention. The batteries are six years old so it is reasonable to suspect them to be the culprit for my problems.

Thanks again
Brian McIlvenna
Brian McIlvenna

Normal rules for polarity conversion apply, should just be a case of reversing the wires from the green (12v) and black (ground) inside the tach.

PaulH.
Paul Hunt

Oh, and reversing the direction of the white through the inductive loop.
Paul Hunt

This thread was discussed between 16/11/2001 and 22/11/2001

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