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MG Midget and Sprite Technical - What might be wrong?

Hello all today is a very special day. My 67 midget now has its first MOT for over 17 years. The car felt great to drive, just like the new car that it almost is but there is one small problem that nearly prevented the test altogether.

The ignition light comes on with the ignition and goes out once the engine's running just as you'd expect but once I'm driving the light starts to glow and actually gets a little brighter as the engine revs increase.

The car has a dynamo, not an alternator, we've fitted a new regulator and battery as part of the rebuild and we've reversed the polarity so that it is now negative earth.

What might be wrong?

If my problem is dynamo related what else would I need to change if I fitted an alternator? Would it be a good idea to replace the dynamo with an alternator in any case?

Thanks for all and any answers.
BillMex

Loose belt? worn brushes in Dynamo?
MK Mike K

Have you flashed the dynamo?

This might help as well

http://www.mgcars.org.uk/mgcc/midgetreg/chargingsystem.shtml
Robert (Bob) Midget Turbo

Thanks for the ultra quick responses Mike and Bob. The belt is correctly tensioned and I think we've replaced the dynamo brushes (I've certainly replaced them in one of my two Midgets but can't be certain which one it was!). However, I should have mentioned that when the light is glowing as the engine is running the battery is discharging. This is why it nearly didn't make the MOT test as with the lights on first thing this morning and the ignition light brightening with the engine speed the battery died VERY quickly.

The article you linked to Bob looks as if it might be helpful and I'm about to print the Lucas manual for a little further bedtime reading.

Meanwhile any more comments will be most welcome just in case I can't find the answer.

Thanks again.
BillMex

Yes I appreciate that the battery is discharging. If you do not know what flashing the dynamo is then you will not have already done it .
Again from our website check here for flashing the dynamo when you change polarity

http://www.mgcars.org.uk/mgcc/midgetreg/positivetoneg.shtml
Robert (Bob) Midget Turbo

Sorry Bob, I should have responded to the first part of your response. Yes, we did "flash" the dynamo and should have said as much before. Apologies again and thanks for your responses.
BillMex

Sorry mate I was determined for you to do a bit of "flashing" :-)

Must admit 99 times out of 100 if it is not flashing the dynamo that is at fault then it comes down to the voltage reg :-(
Robert (Bob) Midget Turbo

Hi Bob, I could never flash for fear of being ridiculed!

I do like the articles that you've provided links to. Are you the "Robert" to whom all emails are directed?

Do you suspect that I didn't "flash" correctly? I plan to try again just in case it makes any difference.

Is there an index of articles available through www.midgetregister.com? Although this address returns a broken link message for me, I have found my way to http://www.mgcars.org.uk/mgcc/midgetreg/index.shtml which directs me to email "Robert" with any input but doesn't seem to explain how I can join the register or link to a list of relevant articles.

Apologies if I'm having a few senior moments this evening.
BillMex

Hi Bill
Yes I am that person, much better I think writing articles about Spridgets than constructing our MGCC Midget Register Website. :-) Another member of the MGCC is at present reconstructing our site ready for a new year launch so I am happy about that and perhaps then Midgetregigister,com will work again..

We like to think that we can help all Spridget owners wehther members or not so we make our site available to all. On the other hand if you are a member of the MGCC then you can be automatically a member of the Midget Register.

But enough of that lets hope we can get your car charging correctly. Let us know how you get on Bill.
Robert (Bob) Midget Turbo

Bob-Bill
If the new regulator is one of the little electronic ones it is possible that it could have been damaged during the flashing process if the engine has been started up with the dynamo wiring connected--They don't seem to like reverse polarity much.
Suggest you flash it with your old regulator first if you do have one of the electronic ones fitted -or start it up with the dynamo disconnected altogether and then flash it running
Just a thought----- Willy
WilliamRevit

Will I haven't tried the electronic regulator before but will take great care considering your advice. THX
Robert (Bob) Midget Turbo

Sounds like the dynamo to me. I had one of the new ones from Moss fail this week after only about 15,000 miles. The brushes were toast. I replaced them only to find that the front bearing had too much play. Those units are made in India and the generator that this one replaced is not really rebuildable due to a worn end piece, a broken field coil connection insulator, etc.

So I will convert it to an alternator, as much as I hate to.

The only regulator failure I have had in 250,000 involved a sticking contact that overcharged the battery. One of the contacts pitted and stuck in the hole in the other contact. Repair was as easy as filing the offending contact smooth.

Pretty sure that yours is either an open field coil, brushes and perhaps, resulting damage to the armature in the dynamo. My only advice is to stay away from the new dynamo made in India that is offered by Moss. Total junk.
Glenn Mallory

Many thanks for the thoughts and comments. The dynamo passed all the recommended tests this morning so we turned our attention to the “new” Regulator/Control Box. Cleaned up all the contacts but on inspection the winding on the second bobbin (the one with just a few turns) was not sitting straight and could have been shorting to the Regulator body. Tidied up and re-fitted and we have new (better I think) symptoms.

The ignition light now goes off with the engine running (as before) and only glows occasionally at 2,500 rpm and/or above, always going out again as the revs fall. Before it would come on and get brighter as the revs increased and then stay on until I turned off. But the battery is now not holding any charge at all. Could I have killed it?

On tick over the ignition light is off but we’re reading marginally less than 12v across the battery.

The battery is now on charge and the plan is to give the car a proper road test to see what happens. The battery was new replaced just a few months ago and has done no road miles to speak of.
BillMex

Bill this is still not right and should be easy to solve with a few simple tests.
The first is I wonder do you have a voltmeter? preferably an analogue meter would be best?

Can you place your voltmeter across the battery and leave in situ. Now start the engine and note the voltage on the meter as you steadily increase the revs on the engine please.
Bob England

Thanks for getting back Bob. We have been using a digital meter though I had noted that the articles suggest using an analogue meter with a needle. I'm guessing that analogue meters must be more sensitive as the digital meter only ever indicates a positive reading of 12 to 15 volts across the battery as we increase the engine revs. It then dropped to around 11.8 volts on tick over.

Curiously, the digital meter continues to read over 12 volts at the battery when we take the revs up to over 2,500 and the ignition light starts glowing (intermittently) . Does this give any further useful information?

Since this morning's post I have exchanged the battery as I found that it was still in warranty. The dealer tested it and found the volts OK (it had been on charge for a couple of hours) but it was only reaching around 200 amps which is insufficient for cold cranking. New battery is now in place but the symptoms have not changed.

Any more thoughts?
BillMex

Hi Bill, keep smiling

OK Bill it is like this

The ignition lamp is like a balance, if the voltage at either side of the lamp is equal then it does NOT light

If however one side the voltage is higher than the other side then it will light up with a brilliance in proportion to the difference in voltage at either side.

One side is connected to the battery, the other side is connected to the voltage regulator output.

So when the ignition is switched on we have full battery voltage at one side and nothing from the voltage regulator therefore we have maximum difference in voltage and the ignition light illuminates at full brightness.

So in your case the lamp is at full brightness when the ignition is switched on. Exactly correct!
The lamp goes out when the engine starts, Battery voltage and generated voltage equal spot on correct.

But as your revs increase the lamp illuminates suggesting that generated voltage is reducing. This should be easy to check by placing your meter across the battery. If this voltage is more than 12.5 (probably 13.5) then it would suggest that the generating system is working and the lamp would not be illuminated, yours is so at the moment I have to assume the generator circuit is not working correctly. (normally with these symptoms it is the brushes in the dynamo not making good contact but is that the case here?

So back to my question what voltage did you measure across the battery when the engine was revving at say 3000 rpm? (the reason why it is better to use an analogue meter is that the reading will be more steady. The regulator is really a switch and is switching on and off many times a minute which causes the digital meter to fluctuate wildly. That said it should be clear to see even with a digital meter whether the voltage is remaining above 13 volts)

Can you confirm this please Bill?

Because if it is remaining above 13 volts then the battery should not go flat and the lamp should not light up unless somehow you have a wiring fault.

By the way another easy check on whether the charging circuit is working is to note the intensity of the headlamps. They should become slightly brighter as the engine revs increase from idle.

Sorry for the boring long post, hopefully my next one will be short and sweet.
Robert (Bob) Midget Turbo

Hello Bob, I’m still smiling and will continue to do so for as long as very helpful folk like you keep coming up with useful suggestions. Your post is certainly not boring, please keep the ideas coming.

I should explain that my 67 is kept at a very good friend’s house as he has much better facilities than I have and is far more competent when it comes to the technical bits. My posts are based on my best understanding of what has happened and I relay the answers (and copy the documents) to Dave when we next meet.

Dave has just read your post Bob and assures me that we did get a constant reading of more than 13 volts with the engine running at or around 3,000 rpm albeit that we were using a digital meter.

I need to explain that we got as far as test 6 in the Lucas manual that is linked from your article on testing dynamos and control boxes. Tests 1 to 5 all gave good results but we got a complete failure at step 6 (erratic voltage measured at any engine speed) which is what sent us into the Control Box looking for problems. What we’re not sure about (and on this occasion Dave isn’t sure which is unusual for him!) is whether or not we put the “dry” card correctly between the “cut-out contacts”. We had to guess what the “cut-out contacts” are as we couldn’t find an illustration that mentions the “cut-out contacts”. Can you point us towards an illustration or perhaps describe where we should have placed the card?

In all events, we found problems with the Regulator as described earlier, hence the discovery that the battery was faulty.

Dave has had another thought that we’d welcome your comments on.

We have fitted a battery isolator as I’ll be sprinting the car (as well as trundling off into the wilderness for week-end picnics in it!) and Dave says that the isolator includes a balance resistor (I think that’s what he called it). Could this be contributing to the problem?

Thanks again Bob.
BillMex

""We have fitted a battery isolator""

That's thrown me mate :-) lol

I have not fitted an isolator switch to a car with a dynamo before so I will have to consider that for a little while?

Yes if you are sprinting then it is required and the scruitineer will have you rev the car whilst he turns off the switch.

When a car is fitted with an alternator, after the battery is disconnected by the isolator switch the alternator is still producing power and will keep the engine running. On a proper FIA isolator switch you put the ignition circuit (white wire power to the coil) through a pair of contacts on the bottom of the switch, these must be closed when the switch is in the run position, and open when the isolator switch is operated off. Now when the battery is disconnected the alternator can also go into surge and blow some of the regulator circuits. To protect the alternator another set of contacts are used at the bottom of the isolator switch that close when the isolator switch is turned to off (opposite of the other one) this is where the load resistor is used (you called it a balance resistor) it is connected to the alternator output at one side and the body earth at the other.

I do not believe this is necessary when a dynamo is used (hopefully someone will confirm or correct me) so for now please remove this resistor circuit.

In fact I think you should simply wire the battery cable through the isolator switch and run the white wire (ignition switch to coil) through a contact at the bottom to cut the ignition circuit.

Back to your comments the cut out is he bobbin on the right as you look at the regulator. I have assumed you have a 2 bobbin regulator.


Robert (Bob) Midget Turbo

As posted earlier on the MGOC forum - where this thread is also ongoing - the resistor is not necessary when using a dynamo.

Has this problem only occurred since fitting the isolator, I wonder?
Dave O'Neill 2

This thread was discussed between 23/12/2009 and 30/12/2009

MG Midget and Sprite Technical index

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