Dynamo Testing

[BOB BLACKSHAW 1Participant @bobblackshaw1]

Hello,  My MGB dynamo is suspect for not charging, as I’ve no crocodile clips on my multi meter can a test be done via the car battery. I’ve seen videos on how to test the dynamo with the engine running but would be more accessible to me via the battery.

Thanks.

 

[Adrian R2Participant @adrianr2]

If it is a dynamo (and not an alternator) then no, as the voltage regulator is between the dynamo and battery and itself could be suspect so testing at the battery won’t really help. The PDF below contains a procedure but do check that is appropriate to your vehicle.

https://www.mg-cars.org.uk/imgytr/pdf/lucas.pdf

If you have an alternator then testing at the battery will be of some use, you should expect to see a voltage increase when the engine is running, likely to around 14v at fast tickover.

[noel shelleyParticipant @noelshelley55608]

Hi Bob,  Yes , just attach the probes to + and – and read DC voltage on the battery, should read about 14V as the engine speed increases. For little more than pence get a pair of small crok clips. It will be POSITIVE earth if a dynamo. There are some tricks to changing a dynamo. Send me a PM with a phone number and we can talk, I know the B inside out. Noel.

[BOB BLACKSHAW 1Participant @bobblackshaw1]

Thanks to Adrian and Noel for your replies, if I have any problems I will PM Noel.

Thanks again Bob

 

 

[MacolmParticipant @macolm]

A quick way to test an ordinary two brushed car dynamo is to run it as a motor. With the belt off, check it is free and rotates easily, and connect it as shown. Beware of supplying half the electrics as well if you can’t be bothered to fully disconnect it . And assuming the battery is healthy.

Connect as shown, and it should run quite slowly, indeed near the cut in speed, and also consume an amp or two at most. If in doubt, supply it via a fuse or more simply by touching with one strand of a piece of ordinary flex.

[Speedy Builder5Participant @speedybuilder5]

Make sure you get the polarity right or you will change the residual magnetism in the dynamo such that it would not charge at all.  I seem to remember this problem (albeit on 3 brush dynamos) of the Austin 7.

Bob

[David SeniorParticipant @davidsenior29320]

Just because a car has a dynamo does not necessarily mean it is positive earth. It probably was originally but there are many people who change the polarity so that they can fit, for instance, a more modern radio.

In the days when we used to sell dynamos we used to include a note in the box explaining how to ‘polarise’ it to suit the required earthing. Though I have to admit I can’t remember the details!

Dave

[not done it yetParticipant @notdoneityet]

Dave, to jog your memory – it just required the field terminal to be flashed from the battery live terminal to set the correct magnetic field.

RAB

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Malcom, that takes me back to the early 1970’s, my elder brother in law was an Auto electrician, and he showed me that method.

Regards Nick.

[Martyn PooleParticipant @martynpoole48399]

I have recently acquired a large 12v dynamo for possible use with a vintage Stationary Engine. Unfortunately, it has no markings on any terminals no makers’ plate or specification and I am not aware of its background or use, so all these notes are very useful.

My question is if it is, by applying the simple motor test, proved working and then tested fully  my first task will be to establish its optimum shaft speed, any suggestions how I can best do this as it is not clear to me what parameter will tell me when it is at optimum?

Also for the motor test, do assume it is positive earth and will it do any harm if I get it wrong?

I ask as World War 2 Jeeps, G-503 (Willys Overland and Ford) were 6vdc but negative earth, so there are negative earth dynamos about, although this is not one as it is too big.

 

Martyn

[old martParticipant @oldmart]

I remember the need to polarise a new dynamo according to the earth polarity. Another way to get higher electrical output would be to change to an alternator, or if originality is paramount, then alternators disguised as dynamos are available, I believe.

[MacolmParticipant @macolm]

We must be talking about an ordinary brush armature machine with a parallel field which is excited via a regulator from the supply. Anything else complicates matters, but 99% of two brush traditional dynamos will be of this simple configuration.

Putting the battery across the field gives the strongest excitation we have available, and as a motor it will run at the speed where the back (self generated) armature voltage comes up to match the battery voltage (at which speed it limits itself). Simplistically, driving it as a generator, this is the cut in speed, below which it does nothing useful.

In practice, this might be set at or below tickover, so as an example for an engine with a useable speed range from perhaps 700rpm up to 5000rpm, we might choose gearing so that 300rpm would drive it this speed. I am sure others can provide better experience, but that is the principle.

The polarity is simply decided by the direction of the residual magnetism in the iron of the field. In practice, this will not change from start-up to start-up. Indeed, a piece of mediocre “part steel” iron is ideal, cheap and in no danger of loosing the residual magnetism. You can swap the polarity as often as you like by simply by applying the desired polarity to the field for a second or two.

[noel shelleyParticipant @noelshelley55608]

Martyn what makes you think it’s a 12v dynamo if you know nothing about it ? It could be a 240v alternator or almost any thing. Commercial vehicles had 24V dynamos, some even had 2 speed units. Give us more details and a picture.  Noel.

[BOB BLACKSHAW 1]

On 22 April 2026 at 07:50 BOB BLACKSHAW 1 Said:

Hello,  My MGB dynamo is suspect for not charging, as I’ve no crocodile clips on my multi meter can a test be done via the car battery. I’ve seen videos on how to test the dynamo with the engine running but would be more accessible to me via the battery.

Thanks.

 

Whats wrong with the old method of connecting an ammeter into the line. somewhere.  Many old cars used to have one on the dash.

[MacolmParticipant @macolm]

If you have an ammeter, it may certainly confirm that the dynamo is charging. However, if there is no reading, you are none the wiser whether the dynamo is faulty, the connection is faulty, the cut out is faulty or the regulator is faulty.

[Martyn PooleParticipant @martynpoole48399]

Apologies for my lack of accuracy and clarity, Noel.

The dynamo was sold to me by a expert in vintage vehicle restoration simply as a “large 12v dynamo”. I believe he knew nothing about it other than that, however, in case my memory has let me down, I will get back to him and ask for confirmation of what he knows. Once I have that I will pass it on and post pictures and give dimensions which I don’t have to hand at present.

[noel shelley]

On noel shelley Said:

Martyn what makes you think it’s a 12v dynamo if you know nothing about it ? It could be a 240v alternator or almost any thing. Commercial vehicles had 24V dynamos, some even had 2 speed units. Give us more details and a picture.  Noel.

If its got a commutator it’s a dynamo

[Oldiron]

On Oldiron Said:

On 22 April 2026 at 07:50 BOB BLACKSHAW 1 Said:

Hello,  My MGB dynamo is suspect for not charging, as I’ve no crocodile clips on my multi meter can a test be done via the car battery. I’ve seen videos on how to test the dynamo with the engine running but would be more accessible to me via the battery.

Thanks.

 

Whats wrong with the old method of connecting an ammeter into the line. somewhere.  Many old cars used to have one on the dash.

Put a voltmeter across the battery, if revving the engine from tickover shows increased voltage it’s charging. Doesn’t tell you how much of course, or if not charging why.

[BOB BLACKSHAW 1Participant @bobblackshaw1]

Thanks for all the replies on this subject.

My MGB has been converted to a single 12v battery that fits in the 6 volt battery box.

I’ve checked my dynamo and that is charging the battery.  I’ve checked the battery and is showing  13 volts when not charging. I’ve also brought a smart charger that shows its fully charged. The charger has a pulse repair option but haven’t tried that as it states fully charged.

This morning I started the car up but it could hardly turn the engine over,if it wasn’t for the electronic ignition no way would it have started. The battery is about three years old always on a battery trickel intelligent charger, the one used for the cartoon cover.

In the past the engine turned over OK, but when the charger shows  fully charged and my volt meter shows 13v is it the battery. Also when it’s started for a few minutes  and then start the engine up again it’s fine.

Many thanks Bob

[Speedy Builder5Participant @speedybuilder5]

Amongst other things , it sounds like a sulphated battery.  When this happens, “leaded sludge”  settles down the bottom of the battery when the battery is at rest – typically over night.  Next morning,  the bottom of the plates short out and reduce the battery output – but it stirs the sludge about a bit when you try to start the engine (Without success).  When left for an hour or so, there is a better chance of the battery providing sufficient current to start the engine.

Sulphating is typical of dynamos overcharging batteries.

Other typical signs is when wife comes back into the house saying the car won’t start – again.  By the time hubby finishes his morning read of the paper and first cup of tea, tries to start the engine and hey Presto! – away she goes. (engine – probably wife too).

Bob  – Happy days with a very old Metro

[Grindstone CowboyParticipant @grindstonecowboy]

Check the voltage drop at the battery terminals when cranking – if it stays reasonably high (above 11 volts), the problem lies elsewhere. It could be that the earth strap from the body to the engine block is damaged or broken, or the positive cable to the starter similarly faulty. Check the connections haven’t corroded. Then you could check the starter itself – maybe worn or sticky brushes.

If the voltage measured at the battery drops excessively – below about 9 volts when turning it over – it’s the battery.

Rob

 

[noel shelleyParticipant @noelshelley55608]

I know these things ! Your battery is too small ! I did the same as you – did away with the 2 x 6v batteries ! I fitted a large 12v in the boot, the type used by the old John Deere 4020 tractor. About 20″ long and about 5″ wide, 2 blocks of wood either side of the hump in the boot and bolted to the rear bulkhead – just extend the – lead from the old battery box through a hole in the floor and on to the new battery. You need 60-70Ah battery or bigger. The starter motor is not as efficient as modern geared units and on a cold damp day the 1798cc engine needs a GOOD battery. The car has little spare space to hide a big battery so 2 were fitted under the rear floor either side of the propshaft when new.  Noel.

PS the original batteries were 80Ah so plan B would be to fit 2 x 12v like you have in parallel  – easy to do.

[BOB BLACKSHAW 1Participant @bobblackshaw1]

This is the second battery in the last 9 years, they seem to be OK when new and didn’t have any problem starting  the car till now. The two single 6v batteries only lasted a few years and are expensive to replace. The single battery that I’ve got  was suggested in a MG magazine as a replacement option. The battery may be fully charged but after a few years it’s past its best.

Bob

 

[john fletcher 1Participant @johnfletcher1]

Big battery and thick leads in the starter circuit, ensure all connections are very clean, apply some Vaseline to all terminals, make sure you have good earth connection, engine block to chassis.  As some one above says check the battery voltage when cranking the engine and when on fast tick over. If the starter motor is turning the engine over forget the brushes and can you still buy them ? Multi – range Clamp ON ammeter are quite cheap now, so its easy to check starting amps and Dynamo as well.In former time one was lucky to get 2/3 years life from a battery, when Sunday morning was time to check SG and top up each cell. John

[duncan webster 1Participant @duncanwebster1]

I suspect modern battery charge controllers are a lot better than the old relay based things and so batteries last longer. I got 13 years out of the one on my vectra, and the one on my Octavia was still going strong after 15 years when if got traded in (someone drove into the passenger door in a car park). I think you can get transistor based controllers for old dynamos, the real answer is to dump the dynamo and fit an alternator, but of course that detracts from originality.

[Author]

Posts