Testing Dynamos

General electrical problems
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Testing Dynamos

Post by john.nash »

Some stuff I've learned about Lucas Dynamo's (well my E3LM, anyway).
I do have some experience with Bosch charging systems, but I do not pretend to be a fantastic electrical type chap, and so this is all fairly straight-forward.

Useful links
Workshop instructions for Lucas Dynamos

The Ariel owners club site also has the workshop instructions for Lucas Dynamos

If you are new to Ariels, then there are two invaluable things you need to make which will aid in testing and troubleshooting
  1. A short piece of wire which will join the dynamo D and F terminals together. I use a connector on each end, which is widened enough to be good firm fit. I also strip a section of insulation off half way along, to allow me to connect a bulb or testmeter
  2. A test bulb. An old (or new) pilot light holder is good for this with a 12v incandescent bulb in it. It needs two wires to the holder, which need to be long enough to reach a bike earth point and the circuit you are testing. You can strip the ends or more usefully you can crimp or solder a crocodile clip to the end of each too. You can also buy a test probe, which is like a pen with one crocodile clip wire - you clip that to earth and use the pen to make the circuit. When testing electrics, though, it's often far easier to be able to clip the wires so that you can use your hands for other stuff ..
To save yourself a full test process.
A good charging system will show a balanced ammeter or a positive charge when running with the lights ON with some revs. The lights get brighter when you rev the bike.
A bad charging system shows a negative charge on the ammeter, at any revs. The lights are dim and do NOT get bright when you rev.

You cannot use a digital multimeter on the battery, with the bike running, as the magneto will play havoc with it. You will need a moving coil type meter but the revving and light test above is pretty good and you can use a test bulb too.

If the charging system was working last time you rode the bike, then these are most likley issues (in order) and can be done without removing the dynamo. Most of these sort of cases, where it was working last time, it will be the dynamo and you should do these steps first:
  1. If the bike has been stood, then polarise the dynamo (see below) and test by starting and reving the bike with the lights on
  2. See Armature below. Take off the wiring and loosen the cover-band enough to slide it off. The surface of the armature, which is the only bit you can see anyway, where the brushes bear on, should be a nice clean copper colour. It is often black with carbon and oil, which must be cleaned off. You can do this by dunking a bit of rag into your fuel tank and then pressing it against the armature as you spin it (yes you can cheat and do it when the bike is running). See brushes below. Check for wear, freedom of movement and that they are bearing fully on the commutator, Reconnect the wiring and test by starting and reving the bike with the lights on
  3. At this point, if it's still not charging then you need to validate whether the issue is with the dynamo or not. See "The best test:the output test" below, which can be done on the bike. if your dynamo does light a bulb (or shows 10v etc) then it's good and the problem lies elsewhere (regulator or wiring). If the dynamo isn't producing then it may now need to come off the bike for further inspection.

Positive and Negative Earth
Dynamos will work with either scheme, but must be polarised to work one way or the other.
Some of the tests may mess with the polarisation, depending on how you are connecting up the power, so it's always advisable to reset it before each output test or before you start the bike with the dynamo on.

There is not much difference between positive and negative earth, in terms of lights or wiring. However, it's pretty important when it comes to the charging system. WARNING: Electronic regulators also care very much and you usually have to order the specific type for your schema and if you connect it the wrong way, even for a split second, then you will likely damage it.

Take a look at your battery. There will be a positive and negative terminal.

Positive is usually marked with a + or a P or painted red.
Negative is usually marked with a - or a N or painted black (or sometimes green).
In simple terms, the difference between the schemes is which terminal is connected to the bikes chassis (or earth). Usually, to save wiring, there will be one (substantial) wire to a convenient bolt that goes into the solid metal of the frame. The other terminal will be connected to all the other wiring on the bike.
On many bikes, people (including me) improve the situation by running additional earth wiring to where electrical components are. Thus I always run one to the headlamp and to the rear light. So, it's not always clear cut.

In addition, if you have the wiring diagram for the bike, then take a look at the battery on that diagram. It will tell you which scheme the bike was originally built with.

If the + terminal is to earth (the three different sized lines) then it's positive earth.
If the - terminal is to earth, then it's negative earth.

Of course, this assumes that nobody has rewired the bike the other way (like I do - as I want all my bikes on the same scheme - makes it easier by the side of the road)

You cannot rely on the "red is positive/black is negative" wiring convention, that you get with modern vehicles.

Lucas dynamos do not have permenant magnets in them, the field coil is an electro-magnet. The dynamo needs enough residual magnetism to get it going. Some field coils hold this better than others. If your bike has been stood for a while then it may not have enough left.
In addition, the field coil needs to be aligned to either positive or negative earth and a dynamo flashed for positive earth will only produce a tiny output on a negative earth bike (and vice versa). ☺
It is always a good thing to polarise the dynamo before testing to ensure it is properly aligned to whichever way you will connect it for testing and has enough magnetism. It is also a good thing, that after performing various testing etc and then mounting it on the bike, to polarise it again just to make sure it's aligned (you may have connected it differently when motoring it etc)
  • Leave the dynamo on the bike.
  • Disconnect all it's wiring.
  • Take a piece of wire and connect it to the battery terminal that is NOT connected to the bike earth(chassis). I have a negative earth bike, so this would be my positive terminal (as my negative is connected to the bike chassis).
  • Take the other end of the wire and just touch the "F" terminal on the dynamo. Don't leave it there, just touch it. You should get sparking.
You can polarise off the bike by using a 6v or 12v battery and connecting one side of the battery to the dynamo chassis and the other side to "F" for a moment.

The best test - Output Test
This test is the best one you can do as the result is conclusive.
If you get a positive result then it means the dynamo is definitely good, on the other hand .....
  • Leave the dynamo on the bike.
  • Disconnect all wiring, from the dynamo.
  • Take a piece of wire and connect it to both the 'D' and 'F' terminals - I had to make something up to do this.
  • Now you need to connect this, such that both terminals go to the bike "ground", with either a mulitmeter (on 20VDC) or a bulb (12v is desirable) in between . In my case, as I am negative earth then I can use the negative terminal on the battery or just a good chassis earthing point. You can use a pilot bulb holder and bulb for this as it has two wires - doesn't matter which one goes the dynamo or the battery.
  • Start bike
  • move it up to a fast idle and the bulb should light. Multi-meter reading will pick up with the revs and 10v should be seen. You can get it higher, but it's not advisable to do so.
The Workshop instructions for Lucas Dynamos gives some guidance on the voltage seen, if you have a mulit-meter connected (as opposed to a bulb). However, if the volts get up to 10 or the bulb is bright then it's safe to assume your dynamo has a healthy output.

You can also perform this test off the bike, but you'll need a separate 6v/12v battery and you will need to connect the earth side of the battery (in my case, the negative terminal on the battery, as the bike is negative earth) to the case of the dynamo. You will also need something to spin the dynamo - usually a power or hand drill on the gear. If you only have a drill that spins at very high revs only, like a dremel, then you probably don't want to use it. However most power drills have a slow speed setting (and in my case, numbers for revs)

The advantage with spinning the dynamo, on the bench, is that you have control over how fast it can go and can also spin it in both directions. You are also not juggling with various wires and trying to raise the revs on the bike at the same time.

Another test .. Motor Test
I have found that this test is not definitive.
The dynamo may "motor" but this doesn't mean it will produce power, when it's on the bike (I know this as it happened to me)
However, it is a useful test for determining which way the dynamo will rotate. There is a little arrow on the case, but it's always good to check.
  • With the dynamo off the bike and a separate battery (6v or 12v) available.
  • As my bike is negative earth, then I connected the negative terminal on the battery to the case on the dynamo.
  • Take that piece of wire and connect 'F' and 'D' ( as you did in the test above) together.
  • Connect it such that both terminals go to the other terminal on the battery, in my case positive.
  • The dynamo should spin. Hopefully in the right direction.
A dynamo that is being turned in the wrong direction will output about -0.2v (yes MINUS) at idle speeds.
If you are not sure what is right for you, then take a look at your bike and figure out which way it will spin the dynamo and compare it the the direction it spins in during this test.

Changing the direction that your dynamo produces power in.
The dynamo will produce power when being spun in one direction only (usually in the direction of the arrow on the case). Spin it the other way and you'll get a tiny reading of minus volts.

I found out how to change it by accident (because I reassembled the dynamo incorrectly ☺)

Inside the cover are the two carbon "brushes" that bear down on the end of the armature. One comes from the field and one goes to the 'D' terminal, on the back of the dynamo. You can ease them out by pulling the spring back. What you need to do is to pull them both out and then simply change them over in their holders. Put the springs back. No need to touch the wiring.
You can also swap over the wires from the field as well.

It doesn't work ...
If the output test produces nothing, then don't panic.
Few things to check.

They aren't really brushes - just lumps of carbon carbon with wires embedded.
  • Undo the band, on the back, and slide it off.
  • Inside, are two holders with two black carbon "brushes" (each have a wire embedded in the top)
  • Check that both brushes are long enough, so that the springs can push them firmly onto the armature.
  • Check that the brushes move easily in their holders. They often get mucky and sticky and may need a clean with some petrol and very light polish with some fine wet/dry paper
  • Check that the brushes make even contact with the armature (this end bit is called the commutator)
Armature (the long thing that spins inside)
The commutator bit of the armature, that the brushes bear against, should be clean and shiny and the spaces in between should not be full of muck. This is the usually the biggest reason for the dynamo not working; you will often find it black (from the carbon on the brushes).
There are instructions in the Lucas workshop manual on cleaning.

Testing other bits
The Workshop instructions for Lucas Dynamos contains some other tests, regarding what you may need to see in terms of voltage output and what it might mean.

Testing the resistance of the Field Coils is pretty easily done and conclusive. The two wires, that come out from the body of the dynamo, go to the field and the values (in ohms) are given in the workshop manual.

I was told that you can test the continuity between the segments on the armature and from each segment to the armature. However, these tests gave me a positive result but my armature was still broken. Be warned.

The armature is fairly easily removed and exchanged.
The field coil can be done without that little lifter thing, but it is a bit of a pig. Expect to use an impact driver and bad language.

There are two bearings, one on each end of the armature. They are fairly tight and you should need a puller to get the smaller one off. Check these for roughness, while you have the armature out, and if they are okay then clean with petrol and repack with HMP grease.
John Nash
AOMCC No.4119
''78 t140 bonneville, '77 BMW R80, '67 CJ750, 196-ish Ural M62 outfit, '51 VH500, '49 project Ariel , '47 VH twinport, '44 Ariel WNG, '42 indian 741b, '41 Ariel WNG and piles of rusty scrap ....
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Post by pete.collings »

A bit of additional information if you use a JG electronic regulator.
I have run all my Ariels with a JG 12v unit, which I found very reliable for my usage of the bikes, and allowed a decent QI headlamp to be used. A downside on higher geared bikes was that you needed to do a fair speed to balance the headlight, on the Huntmaster this was aproaching 40mph, so I used a decent capacity battery. I have heard it said that if you do a lot of fast distance touring, JG units can be hard on batteries, although I have not experienced this myself.
I understand that with the JG unit (and possibly others), when the battery is fully changed and there is no other load on the system, the regulator cuts off any feed to the field coil, so the dynamo is running idle and not providing any change. This can allow the brushes to glaze over after a period of time, and when the bike is next started, no charging occurs. This is especially so if the bike has not been used for a while, and rather inconvenient if you are about to take it for a MOT!
Removing the dynamo end cover, starting the bike and gently squeezing the brushes inwards can remove the glazing. Taking the bike for a rerasonable length run can also restore charging, the ammeter will suddenly spring into life (usually a high charge which quickly drops).
As said earlier, these are my experiences with dynamos and JG units, and may not be applicable to other electronic regulators, but if your dynamo does not charge even though all the other tests or OK, as outlined by John in the post above, they may be worth thinking about.
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Re: Testing Dynamos

Post by adrie.degraaff »

Testing your dinamo the old way;
connect the 2 wires to the outside of the dinamo, make a pulley on it, wind a rope with a wooden grip round the pulley (like a outbord engine), pull in the direction of rotation, a good dinamo will block now totaly pulling the wooden grip from your hand.
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Post by nevhunter »

Pretty comprehensive John.
Regarding the commutator, it should be undercut. The gap between the segments has "bakelite?) in it. It is not enough to just machine it true.. (Should be done in a steady) .
The gaps are undercut using a hacksaw blade that has been thinned down on the edges to reduce the thickness of the cut. Make a special handle for the blade and you only need a small length of it. About 5'. If you don't do this the bakelite will cut away the brushes. The copper is softer. Go about 1/2 mm below the copper . Clean any burrs with fine glasspaper and blow clean. You only need to do the area where the brushes run.
A bike that has sat for a while may be reluctant to charge initially. (may taker a few revs to start it). This is due to the residual magnetism being too little. There is no permanent magnet in a generator and the residual magnetism can be quite small in soft steel.
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Re: Testing Dynamos

Post by Richard.Carrigan »

This may be a dumb question but.. should a dynamo that when off the bike motors strongly in an anti-clockwise direction, still generate electricity when driven in a clockwise direction?
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Re: Testing Dynamos

Post by Roger Gwynn »

No, it won't. This assumes when testing on the bench the dynamo is wired with the same polarity for earth as it is on the bike. Either swap the polarity of your battery, not possible if you have an electronic regulator, or re-polarize the dynamo, or swap the field coil wires around or swap the brushes over, see John Nash's comprehensive instructions above.
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Re: Testing Dynamos

Post by will_curry »

The only way to change the direction of motoring is to swap either the field coil connections
or the commutator connections inside the dynamo.
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Re: Testing Dynamos

Post by bevanclark »

Just to add my recent experience: my return pump failed resulting in oil being pumped everywhere, including into the magneto and even up into the dynamo. Despite this, both continued to function perfectly. However, I was uneasy that this may not continue so sent the magdyno away for an expert to examine. Turns out that crud had been pumped into the bearings, so just as well I was cautious. On return, the mag was fine but the dyno produced no volts at all, even though all resistances tested ok. Flashed the dyno to no result. It was only after I removed and motored the dynamo for a few seconds that it then decided to start generating.
Cheers, Bevan
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Re: Testing Dynamos

Post by nevhunter »

Loss of residual Magnetism in the generator. Even Bikes left unused will suffer from it sometimes. Nev
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