M18 Dynamo Charging Problems

[Bomber Bri]

I have a 1948 AJS M18. It is a -ve earth system which was working fine until a few months ago.

Since repolarising the dynamo after it was wrongly connected I have had the readings on the ammeter in a -ve reading. I have cleaned the comutator and connections and tested the dynamo with an ammeter on low tickover getting a 6.15v reading, raising the revs slightly to a reading 0f 9.36 before knocking off, satisfied the dynamo was not at fault.

Next was cleaning the rectifier, getting a nice green spark on the connections when the bike was running on tickover. I have checked all the connections for a short from the dynamo to rectifier but nothing

When riding the ammeter was reading 0 until lights are turned on then it went to -2 on side light and -4 on main. I left the lights on to discharge the battey then ran the bike but lights failed..... no charge after a couple of miles. Since cleaning the comutator, bushes and connections the ammeter reads -2 when the bike is running - 4 with sidelight and -8 on main..... I have been told to reverse the ammeter wires but would than not just reverse the ammeter readings i.e +2 tickover, +4 sidelight and +8 main?......

Has anyone else had the problem? I would like some ideas from people in the know before I start spending lots I dont need to.

Any ideas appreciated, thank you

Bri

[Groily]

There is a number of things here . . .
I think it would be good to go back to first principles. This is a good link for what to do and how:
https://docs.google.com/document/d/1KAP ... nqxAp/edit
It also includes testing the voltage regulator for function (not the rectifier, which you won't have one of).
Reversing polarity normally requires reversal of ammeter leads, yes - but if you were negative earth before the dynamo polarity got reversed to +ve and the readings are now wrong, are you sure you repolarised it correctly? Or are you just seeing Negative on the ammeter to show discharges, which is correct, while the dynamo isn't charging at all?
A reading of -2 amps with engine running and lights off is weird given you have a magneto on there so there should be no load on the electrics - the other negative readings with the engine running suggest it's not charging, hence the minus readings. OR, if the readings ARE truly reversed and it is actually charging, that the voltage regulator isn't regulating as you would never want to see an 8 Amp 'charge'. The charge rate shouldn't vary that much at rpm above about 1500, but will show strongly negative at tickover with lights on, bouncing back to a moderate charge with some revs.

Your 6v rising with modest revs to 9v - with the potential to go far higher with more revs? - sounds OK, if you were doing that with F and D bridged at the dynamo and were using a VOLTmeter to measure it.

[SPRIDDLER]

Welcome to the Forums!

Just to be quite sure............ Do you have the original electro-mechanical regulator or a modern solid state regulator?
Subjecting some solid state regulators to reverse polarity can ruin them.

[Mollbhan]

My AJS 18 wouldn,t charge , dynamo fine, all wiring seemed ok, new regulator made no difference, turned out the old half charged battery showing only 3 volts was the problem, new battery fitted and all was well. I had always assumed a dynamo would charge a battery from flat.

[Harry44]

My AJS 18 wouldn,t charge , dynamo fine, all wiring seemed ok, new regulator made no difference, turned out the old half charged battery showing only 3 volts was the problem, new battery fitted and all was well. I had always assumed a dynamo would charge a battery from flat.

The ability of a dynamo to charge a flat battery depends on the internal wiring of the regulator. If the output from Dynamo D is fed back by the regulator to Dynamo F. It can charge from zero volts.

But if the regulator can only send battery voltage to Dynamo F, nothing will happen.

[Bomber Bri]

Thank you all for taking the time to reply and giving me some ideas

Cheers

Bri

[Bomber Bri]

Welcome to the Forums!

Just to be quite sure............ Do you have the original electro-mechanical regulator or a modern solid state regulator?
Subjecting some solid state regulators to reverse polarity can ruin them.

It is still the original MCR2 rectifier, I have cleaned the contacts and am getting a healthy green spark between them with the engine running

[Groily]

Useful info to have.

With the mechanical Voltage Regulator, the electrical system will operate with a flat battery or no battery at all, as indeed is the case with the better electronic ones.
The mechanical ones also aren't polarity sensitive, ie will work with either earth without anything needing to be done.

If your system is working, you should have lights with the battery disconnected as long as you give her some revs to get the dynamo to cut in. It won't matter whether the dynamo is polarised to + or - earth if there's no battery in play - and you'd see nothing on the ammeter either as that measures what's happening to a battery that's not there. The lights would be flickery - but they should be there.

So, if you run the engine without the battery, do you get any lights?

If you do, you're on a winning path because it says the dynamo is working, and also probably that the MCR2 is as well, if the lights brighten and then stabilise with higher rpm (rather than blowing due to excessive voltage).

If you don't get any lights, then you need to revert to the dynamo and do the sorts of tests in the link in my previous comment here. It all starts with bridging D and F at the dynamo and measuring from the bridged wire to earth with a voltmeter or using a bulb. (12v bulbs are easier for this as they illuminate a lot more readily than 6v ones if going that way.)

Let's assume the dynamo still produces the sort of output you mentioned at the beginning, and more with more revs. So the next step is to see if the voltage regulator is working. The easiest way to do that is to measure the voltage coming out of the A wire from the MCR2 with the engine running. It should build up with rpm until it stabilises at about 7.2 volts average and goes no higher if more rpm are applied. Again, a bulb can be used - if it gets bright with medium revs but then settles and doesn't blow, probably OK.
If the voltage just keeps on going up - the regulator isn't doing its job.
If there is nothing to see on the A wire, then it could be a number of things, from a sticky cut-out via dodgy connections in the regulator unit . . .

If you have a decent output on A, then we can start thinking about the ammeter, the battery polarity etc.
Your A wire from the regulator will go to the ammeter (or maybe to the main switch for the lights with a short link to the ammeter). From the other side of the ammeter the main LIVE lead goes to the battery. You choose which side of the battery you want to be live, and then you make sure that the DYNAMO is in tune with that - by 'flashing the field'. No magic, you just briefly touch a wire from the side of the battery you want to be live to the F terminal on the dynamo. Just for a second or two. There'll be a sparky flash, that's OK.

Then, turn the lights on with the engine stopped and see which way the ammeter needle goes. If it goes the wrong way, swap the wires on the terminals at the back. Obvs you want a discharge to show.
Then, the exciting bit, see whether anything happens at the ammeter with the engine running and the battery in the circuit. It should show a small charge if the battery's charged (maybe less than an amp) - and should continue to show a small charge or at least no discharge as you put the lights on and give the engine a few revs. At tickover with the headlight on there'll be a big discharge, which should disappear with revs, and there'll probably be a small discharge at tickover with the lights off, because of the way the mechanical cut-out / cut-in arrangement is designed. Voltage across the battery terminals at tickover will be whatever its charge level is, but with medium rpm should be 7v +, so that's another thing to measure if you want.

If the regulator is working properly, and the battery is reasonably well charged, you won't see a large positive reading with the lights off or on. And nor do you want to . . . If the ammeter acts a bit like a mini rev-counter, the regulator isn't regulating and you're overloading the dynamo and at risk of blowing bulbs.

Turning to your little green sparks -I'm not sure quite what they are telling us.
There are two things that should happen in the MCR2. One side is the Cut-Out which connects and disconnects the battery from the dynamo as required. (If that wasn't there, the battery would be permanently connected through the regulator to the dynamo and would try to drive it as a motor with disastrous results, so the battery is 'cut out' while its voltage exceeds that being generated by the dynamo, and is 'cut in' when dynamo voltage exceeds battery voltage.)
There shouldn't be sparks across the cut out.
In 'F A D E' terms, the cut-out connects and disconnects 'A' (which is the main power line that goes to the ammeter and battery) to/from 'D'.

The other side is the regulator, which has the very rapidly opening and closing contacts that are probably providing your green sparkfest.
They should only show above 1750-ish engine rpm though, because below that speed the regulator shouldn't be operating - the dynamo voltage isn't high enough to need to be managed.
Reverting to F A D E - What your regulator contacts are doing is managing a connection between D and F of the dynamo: when they are shut the output from D is being fed into F (as well as going off to charge the battery and provide lights etc through 'A'). Feeding F creates the electro-magnetic field which the armature spins in - the stronger the field the higher the output from D. When the regulator points are open, the feed from D to F is interrupted, when closed it's restored and its the relative length of 'time shut' to 'time open' which determines how much output the dynamo is allowed to deliver.

Hope you are able to get the system to work!

[ChrisTheChippy]

I think that is one of the most comprehensive and comprehensible replies I've ever read on the subject. I learnt a huge amount from that . Thanks. Cheers Chris

[Radcap]

Just a thought? Have you checked the polarity at the AMP meter ? ( there directional )You could be conected wrong ? Swap the terminals over ???? Maybe try a clamp on ammeter to test.