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Dynamo,again

Can someone please advise what my chargeing cct problem might be? I've read the interesting stuff from the thread on dynamos in May, but my symptems are: when the revs are increased the cut out pulls in and the red light goes out initially, but as the revs increase the light comes on again and gets brighter in proportion to the revs. There in no charge shown on the ammeter. Dynamo produces 20v, cut out cuts in and drops out and is earthed ok,wiring checks out and I've tried another switch panel with the same result. Have I got something connected the wrong way round which I am not seeing?
Dynamo is C35A with resistor in terminal box and cut out is CF3.

Re: Dynamo,again

Rod. When I had a similar problem, I found that the earth brush (connected to the end plate) was not making firm or sufficient contact with the commutator - a bit a judicious realigning of the spring sorted it. Difficult to do is situ so best to take dynamo off to have a proper look

Not sure if that is your problem but worth a shot

Re: Re: Dynamo,again

Thanks for the response David. I thought it would be something simple. I'll take the dynomo off yet again and try it.
Regards, Rod

Re: Dynamo,again

Hello Rod,
Remote diagnosis is fraught with difficulties but may I make the following suggestions/observations:-

Firstly, the "Ignition Warning Lamp" is wired between the D+ Dynamo Output Terminal, and the "A" line which is the main battery feed via the "Ignition Switch" in the Switchbox. It is effectively across the Cut-Out contacts.

When the Ignition is turned "On" there is a path from the Battery supply ("A" Line) and back through the Dynamo and its windings to Earth. The Ignition Lamp therefore glows quite brightly.

With the engine started the dynamo output increases with increasing engine RPM. The Ignition Lamp will begin to glow slightly less brightly as the current flowing though it is a product of the difference between the Battery Voltage and the Dynamo Output Voltage.

When the Dynamo Output is of a sufficient level (say 6.8 volts - chosen as a value above the voltage of the battery) the Cut-Out contacts will close due to the the critical flow of current in the Shunt Coil of the Cut-Out.

The closing of the Cut-Out contacts effectively shorts out the Ignition Lamp and it becomes extinguished.

In simple terms, Dynamo Current will then flow from the Dynamo to the Battery.

Now in your case, you descrbibe a situation where following that first stage, the Ignition Lamp comes back on at higher engine RPM and the brilliance varies "upwards" with increased RPM.

For that to happen, the Cut-Out contacts cannot be maintaining a "short" across the lamp.

So we must establish why that is so!

You also say that you have measured Dynamo Output at 20 volts but do not described the circumstances under which such a measurement was made. I would have to presume that the 20 volts figure was taken with the Dynamo Field circuit fully energised and the Dynamo running open circuit.

What could be wrong?

Well, if you are managing a good steady 20 volts out of the Dynamo at moderate engine speed, I doubt if there is too much wrong with the Dynamo, although the "earth brush" inspection which you have been recommeded to perform could still be worthwhile, because anything which may present a higher than intended resistance once current starts to be drawn from the Dynamo would have to be suspect. Clean and free brushgear is important, although I would have thought that you would have probably had an "erratic" 20 volts if there was anything seriously wrong there.

Clearly, there must be some output from the Dynamo, because there is variation in the brilliance of the lamp.

I would want to be certain that the Cut-Out Contacts were nice and clean and have good "follow" so that you can be sure they are making good contact. Go easy on the contacts, some fine wet and dry paper, used dry, should do the trick.

If you have a good voltmeter, I would be inclined to check the actual Cut-Out settings. They are described in Woodrow's "Austin Seven Manual" (the Red Book) and are not really all that difficult.

I assume that you see a moderate discharge on the Ammeter when you first switch on?

Please feel free to email me if you have any queries in relation to the above, and maybe, if we both have a copy of your wiring diagram at hand, we could work the thing through.

I am sorry the above is such a ramble, but I am just thinking out loud.

regards,


Barry R

Re: Re: Dynamo,again

Hello Barry. Thanks for your input.
Yes the dynamo produces 20v with D and F terminals linked and all car wiring disconnected. Incidently it also motors nicely on the bench.
Yes there is a small discharge on the ammeter when the coil is energised.
Difficult to say accurately what the cut out pull-in voltage is as the volts rise very quickly from 2 to 20 or so, but I think it's about 8.
Taking a line direct from battery+ to the D terminal on the cut out produces a positive pull-in and drop out with the A terminal disconnected.
I'll try fettling the points and see what happens.
Regards,
Rod

Re: Re: Re: Dynamo,again

Problem solved.
The earth brush was all ok, but Barry, your analysis was spot on: there was something wrong with the cut out points. I cleaned them up with wet and dry, but with no improvement, so wasted most of the week-end looking for other things until in desparation I tried pressing the already closed points with my finger. The light went out and the ammeter began to register. I bent the contact arms so that the system appears now to be charging, though the cut-out doesn't drop out until either the engine stops or it is switched off; but I can live with that.

Many thanks for your responses gents, I don't think I would ever have got there without your direction.
It just shows that you can't assume a cut-out is servicable just because it pulls in and drops out correctly.

Kind regards, Rod

Re: Dynamo,again

Sounds like the spring is knackered.

Mick

Re: Re: Re: Re: Dynamo,again

Hello Rod,
I am glad that you are making progress.
It does seem as though your Cut-Out needs to be put into proper adjustment.
I personally would be worried about the Armature "hanging on"
I will send you some adjustment instructions as soon as I can arrange a scan.

In brief though, the whole thing involved getting the "pull in point" right which can usually be achieved by adjusting the "rest" air-gap between the armature and the core.This is achieved by careful adjustment of the backstop against which the armature is held under spring tension. when at rest.

The actual "cut in" point needs to be somewhere in the order of 6.8 volts. If you have a good voltmeter, you should be able to measure the Cut-In point as follows.

Connect the Meter between the D+ terminal on the Cut-Out and earth. Slowly raise the engine speed up using either the hand throttle control or by winding up the throttle screw until you note a slight flick on the voltmeter. This should occur at the said 6.8V, and the meter will "steady" at this voltage.

Some people will advise various voltages which range between 6.5 and 7 volts. The important thing is to arrange the cut in point to occur at a point of just a bit more than what you would anticipate a fully charged battery to be (say 2.2v per cell = 6.6v for a six volts system) So I generally aim at about 6.8v.

If the Cut In voltage is found to be too high, then you need to bring the armature closer to the core by very deft adjustment of the backstop. If the Cut In voltage is too low, then you need to slightly ease off the backstop.

As far as the drop out voltage is concerned, once you have the Cut In point right, a little tickle up of the armature spring ( a bit of extra tension will mean that the drop off point will occur "earlier") should produce the right result, although you might need to re-check the the Cut In point in case that has been affected by the adjustment of the spring.

The spring can loose its tension, but unless obviously rusted or obviously stretched, is probably OK, but in need of a little coaxing.

If you have no voltmeter, but are concerned lest the Cut-Out is not working, or is maybe set to cut in at too low a voltage, you can do a “dynamic” test using the Ammeter Lord Austin kindly ensured was fitted to your dashboard.

Simply bring the engine up to around normal “cut-in” speed with the lamps and all other electrics (other than ignition of course) “off”
The ammeter should be observed to “kick –up” into the charge area and show a small charging current. If the ammeter kicks “down” slightly into the discharge side, then the Cut-Out is operating at too low a voltage from the Dynamo.

All the above is a mouthful to explain, but is a lot easier in the practice.

You seem to have got "the follow" on the contacts right, and I would now urge you to actually check out the adjustments.

Best regards,

Barry R

Re: Re: Dynamo,again

Just returnede to this after a couple of weeks away.

The only way I could get the contact to operate at about the correct voltages was to disassemble the armature so that I could access the points for a proper facing job and pack the pivot end of the armature so that it lay square to the coil core; this also having the effect of increasing the spring tension.

Now it closes at about 7v and opens at about 4v, (not ideal, but well above tick-over speed). So I'm going to stick at that.

Many thanks for the help folks; much has been learned.

Regards,

Rod Griffin