Dynamo Problem on Star Twin

[starfield]

Some help appreciated with an odd problem on my A7 Star twin with 6v lucas dynamo and solid state regulator.

When you first start up the bike everything works fine. Ammeter initially shows a small charge on no load then settles to 0 charge. Switch on the lights and it goes to about 6a discharge on tickover but quickly settles to a charge when you rev the engine, slowly settling to balancing if you hold the revs steady. I would regard this as text book performance.

However, Take the bike on the road and within minutes the ammeter is showing a discharge of maybe 1a which increase to perhaps 2-3a as the revs rise to a 50mph or so cruising speed. This is with none of the lights switched on. This situation lasted for the whole of a 100 miles trip yesterday and I have seen similar behaviour before but not observed it so closely. There was still plenty of life in the battery at the end of the trip. If I put the lights on the discharge rate will increase but not in proportion to the additional load.

What prompted me to check more carefully was a flat battery after a daylight run of perhaps 45 miles with the headlight on. Clearly the discharge is real and I am concerned about what it is doing to the dynamo as well as the risk of doing  a night journey until its is fixed.

The regulator was new about a year ago bought by the previous owner and not sure of the make yet as its inside a tin box. The 6v battery is also about a year old and is one of the multi cell jobbies sold by Burlens. When I recently recharged it with my clever electronic charger it came up green indicating it achieved a full charge. All the connections appear to be correct and sound.

Any suggestions appreciated

Thanks

Peter

[bsa-bill]

Hi Peter - is the stop lamp coming on when your riding the bike (needs brake switch adjusted maybe)

[BSA_54A10]

yes, the stop lamp is a prime culprit I do have to adjust mine from time to time.
After that it can only be a short with a loose wire , Horn button ? power feed to horn ? Light switch or more oft dipper switch.

[trevinoz]

The stop light is normally connected at the battery and does not show a discharge on the ammeter when it is operated.
Probably the best way to go is to test the generator then have a long hard look at the regulator.

  Trev.

[starfield]

Tried the following. Removed the rear and sidelight light bulbs completely. No difference, ammeter balanced at tickover, 3amp discharge at 45-50mph. Then disconnected the dynamo by pulling out the field and dynamo leads obviously no charging but still a 3a discharge at 45-50. This now seems to eliminate the dynamo.  Horn and stop light are wired direct from battery so would not show on ammeter anyway.

The only other thing I noticed was that putting on the headlamp only took the ammeter to 6a discharge whereas I would have expected it to go to 9a if something was already consuming 3a.  It might suggest the discharge is through the headlight circuit. However, if there was a short it would be constant surely, not influenced by engine speed and in this case by actual road speed. Revving the engine at standstill does not produce the problem. That's the thing which really bugs me

I fully accept its a wiring fault and it does appear to be a home made loom but not sure yet where to start. I guess its a process of elimination. One thing I did not test was disconnecting  the regulator completely but that will have to wait until tomorrow as its raining here now.

Peter




Peter

[chaterlea25]

Hi Peter,
I think vibration is making your ammeter tell lies??
Wire in a volt meter (multimeter) and  tie it to the headlamp or tank and go for a spin

HTH
John

[trevinoz]

Peter,
             You are putting the cart before the horse.
First you must establish whether the generator is any good by disconnecting it and going through the test procedure.
If it is OK, the next step is the regulator.
If you connect an ohmmeter to the "D" & "F" wires after disconnecting, you should get a reading very close to 0.
A discharge of 3 amps at speed could just be the bike vibrations as you would expect a wiring fault to be constant.
If in fact there is such a discharge, where is the current going?
A short circuit normally blows the fuse or the wire melts with a heavy current.
3 amp discharge indicates a resistance of 2 ohms whereas a short should be 0.
Check your wiring after a run because if there is a high resistance short, there should be some heating at the location.

   Trev.

[starfield]

Wired in my mulitmeter and took it for a ride. With all lights off it charged steadily at 7.3v at road speeds, ammeter needle slowly creeping to a 2-3a discharge. Took it for another ride with the battery and dynamo disconnected, 2-3a discharge again. Since there was absolutely no source of power to make the ammeter move I guess it has to be vibration  on the needle.

Very odd but at least I have an answer and the other tests have established there is not much wrong with my electrics though I think I will be a bit wary of night rides for now until I have built up some confidence with them.

Thanks for your patience.

Peter

[bsa-bill]

never tried it myself Peter but other members here have talked about an screw adjustment under the ammeter to damp the needle

[starfield]

Hi Bill,

This was a coment made by Sean Hawker some time ago. However, it seems only to apply to genuine Lucas ammeters and I doubt mine is of this type but I will look.

Peter