Stuttering with lights on - Charging problem?

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
schrottschlecker
Posts: 46
Joined: Tue Nov 14, 2017 8:36 am
Location: GERMANY

Stuttering with lights on - Charging problem?

Post by schrottschlecker »

Dears,
I have successfully started my Model 18S after restauration and it runs pretty good, but with lights on it looses power and starts spitting and stuttering. I have the original 1958 setup:
- Alternator Lucas RM15 with 3 cables
- 6 Volts coil ignition
- New 6 Volts 11Ah gel-battery
- New wiring harness from Jampot spares
- New Lucas PRS8 ignition/lighting switch from Jampot spares
- Original Lucas NOS rectifier
I checked with a fully charged battery (6,7V) and a multimeter on the battery poles. When I switch on the ignition, it shows 6,4-6,5 Volts. The amperemeter slightly moves from Zero. When I switch on the light, the voltage goes down below 6 Volts and the pointer in the ammeter disappears completely. When I start the engine (very good starter!), the multimeter shows no difference, even at higher revs. When I disconnect the 3 cables from the stator, there is no difference. So I am guessing there is no charging.
How can I check if the alternator is charging?
Is there a chance to get it reliable with the original setup? If not, which upgrade would you recommend?
Thanks for any feedback.
Greetings from Germany,

Christian
Mick D
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Location: Aberdeenshire

Re: Stuttering with lights on - Charging problem?

Post by Mick D »

schrottschlecker wrote: Sun Jul 11, 2021 12:27 pm Is there a chance to get it reliable with the original setup?
Hi

Yes it is, that's how I run my G3, though I do very little night riding.

I would connect up the alternator again, start the engine and measure the DC output from the rectifier. If you see zero volts you need to work towards the alternator, checking the connections and resistance of the alternator. If you see around 6V the issue is likely to be a wiring fault in the lighting circuit. The ammeter on my bike deflects to about -5 Amps when I switch on the lights, (engine off), going full scale suggests you have a very high drain, approaching a short.

Regards Mick
Reynard24
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Location: CAMBRIDGESHIRE UK

Re: Stuttering with lights on - Charging problem?

Post by Reynard24 »

You can get reliable electrics with a 6V system but a 12V conversion is so easy and well documented on these forum pages and with the alternator this allows the use of halogen lamps for better lighting and battery charging is so much better controlled by using a combined rectifier/regulator or even a zener diode rather than switching the alternator output through the PRS8.
To check the alternator is charging you need to reconnect it and then measure the voltage across the battery with the engine running at fast idle and all lights off - you should then see a voltage of around 7.2 - 7.5 volts. If the voltage is the same as with the engine off i.e.6.XV, then it will be either the rectifier or the alternator. A modern encapsulated rectifier is around £5 from most parts suppliers, cheaper from electronics specialists, and will be much more reliable than the old type plate rectifiers and I would swap that out as a matter of course.
The high drain shown on the ammeter is most likely a chafed wire in the loom - my guess would be either somewhere between the tank and headlamp where it runs close to the steering head or under the rear mudguard where it can be rubbed by the tyre if not secured out of the way and then shorts against the mudguard. I had this issue last week on a dynamo equipped bike and the current draw when switching on the lights was enough to drop the battery voltage but not enough to take out the fuse. The cause was a slightly chafed wire to the rear lamp.
Groily
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Location: NORMANDIE FRANCE

Re: Stuttering with lights on - Charging problem?

Post by Groily »

I'm very much with Reynard on this. It simplifies the wiring, improves reliability, controls the charge more precisely and it's easier to get bulbs and so on.

The only little caveat might be that with coil ignition to support, a 35/35W halogen is about right, or you could be at risk of being 'marginal' running around in town or on slow jaunts. That was my experience with a Beesa single configured exactly as proposed, anyway. The 50W lamp I tried first was a bit too optimistic! (But is great on bikes with magnetos . . .)
Of course, upgraded stators and rotors are available for those who want to invest in them, or indeed a combined regulator / rectifier is available at 6v too, which suits some people very well as the engine needs fewer rpm to support the lower system voltage.
And then there's always the LED option . . .
We are spoilt for choice these days frankly, compared to what we had to struggle along with in the old days!
schrottschlecker
Posts: 46
Joined: Tue Nov 14, 2017 8:36 am
Location: GERMANY

Re: Stuttering with lights on - Charging problem?

Post by schrottschlecker »

Dear all,

thanks for the feedback so far. Pardon me for being an electric-fool, but where and how do I actually measure if the alternator charges? I have attached the PLUS-cable from the multimeter (set to DC Volt) to the frame (ground) and the MINUS into each one of the 3 cables when engine was running. No reaction :cry: Was that correct?
How to measure DC on rectifier? MINUS cable on the purple connector on the rectifier (see photo)?
IMG_1801.JPG
Wrt the "chafed" wire: wiring harness is 8 miles old as the rest. I can't see any damage. How can I check the high drain or short?

Thanks for being patient. The stomp of the engine motivates me :lol:

Cheers,

Christian
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Mick D
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Re: Stuttering with lights on - Charging problem?

Post by Mick D »

Hi

The DC terminals on the rectifier are the mounting stud and the central tag.

You can do more diagnostics by removing bulbs from various lamps and seeing what effect that has on the ammeter indicated drain. I assume all lights were working when you saw the heavy discharge on the ammeter.

Regards Mick
schrottschlecker
Posts: 46
Joined: Tue Nov 14, 2017 8:36 am
Location: GERMANY

Re: Stuttering with lights on - Charging problem?

Post by schrottschlecker »

Mick D wrote: Mon Jul 12, 2021 6:46 pm Hi

The DC terminals on the rectifier are the mounting stud and the central tag.

You can do more diagnostics by removing bulbs from various lamps and seeing what effect that has on the ammeter indicated drain. I assume all lights were working when you saw the heavy discharge on the ammeter.

Regards Mick
Yes, all lights are working. I will take them out one after another and watch the ammeter.
To check the alternator, I attach multimeter PLUS to the frame while engine is running ... and where does MINUS attach to?
Groily
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Location: NORMANDIE FRANCE

Re: Stuttering with lights on - Charging problem?

Post by Groily »

To check the alternator, which you need to look at if there is no DC voltage coming off the rectifier live wire (meter connected between it and earth, engine running), you need to measure across the pairs of (disconnected) alternator leads on the AC volts scale. (Not from leads to earth, no.)To do it properly, you need to put a 1 ohm 100W 'load' resistor across the pair being tested first, and measure the output against Lucas specs for the particular pair of wires being tested. That needs the wires to be identified according to their colour code (varies between models) and the voltages specified vary between models. So that's not that simple if you don't know the colours, or can't figure out which of the three wires is the 'common' one so-called. Colours can in any case be murky to tell apart, especially where light, medium and dark green are concerned.
However, if you just check for simple AC output by putting the meter probes across each pair of wires with th engine running, you should see load-free volts being delivered, although it's not definitive of the true state of affairs when loads are switched on.
Some Lucas testing procedures can be seen here, done 'their way': http://www.brightsparkmagnetos.com/libr ... 20RM12.pdf
Last edited by Groily on Tue Jul 13, 2021 8:00 am, edited 1 time in total.
Reynard24
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Location: CAMBRIDGESHIRE UK

Re: Stuttering with lights on - Charging problem?

Post by Reynard24 »

You cannot test the alternator output using DC Volts, you either need to use AC Volts across each pair of coils or DC Amps at the output of the rectifier.
As a quick check you can try DC volts to check charging- connect the meter across the battery - one lead on each battery terminal, it doesn't matter which way round you connect the leads as only the polarity will be different, the value will be the same. With the lights off and engine not running you should see +/-6.XVolts and with the lights off and engine running at fast tickover then the reading should be +/-7.XVolts if the alternator and rectifier are working.
If you are an electrical novice then Al Osborn of AO Services has some very good details on how to check an alternator and rectifier on his website www.aoservices.co.uk and click on the orange "electrical advice & info" diamond in the top right hand corner - this will take you to a second page with PDF downloads on how to check the alternator.
schrottschlecker
Posts: 46
Joined: Tue Nov 14, 2017 8:36 am
Location: GERMANY

Re: Stuttering with lights on - Charging problem?

Post by schrottschlecker »

Dear all,

I was afraid that it would not be that easy. I try my best and keep you posted.

I will probably go for the 12 Volt conversion and search the forum for advice. Need to see if parts are available in Germany since Club spares does not sell outside of UK any longer.

Christian
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