Technical question about 350 Scrambler 1974 (Mototrans)

[Rocla]

Hello everybody,

I am wondering if it is normal that my Scrambler (350 Ducati/Mototrans 1974) battery voltage is fluctuating a lot when the engine is running: it fluctuates from 5 volts to 18 volts (at about 1000 rpm). It it symptomatic of something wrong? Best regards

[ducwiz]

Hi,

no, that's totally "unnormal".
I suppose your Mototrans bike has a 12 Volt system ex-factory. In that case, and if your battery is in a proper condition and charge state, the voltage should read ~12.5 V with engine off, and increasing up to 14.4 Volt when the engine runs above 3000/min.
Sometimes, cheap electronic DMMs are not able to display slightly varying voltages in a continuos manner, which may be a case of trouble here.
Please, give us some more information about the alternator, R-R, the battery (typpe/age) and the DMM you use for the measurements.

Hans

[Rocla]

Thanks Hans,
Here the data:
Battery GTX 12v/6a (brand new)
Voltage measured engine off: 12,34v
At about 3000rpm (approximately 3000rpm), the battery voltage is beetween 0,2 and 19 volts.

Your diagnosis?

[ducwiz]

Can you please post an image of your DMM, or quote at least brand and type?

Hans

[Rocla]

[img]

Multimètre.jpg

[/img]

Do you have some doubts about this appliance with such figures?

https://www.amazon.fr/Multim%C3%A8tre-n ... B003NEGZLS

[veloduke]

I agree with Hans' diagnosis. I have seen multimeters display complete garbage because the display either cannot keep up with changing voltage, or be affected by electrical noise (such as might be created by the ignition coil).
Sometimes an analogue meter can give a better clue as to what is happening.

You could try to measure the current flowing in/out of the battery instead of the terminal voltage. That would tell you if your battery is charging or not. There is a 10A current capability with that meter. Don't put the probes across the battery when the meter is set up to read current!!!

[ducwiz]

Well, this is one of the cheapest DMMs I have seen in years ...
I don't know this brand, have no experience with it, but many doubts
This analog meter https://www.reichelt.de/de/fr/multim-tr ... GE=FR&&r=1 is part of my workshop. Due to the sluggish action of its pointer it has a very stable display of the average voltage value.
A DC current measurement would show if the battery is charged at higher revs. Hopefully, your DMM gives a steady display in the 10Amp range.

Hans

[Jordan]

I heard it said that analog meters are better at "trending" readings than digital meters.
A bunch of flashing numbers are not so easy to make sense of.

A varying voltage reading could be due to a poor connection somewhere?

[Rocla]

Many thanks Hans, Ducwiz, Jordan and Veloduke. If I understand well, there are maybe 80% likelihood of a wrong display, but 20% of a possible bad connection somewhere on the bike?

[ducwiz]

imho possible reasons for the jumpy display of readings:

- DMM of inferior quality
- R-R malfunction
- problem with the battery (bad/broken contact between a terminal/pole and the inner Pb plates)
- bad connection or broken wire in loom

Ranking of likelihood: top to bottom.

Is the original Motoplat R-R still in use, or some replacement?

Hans