Faulty reg/rec or alternator or both?

[Baz]

Hi guys, ive got a 91 NC30 and having a few starting problems.
When I try to start with a fully charged battery the voltage drops down to about 4v and bike really struggles but then voltage slowly climbs and when it gets to about 12v fires up. If I leave it stood all day not on charge it then wont start. I have looked at the reg/rec and one of the terminals is a bit burnt. I have also tried testing the 3 yellow wires on the alternator but the readings on the multimeter dont change at all and also dont change if I put the probe from yellow wire to earth. I'm not sure if I'm testing this correctly but if it is faulty are these the readings I would get?

[magg]

Batteries usually do not drop to 4 volts and then recover to 12 volts when cranking. How are you measuring the battery voltage (AC/DC)? What were you trying to measure on the three yellow wires (voltage AC/DC or resistance), was the rect/reg disconnected?

[Baz]

I have a little digital display that shows time, temp and battery voltage. It is connected directly to the battery so I assumed it must be a correct reading. When testing the yellow wires the reg/rec was disconnected but I thought I was testing the other side of the circuit? (Can you tell I'm not an expert yet?) and I tried the multimeter on on ohms setting. Wasn't sure what number to put the tester on so I tried it on 2, 20 & 200??? The multimeter just read '1' and didn't change no matter where I put the probes.

[magg]

Has your battery voltage monitor always started at 4 volts and slowly increased to indicate 12 volts or is this a recent development.

Does your multimeter indicate 0.00 ohms when you short the probes together. '1' is an indication of infinite resistance.

With rect/reg disconnected resistance between each pair of yellow wires should be 1-2 ohms on 2 ohm range. If not then the alternator wiring needs checking There is a connector, located in the V between the front and rear cylinder pairs, that is a known weak point. If this connector is ok then the alternator is suspect although it is unusual for all three yellow wire pairs to read infinite resistance.

[Baz]

Thanks for the info. I will check it tonite and post the results. The battery has only just started to show low volts and then climb. Before it would show about 12.5 and drop to 12.2 when I pushed the start button. Somebody has suggested to me it could be a bad earth but he is a car mechanic and doesn't work on bikes. ??

[magg]

I suggest you measure the battery voltage with your multimeter when the engine is running, should read 14-15 volts at 5000 rpm. Does the engine start using the battery or do you need to recharge it.

[Baz]

It starts off the battery but its kept on a trickle charger. I will get the multimeter on it tonite and post the readings.
Thanks.

[Baz]

I have just checked the battery with the multimeter and it reads 12.95v at 5000rpm
I also rechecked the yellow alternator wires. I tested the multimeter first, set at 2 on the ohm setting it reads 1 and when i put the probes together it does go to 0.00 I then tested the wires and on all the wires (2 at a time obv) it stays at 1.
I havent got time to strip the bike tonight to look for the connection between the V but will get that checked tomorrow. Any other suggestions now that we know the voltage at 5000rpm is only at 12.9v? The battery is less than a year old and was not a cheap one so I would guess thats ok unless other electrical faults could have f*#ked it up?

[bikemonkey]

My lithium ion battery did this after a loose connection meant the bike was effectively running total loss and the bike flattened it completely.

I had a month of the bike taking 5 minutes to start as the battery cranked the starter harder each time I prodded it.

I soon got fed up and got a new battery though, didn't get a lithium one though. Too expensive really and not much advantage other than weight.

[Baz]

my battery is a Motobatt and is yellow. I cant remember the model number but I do recall there being a choice of 2 that would fit and I went for the dearer one. It was recommended by someone on this site and seeing as the majority of you know a shit lot more than me about NC30's I thought it would be a good choice.