C72 Basic Current Draw

[Muddy]

G'day Guys

Just got my ‘64 C72 Dream rebuilt and started (with no oil leaks - now). However, I was surprised at the current draw and rate of voltage drop at the battery when I turned the ignition on and before I started the engine. So if you have any electrical knowledge and/or experience and can assist, please let me know.

To start with, I'm a Mechanical Engineer with only limited electrical experience, but I have a basic understanding of DC electrics, know how to use a multimeter and reckon I can read a basic electrical schematic.

So this is how I see it - I reassembled the bike after a rebuild and the dreaded initial restart oil leak (see other posts). The bike has no light bulbs installed yet and has no real electrical accessories fitted - just enough stuff to get the motor running. I had a new, fully charged battery and went to do the initial restart using the starter motor. The solenoid would engage but the starter motor would not spin (it had previously). With the ignition still turned on, I checked the battery voltage using a multimeter and saw it dropping by about 0.01 volts each few seconds and was reading about 12.5 volts, so I blamed the low voltage for the starter motor not spinning. So I recharged the battery and went to work trying to find the current drawing dropping the battery voltage.

The current draw was only evident when the ignition was turned on and appeared to be about 2.3 amps. I was surprised how high this was - but have nothing to gauge if this is normal or not. So I went about trying to determine what was causing the current draw, thinking it was excessive. I established the current draw was only on the black power cable off the ignition switch - everything else was dead – zero current draw.

On the black power cable, there is no neutral light bulb fitted, so pretty confident the neutral switch is no problem. The horn is not fitted, the rear stop light switch is disconnected and so is the winker relay. That only just leaves the ignition coil, condenser and points remaining on the circuit. I pulled the green outlet cable off the coil and measured the 2.3 amps on the outlet of the coil on the way to the condenser and points. I had relocated the condenser and blamed that it was not earthing properly and checked that this was all OK by doing a continuity check to earth. I turned the motor over a few times to see if it was a points-position issue, but nothing seem to change.

So after all this and out of shear frustration, I kick started the bike and away it went. The voltage at the battery seemed pretty constant while running and it ran just fine. I went for a couple of rides up and down the street and all ran fine. Now I pretty sure the starter motor problem is actually related to the starter motor itself rather than an electrical circuit problem.

So after all this, I guess my questions are:

What would be the expected current draw, with the ignition turned on and just the ignition coil, points and condenser in the circuit? 2.3 amps just sounded too high, but is this normal?

If the current draw is excessive what should I be looking for/checking to try and rectify it? Is this a condenser issue?

Hope you electrical techno gurus can help.

[48lesco]

Mud - One M.E. to Another... The coil primary winding (in series with the points) should have about 3-4 Ohms resistance. Put 12V on that and you get 3-4 amps if the points happen to be closed. Open them up and you should get no amps. The condenser itself should show infinite resistance between it's wire and ground.
-48

[Muddy]

Hey 48

MEs - we'll rule the world one day! Thanks that is great info. I'll do some more testing on the weekend and see what I come up with, but that is a great start and gives me some confidence that things are more normal than I first thought.

Greatly appreciated.

[Muddy]

Hey 48

You don't happen to know the expected resistance of the starter motor by any chance do you?

[48lesco]

No not off hand but I wouldn't think it would be much maybe a couple of Ohms assuming good contact between the brushes and armature. I have one I can check tonight. What is the problem, not cranking?
-48

[Muddy]

Yeah, not cranking. Have not done too much yet but will have a look at it today. I was gunna check open circuit and short circuit and the brushes. I remember one brush being a little sticky on reassembly.

Thanks for your help.

[Muddy]

G'day 48 and thanks for your help, but don't worry - it was a sticky earth brush. Freed it up and all looks OK.