Voltage drop at coil

[Barnsey]

My on going investigation is showing that I have a voltage drop of around 4.5V at the coil, and 2A, even with a freshly charged battery. Google AI reckons this might prevent starting. Certainly, it fires the first time I kick it, but, if I don't catch it,  then it is all over on the second kick, and nothing from then on. 

 The resistance of the whole loom at the battery, is 3 Ohm and the coil is 2 Ohm, so the numbers stack up. Maybe I need to whittle down that extra Ohm. 

And, yes, I did clean the points.

I'm half tempted to do a 12V conversion, even if I don't do a full electronic ignition conversion.  These stray fractions of an Ohm would be less of an issue. 
 

[rhyatt]

i feel we may need to clarify "voltage drop"

your battery (fully charged and connected to the bike - nothing turned on) should show about 6.5volts when the red and black croc clips of your meter on the 20volt scale are connected straight across the battery.
ok - turn the ignition key ON and watch the ammeter kick to the - side , showing you have a discharge - ensure the points are closed will confirm this.
Check what the battery voltage has dropped to at the meter - it will drop as the primary of the HT coil is consuming power - maybe 1 volt ish .
but it is the time that this voltage stays stable that will show if your battery is good or bad health.
if it stays stable for a few minutes (bear in mind the HT coil will start to get warm , so don't leave too long) then you are probably fine
you can try to turn the headlamp on as well and this will make the ammeter kick further into the - side , the battery will drop further and if it stays stable for a little while , then you have a happy battery.

are you saying that your battery drops from 6.5v to 4.5 volts and stabilises there with stuff turned on

I spent years as an AA patrol sorting out dead cars with ignition and charging faults and my best friend was my £2.50 HT neon tester that went in between the HT cap and the top of the plug - showing me (without hesitation if the car's issue was loss of spark or something else.

trouble was , I got the car running and waved them off into the sunset leaving my Neon tester on their car - must be hundreds out there!!

or are you saying that the voltage drops to 2 volts showing a volt drop of 4.5v ?? if this was the case, then the battery is not healthy at all (but was it fully charged before you did the test??

you must never do charging checks or battery check if it has not been on charge for a good while

[limeyrob]

You may have something shorting to earth. Disconnect everything except the coil and wire it direct then see what happens. It should run fine without charging for a good 30 mins.
Its also possible the points are shorting under the little plastic ferrule but this usually causes no spark.

[idie]

Where were you based as a Patrolman?

[rhyatt]

Central south west London, Surrey and West country

[iansoady]

I was an AA patrol for a year in the early 70s (based in Birmingham) but left when they brought Relay in as I didn't want to be a glorified lorry driver. The crunch was when I was called 10 minuted before the end of my backshift when they wanted me to take a broken down car to Leicester, which I politely refused.

[Derek996]

Barnsey states that the voltage at the coil drops to 4.5V. He didn't say that the battery voltage drops to 4.5V. It's possible to have a high resistance somewhere between the battery and the coil causing the volt drop, like the ignition switch. He should check the resistance of the loom from the coil back to the battery.

[idie]

I was based in Basingstoke
 And North Hampshire. 26 years with the company last 3 years on the Technical Help Desk before taking early retirement.

[iansoady]

If he's measuring voltage at the coil, he'll get a low reading if the points are closed......

[rhyatt]

with the points OPEN, the voltage at the coil should be virtually the same as that voltage across the battery.

the best/only scale to use to discover "volts drop" is the 20V voltage scale as this is when the circuit under test is actually working

end to end with the 2 clips - 1 on battery + and the other to the points wire or (if points are held open) the actual point face.
any voltage drop will show on the meter - any more than 0.5volts is excessive , preferably 0 volts

[Barnsey]

I powered the bike up tonight, from a bench top PSU, hense no voltage sag at the battery.  The PSU is giving 6.4V and 2.01 A.   The voltage drop across the coil, with the points closed is currently 3.7V.  I would expect it to be more like 5V.  With the points open I have the full 6.4V on the coil. 

I am losing almost nothing in the points (I did clean them), but, weirdly, I am losing about 0.6V where the distributor mates to the engine block, so, maybe I need to take the distributor off and give the mating surfaces a good clean.   However, this would only get the coil drop up to around 4.2V, so it isn't the whole picture. 

[rhyatt]

there are always 2 sides to a voltage drop - the live side and the earth side

from the look of your reading with points open and points closed , you're pretty much ok - 6.4 going down to 3.7 v points closed - i would prefer the 3.4 to be about 4.9 v ish really.

the 0.6v concerns me - try this test--
meter on 20v scale
any colour clip on the earth terminal of the battery ( are you POS earth if so red clip for ease)
and the black clip on the cylinder head fin  or cylinder head nut - in other words on the engine - not the frame
good and tight

everything off no ignition  or anything

now turn your ignition ON with points OPEN - what reading do you get on the meter ?
now points CLOSED , what reading do you get on the meter ? (I am assuming 0.6v)??

let us know

[Barnsey]

Okay,  I've finished for tonight, so I'll take a look tomorrow after work. 

[Derek996]

The 0.6V would indicate a resistance on the earth side of the contacts. With the points closed I would expect the voltage across the coil to be the same as the battery volts assuming no resistance in either the supply side from the battery or the earthing of the distributor. Obviously with the load from the coil the battery volts will have dropped a bit but I'd expect it to be at least 5V depending on the state of the battery.
The other 0.7V - 0.8V drop can only be due to a resistance in the supply side. As I said previously, check the resistance between the coil and the battery - with the battery disconnected. Also with the battery connected and the ignition on check the voltage measured between the battery terminal and the coil with the points closed and with them open. This will give you an idea of any losses in the supply side.

[Barnsey]

I took some measurements after work tonight.  I have the full 6.5V on the engine block, but I am losing 1.5V between the engine and the coil positive.  Most of this appears to be between the engine and the distributor. 
If I measure between the engine and the coil positive I am seeing the 5V which I want.  I guess that this also implies that I am losing 1.5V on the negative side. 

It seems to me that there are three possible solutions. 

1/  Clean and reseat the dizzy, and go around the wiring remaking all connections.  This might become a full loom replacement, and switch replacement, as on my other bike. 

2/   DO a 12V conversion, as a 12 V coil has a higher internal resistance, and all these other traces of resistance will have less effect. 

3/  Go the whole hog, and do a full electronic conversion with 12V to boot.  This should knock it into a cocked hat as there will be no current through the dizzy, all current going via the magic black box. 

   Since advice is in that direction, I will persist with 1 for the moment, but I am strongly tempted to skip 2 and go straight to 3.