Joined: May 2018 Posts: 2,104 Threads: 110
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Location: Llandrindod Wells
Car type: 29 Special, 30 RK, 28 C Cab
Hi All
I'm a bit worried by the current draw shown on the ammeter when I switch on the ignition on my 1930 RK. It reads 6 amps discharge (without the engine running). I've established that when the points are open the discharge is pretty much zero so I reason that it can only be the ignition circuit that is causing the high current draw.
I thought I'd found the answer when I tested the coil and found the resistance between the + and - terminals was 1 ohm. I reasoned that from ohms law 6 volts divided by 1 ohm gives 6 amps. I thought the coil was faulty. So I purchased a new coil from a reputed supplier and found that too had a resistance of just 1 ohm and apparently that is normal for the coils they sell.
The car starts, runs and has no ill effects from the high ignition current apart from the fact the dynamo struggles to counter the discharge unless its revved.
Question is 6 amp discharge normal? Is there anything else that could be drawing the current?
Cheers
Howard
Joined: Aug 2017 Posts: 984 Threads: 6
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Location: Scottish Borders
Once the engine is running the coils open and close continually. This reduces the average current probably to less than 3 Amps.
6 Amps with the points closed is about what I would expect expect.
Jim
Joined: Aug 2017 Posts: 3,388 Threads: 105
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Location: Darkest Bedfordshire
Resistance between + & - terminals of a working coil is typically 0.2 - 2 ohms. From either terminal to the HT connection 6000 - 15000 ohms (mine was 8570 when last I checked).
It's a bit late to go down and look now but I'm pretty sure my ammeter shows a generous discharge when ignition is switched on before engine start.
Joined: Aug 2017 Posts: 925 Threads: 74
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Location: Essex
I find the handbrake puts the brake lights on and draws about 6 amps until I start the engine. Apparently it shouldn't, so I'd like to see a photo of someone's.
Joined: Aug 2017 Posts: 425 Threads: 30
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Location: Wellington, NZ
Isn't it normal for the current draw to be high with the ignition on and the engine not running because the points will be closed so all the current runs through the coil?
With a 1 ohm coil you'd get 6 amps from a 6 volt battery (from ohms law). That's normal. It's the points opening that induces the high voltage in the secondary of the coil to make the spark. If you look at the distributor you can see the lobes open the points and most of the time they are closed.
I was always told with with older cars not to leave the ignition on for any period of time exactly for this reason, the coil will get hot with all the current going through it.
As Jim mentions once the car is running the points are switching on and off so the total current will go down.
Simon
Joined: Aug 2017 Posts: 984 Threads: 6
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Location: Scottish Borders
An advantage of the Accuspark electronic ignition is that it switched off the coil current after a few seconds. On switch on the ammeter shows the 6 to 8 amp discharge and then drops to zero. This means inadvertently leaving the ignition on won't cause damage to the coil and reduce the danger of flattening the battery.
Jim
Joined: Aug 2017 Posts: 780 Threads: 26
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Location: On a hill in Wiltshire
That would be a normal sort of reading on both my cars.
It all goes positive when the engines are running, I have never bothered about it.
Simon
Joined: May 2018 Posts: 2,104 Threads: 110
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Location: Llandrindod Wells
Car type: 29 Special, 30 RK, 28 C Cab
Thanks All
Reassuring.
I know that when running and with the dynamo disconnected the draw drops to about 2 amps so everything seems normal.
I intend to run unmodified at VSCC events so the luxury of electronic ignition is not for me. However I believe that a solid state cut-out and even regulator may be allowed. I have a diode based cut-out ready to fit as a spare.
Cheers for all the advice
Howard
