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David Geach
New User
Username: davidgee

Post Number: 3
Registered: 02-2022
Posted on Thursday, 24 November, 2022 - 23:49:   Edit PostDelete PostView Post/Check IP

Hi All, I am reinstalling the magneto on my 20/25 GGA60. It was not working when I purchased the car, so it appears to be missing a lead from the Switch Box. The lead is shown as Black on the Electric Wiring Diagram. The lead from the magneto to the HT Distributor is installed. I have refurbished and rewired the switch box, and I don't remember the black wire being there. What does the black wire connect to inside the switch box, is it 12V or earth. Also is it permanently connected to either of these or is connected/disconnected when the ignition switch is turned on. David
application/pdfWiring Diagram
Wiring Diagram.pdf (752.3 k)
application/pdfWiring Diagram
Wiring Diagram.pdf (752.3 k)
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Jim Walters
Frequent User
Username: jim_walters

Post Number: 373
Registered: 01-2014
Posted on Friday, 25 November, 2022 - 06:00:   Edit PostDelete PostView Post/Check IP

The black wire from the mag goes to ground when the switchbox is turned off or to battery. The points in the mag are grounded when the switchbox is in those positions which prevents mag operation. When the switchbox is turned to mag, the circuit is open which allows the points to function allowing the mag to fire. So whichever terminal on the switchbox corresponds to those parameters is where that wire connects to.

SRH8505 SRC18015 SRE22493 NAC-05370
www.bristolmotors.com
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David Geach
New User
Username: davidgee

Post Number: 4
Registered: 02-2022
Posted on Sunday, 27 November, 2022 - 20:44:   Edit PostDelete PostView Post/Check IP

Hi Jim, Many thanks for your kind response. So by my reading of your email, by earthing the black wire stops the magneto from providing a spark to the distributor and thus stopping the engine. I also read by Earthing the black wire I assume this does not cause any damage to the magneto. I just did not want to damage the magneto, especially as I've paid alot of money to have it checked over. Unfortunately I have just rewired the switch and fuse boxes, but I missed off this lead, as it was not there when I started the job. Removing the switch box is a fiddly job due to lack of room. But at least I now know what to do. Many thanks again.
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Jim Walters
Frequent User
Username: jim_walters

Post Number: 374
Registered: 01-2014
Posted on Monday, 28 November, 2022 - 04:04:   Edit PostDelete PostView Post/Check IP

You are correct, earthing this wire stops the magneto from working. It is designed to work this way and will not damage the mag.

SRH8505 SRC18015 SRE22493 NAC-05370
www.bristolmotors.com
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David Geach
New User
Username: davidgee

Post Number: 5
Registered: 02-2022
Posted on Tuesday, 29 November, 2022 - 05:16:   Edit PostDelete PostView Post/Check IP

Hi Jim. Many thanks for your help, it is appreciated. I will now have a go at installing the mag. Great website/forum. David

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